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Twisted pair cables support a wide variety of fast, modern network standards. Twisted pair cabling is
composed of the following components:
 Two wires that carry the data signals (one conductor carries a positive signal and one carries a
negative signal). They are made of 22 or 24 gauge copper wiring.
 PVC or plenum plastic insulation surrounds each wire.
 Two wires are twisted to reduce the effects of electromagnetic interference (EMI) and crosstalk.
 Multiple wire pairs are bundled together in an outer sheath. Twisted pair cable can be classified
according to the makeup of the outer sheath:
Shielded Twisted Pair (STP) has a grounded outer copper shield around the bundle of
twisted pairs or around each pair. This provides added protection against EMI.
Unshielded Twisted Pair (UTP) does not have a grounded outer copper shield. UTP cables
are easier to work with and are less expensive than shielded cables.
The table below describes the different unshielded twisted pair (UTP) cable types (categories):
Type
Connector
Cat3
RJ-45
Designed for use with 10 megabit Ethernet or 16 megabit token ring.
Cat5
RJ-45
Supports 100 megabit Ethernet and ATM networking.
Cat5e
RJ-45
Similar to Cat5 but provides better EMI protection. Supports 100 megabit and
gigabit Ethernet.
RJ-45
Supports 10 gigabit Ethernet and high-bandwidth broadband
communications.
Cat6 cables often include a solid plastic core that keeps the twisted pairs
separated and prevents the cable from being bent too tightly.
Additional standards for Cat6 include Cat6a (advanced) and Cat6e
(enhanced), which provide better protection against EMI.
Cat6
Description
Each type of UTP cable can be substituted for any category below it, but never for a category above. For
example, Cat6 can be substituted for a task requiring Cat5e; however, neither Cat5 nor Cat3 should be used
for this particular task.
The RJ-45 connector is used with twisted pair cables to establish network connections. An RJ-45 connector
has the following characteristics:
 Has 8 individual connectors
 Supports up to 4 pairs of wires
 Uses a locking tab to keep the connector secure in an outlet
Ethernet specifications use the following pins on RJ-45 connectors (Tx is a pin used for transmitting and Rx
is a pin used for receiving):








Pin 1: Tx+
Pin 2: TxPin 3: Rx+
Pin 4: Unused
Pin 5: Unused
Pin 6: RxPin 7: Unused
Pin 8: Unused
When connecting devices in a LAN, you will need to use different types of Ethernet cables. You will need to
know the pin positions of the cable types to differentiate them from each other. The types of Ethernet cables
used for LAN connections include the following:
 Use a straight-through Ethernet cable when connecting the following devices:
Workstation to hub
Workstation to switch
Router to hub
Router to switch
 Use a crossover Ethernet cable when connecting the following devices:
Switch to switch
Switch to hub
Hub to hub
Workstation to router
Workstation to workstation
Router to router
You should also be aware of the following when making LAN connections:
 Through Auto-MDI/MDIX, newer switches can determine what type of Ethernet cable is needed and
will internally change the sending/receiving pin positions as needed.
 Some Cisco routers provide a generic Attachment Unit Interface (AUI) port. The AUI port is
designed to connect to an external transceiver for conversion to a specific media type, such as
coaxial or fiber optic.
 To support LAN distances above twisted pair Ethernet limits (>100 meters), use the switch's SFP slot
(a Gigabit uplink port) and fiber optic media.
Fiber optic cables use two fiber strands to connect computers together. One strand transmits signals and the
other receives signals. Fiber optic cabling is composed of the following components:
 A plastic or glass core that carries the signal.
 A cladding surrounding the core that maintains the signal as the cable bends.
 A sheath that protects the cladding and the core.
Fiber optic cabling offers the following advantages and disadvantages:
Advantages
 Completely immune to EMI
(electromagnetic interference)
Disadvantages
 Very expensive
 Difficult to work with
 Highly resistant to eavesdropping
 Supports extremely high data transmission
 Special training required to attach
connectors to cables
rates
 Allows greater cable distances without a
repeater
Multi-mode and single-mode fiber cables are distinct from each other and are not interchangeable. The table
below describes multi-mode and single-mode fiber cables:
Type
Description
 Transfers data through the core using a single light ray (the ray is also called a
mode)
Singlemode
Multi-mode
 The core diameter is around 10 microns
 Supports a higher bandwidth than multi-mode cables
 Cable lengths can extend a great distance
 Transfers data through the core using multiple light rays
 The core diameter is around 50 to 100 microns
 Cable lengths are limited in distance
Fiber optic cabling uses the following connector types:
Type
Description
 Used with single- and multi-mode
cabling
 Keyed, bayonet-type connector
 Also called a push-in and twist
connector
ST Connector
 Each wire has a separate connector
 Nickel plated with a ceramic ferrule
to ensure proper core alignment and
prevent light ray deflection
 As part of the assembly process, it is
necessary to polish the exposed
fiber tip to ensure that light is
passed from one cable to the next
with no dispersion
SC Connector
 Used with single- and multi-mode
cabling
 Push-on, pull-off connector type that
uses a locking tab to maintain a
connection
 Each wire has a separate connector
 Uses a ceramic ferrule to ensure
proper core alignment and prevent
light ray deflection
 As part of the assembly process, it is
necessary to polish the exposed
fiber tip
 Used with single- and multi-mode

LC Connector



cabling
Composed of a plastic connector
with a locking tab, similar to an RJ45 connector
A single connector with two ends
keeps the two cables in place
Uses a ceramic ferrule to ensure
proper core alignment and prevent
light ray deflection
Half the size of other fiber optic
connectors
 Used with single- and multi-mode
cabling
 Composed of a plastic connector
with a locking tab
MT-RJ Connector
 Uses metal guide pins to ensure
proper alignment
 A single connector with one end
holds both cables
 Uses a ceramic ferrule to ensure
proper core alignment and prevent
light ray deflection
The following table lists several common connectivity devices used within a LAN:
Device
Hub
Description
A hub is the central connecting point of a physical star, logical bus topology. Hubs manage
communication among hosts using the following method:
1. A host sends a frame to another host through the hub.
2. The hub duplicates the frame and sends it to every host connected to the hub.
3. The host to which the frame is addressed accepts the frame. Every other host ignores
the frame.
Hubs are Layer 1 devices; they simply repeat incoming frames without examining the MAC
address in the frame.
A bridge is a data forwarding device on a network. You should understand the following key
concepts relating to the operation of bridges. Bridges:
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

Connect two media segments that use the same protocol.
Examine the source address to determine the media segment of network devices.
Operate at layer 2, the Data Link layer, of the OSI model.
Maintain a table of device addresses and their corresponding segments.
Allow each segment connected by a bridge to have the same network address.
Prevent messages within a media segment from crossing over to another segment.
Bridges offer the following advantages:




Wasted bandwidth is prevented by eliminating unnecessary traffic between segments.
Maximum network length is increased.
Packets for multiple upper-layer protocols are forwarded.
Segments with dissimilar transmission media and media access methods can be linked.
Bridges have the following limitations:
Bridge

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


Multiple architectures cannot be linked because different frame types are used.
Upper-layer protocols cannot be translated.
Packets for different networks cannot be forwarded based on the network address.
Broadcast packets are not filtered.
Forwarding frames between segments introduces latency.
Use bridges to isolate traffic to a segment, to prevent unwanted traffic from crossing over to
other segments, or to slow WAN links. When designing the placement of bridges on the
network, follow the 80/20 rule:
 At least 80% of network traffic should stay within a segment.
 No more than 20% of network traffic should pass through the bridge to another
segment.
A bridge builds a database based on MAC addresses to make forwarding decisions:
 The process begins by examining the source MAC address of an incoming frame. If
the source address is not in the forwarding database, an entry for the address is made
in the database associating the MAC address with the media segment.
 The destination address is then examined:
If the destination address is not in the database, the frame is sent out on all
segments, except for the one on which it was received.
If the destination address is in the database, the frame is forwarded to the
appropriate segment so long as the segment is different than the one on
which it was received.
Broadcast frames are forwarded to all segments except the segment on
which the frames were received.
A switch is a multiport bridge. It provides the same functionality, but with a higher port
density. In addition, switches provide features that cannot be found in bridges. Switches have
replaced Ethernet hubs and bridges in most network applications. Switches:
 Manipulate Ethernet frames at the Data Link layer of the OSI Model. A switch
examines the Data Link header within the frames it receives to determine how each
frame should be processed. This information is used by the switch to do the
following:
Learn connected device MAC addresses
Forward frames
Filter frames
 Connect multiple segments or devices and forward packets to only one specific port.
 Connect a single device to a switch port or multiple devices to a switch port by using a
hub.
Switches offer the several advantages over a non-switched network. Switches:
 Connect multiple segments for devices and forward packets to only one specific port.
Switch

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This is called microsegmentation.
Produce less latency than other segmentation solutions.
Can be used to provide collision-free networking if only one device is connected to
each switch port.
Create separate collision domains.
Provide guaranteed bandwidth between devices if dedicated ports are used.
Enable full-duplex communication.
Can simultaneously switch multiple messages.
Support rate adaptation, which allows devices that run at different speeds to
communicate with each other. For example, 10 Mbps, 100 Mbps, and 1000 Mbps
devices can communicate with each other when connected to a 1000 Mbps switch.
Can connect a single device to a switch port, or can connect multiple devices to a
switch port by connecting it to another switch.
Different types of switches can be implemented. Switches can be categorized according to the
layer of the OSI model in which they function. Two common classifications include:
 A Layer 2 switch operates at the Data Link layer of the OSI model to process frames
within a single physical network segment. This is the most commonly implemented
type of switch.
 A Layer 3 switch provides all the functionality of a Layer 2 switch but also provides
routing functionality at the Network layer of the OSI model. This allows the switch to
process frames within a network segment (as a Layer 2 switch does) and to route
packets between network segments (as a LAN router does). Layer 3 switches are
sometimes called multilayer switches because they function at multiple layers of the
OSI model.
A router is a Layer 3 device that sends packets from one network to another network. Routers receive
packets, read their headers to find addressing information, and send them to their correct destination on the
network or Internet. Routers can forward packets through an internetwork by maintaining routing
information in a database called a routing table. The routing table typically contains the address of all known
networks and routing information about that network, such as:
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
Interface
Routing Path
Next Hop
Route Metric (Cost)
Route Timeout
Routers build and maintain their routing database by periodically sharing information with other routers. The
exact format of these exchanges is based on the routing protocol. The routing protocol determines:
 The information contained in the routing table.
 How messages are routed from one network to another.
 How topology changes (i.e., updates to the routing table) are communicated between routers.
Regardless of the method used, changes in routing information take time to propagate to all routers on the
network. The term convergence is used to describe the condition when all routers have the same (or correct)
routing information.
Routers provide more functionality than either switches or bridges. For example, routers:
 Support multiple routing protocols for better flexibility.
 Provide more features than switches or bridges, such as flow control, error detection, and congestion
control.
 Provide multiple links between devices to support load balancing.
 Can connect different network architectures together. For example, a router could be used to connect
an older Token Ring network to an Ethernet network.
Because of their enhanced features, however, routers are also more expensive and more difficult to
configure.
When learning about TCP/IP protocols, it is common to use a theoretical layered model called the TCP/IP
model (also known as the Department of Defense (DoD) model). The TCP/IP model classifies and organizes
the tasks that hosts perform to prepare data for transport across the network. You should be familiar with the
TCP/IP model because it is a widely used method for understanding and talking about network
communications. However, remember that it is only a theoretical model that defines standards for
programmers and network administrators, not a model of actual physical layers.
Using the TCP/IP model to discuss networking concepts has the following advantages:
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

Provides a common language or reference point between network professionals
Divides networking tasks into logical layers for easier comprehension
Allows specialization of features at different levels
Aids in troubleshooting
Promotes standards and interoperability between networks and devices
Provides modularity in networking features (developers can change features without changing the
entire approach)
However, you must remember the following limitations of the TCP/IP model:
 TCP/IP layers are theoretical and do not actually perform real functions.
 Industry implementations rarely have a layer-to-layer correspondence with the TCP/IP layers.
 Different protocols within the stack perform different functions that help send or receive the overall
message.
 A particular protocol implementation may not represent every layer (or it may spread across multiple
layers).
The layers of the TCP/IP model are as follows:
 The Application layer (also called the Process-to-Process layer) corresponds to the Session,
Presentation, and Application layers of the OSI model.
 The Transport layer (also called the Host-to-Host layer) is comparable to the Transport layer of the
OSI model and is responsible for error checking and reliable packet delivery. This is when the data
stream is broken into segments that must be assigned sequence numbers so that the segments can be
reassembled correctly on the remote side.
 The Internet layer is comparable to the Network layer of the OSI model. It is responsible for moving
packets through a network. This involves addressing of hosts and making routing decisions to
identify how the packet traverses the network.
 The Link layer corresponds to the functions of the Physical and Data Link layers of the OSI model. It
is responsible for describing the physical layout of the network and how messages are formatted on
the transmission medium. Sometimes this layer is divided into the Data Link and the Physical
layers.
The TCP/IP model focuses specifically on the functions in the Internet layer and the Transport layer. All other
functions of the traditional OSI model are encompassed in the first and fourth layers.
The following table compares the functions performed at each TCP/IP model layer:
Layer
Application
(Process-toProcess)
Description
The Application layer contains high-level protocols used by processes (applications)
running on a host for network communications. The Application layer integrates
network functionality into the host operating system and enables network services. The
Application layer does not include specific applications that provide services, but rather
provides the capability for services to operate on the network.
Processes operating at the Application layer on the source host send data to other
processes running at the Application layer on a destination host. For example, a Web
browser on a client system can send an HTTP GET request to the Web service running
on a network server to request that it send a particular Web page.
Processes running on the source host produce the data to be transmitted and encode it
using the appropriate Application layer protocol. Some commonly-used Application
layer protocols include FTP, HTTP, Telnet, SMTP, DNS, and SSH. Once encoded, the
data is then sent to the Transport layer where it is encapsulated using the appropriate
Transport layer protocol.
The Application layer in the TCP/IP model corresponds to the Session, Presentation,
and Application layers of the OSI model.
The Transport layer is responsible for error checking and reliable delivery. The
Transport layer provides the following key functions:
 The sending Transport layer receives a stream of information from the
Application layer and breaks it into smaller chunks called segments.
Segmentation is necessary to enable the data to meet network size and format
restrictions.
 The receiving Transport layer uses packet sequence numbers to reassemble
segments into the original message.
 The Transport layer establishes a communication channel that can be used to
transfer data to a remote host.
Protocols that are associate with the Transport layer include:
 Transport Control Protocol (TCP):
Transport
(Host-toHost)
TCP creates a connection-oriented communication channel. Prior to
transmission, TCP negotiates a connection with the remote host
using a three-way handshake:
 The source host sends the destination host a TCP SYN
message.
 The destination host responds with TCP SYN/ACK
message.
 The source host responds with a TCP ACK message.
TCP uses acknowledgements after each packet is transmitted to
ensure that the data arrived correctly. Any missing, damaged, or
discarded packets are retransmitted.
TCP ensures a high degree of reliability. However, it also incurs a
degree of latency due to the extra overhead required to ensure data
integrity:
 TCP is most appropriate for communications where data
integrity is more important than transmission speed.
 For example, when saving a file on a network server using
the SMB protocol, a few milliseconds of latency is of little
concern, but the integrity of the data is critical.
 User-Datagram Protocol (UDP):
UDP uses connectionless communications.
Unlike TCP, UDP does not set up a connection nor does it use
acknowledgements to ensure the data arrived properly.
UDP assumes that lower level protocols can reliably deliver packets
to the destination host.
This protocol is most appropriate for application-level processes that
require low-latency transmissions and can tolerate a degree of
missing or out of sequence packets.
UDP is commonly used by streaming audio, streaming video, and
Voice over IP (VoIP) applications.
The Transport layer uses the concept of the port to enable application-to-application
communications between hosts. A port is a number that is logically assigned to each
service running on a system. Using ports allows a network host with a single IP address
to provide multiple services, each sending and receiving data on its own port. The
Transport layer header applied to each segment before transmission identifies the
source port on the sending host as well as the destination port on the receiving host.
Standardized port numbers have been defined for well-known services. For example:
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FTP: 20 and 211
SSH: 22
SMTP: 25
DNS: 53
HTTP: 80
POP3: 110
IMAP: 143
HTTPS: 443
The Transport layer is comparable to the Transport layer of the OSI model.
The Internet layer is responsible for forwarding packets through multiple networks.
This process is called routing. The Internet layer manages the host addressing and
routing decisions to identify how packets traverse networks. Protocols that reside at the
Internet layer include:




Internet
Internet Protocol (IP))
Address Resolution Protocol (ARP)
Internet Control Message Protocol (ICMP)
Internet Group Management Protocol (IGMP)
The Internet layer uses logically-assigned IP addresses to uniquely identify networks
and network hosts. Each address assigned to a host identifies:
 The network the host resides on.
 The host's unique identity on that network.
The Internet layer header applied to each packet before transmission includes the
source IP address of the sending host as well as the destination IP address of the
receiving host. When transmitting data, the Internet layer uses the source and
destination network addresses to determine whether the hosts reside on the same
network or on different networks:
 If they reside on the same network, the data can be sent directly to the
destination host.
 If they reside on different networks, the Internet layer can forward packets from
router to router until they reach the appropriate destination host.
Key Internet layer functions include:
 Maintaining addresses of neighboring routers.
 Maintaining a list of known networks.
 Determining the next network point to which data should be sent. Routers use a
routing protocol to take into account various factors, such as the number of
hops in the path, link speed, and link reliability to select the optimal path for
data.
The Internet layer is not concerned with reliable delivery of information. Instead, it
relies on the Transport layer to establish a host-to-host communication channel and
ensure information arrives correctly at the destination host.
The Internet layer is comparable to the Network layer of the OSI model.
The Link layer is responsible for describing the physical layout of the network and how
messages are electrically transmitted. It is used to move information between hosts by
controlling how individual bits are transmitted and received on the network medium.
Each host is uniquely identified at the Link layer using a Media Access Control (MAC)
address. Every network interface has a physical MAC address assigned to it by the
manufacturer. This address is stored in the firmware of the network interface itself.
Theoretically, no two network interfaces in the world should have the same MAC
address assigned.
Link
Unlike an IP address, a MAC address only identifies the host. It does not identify the
network where the host resides. As a result, the link layer is not concerned with which
network the sending and receiving hosts reside on. It simply transmits data from
interface to interface using electrical signals on the network medium.
The Link layer converts the data to be transmitted into frames by adding a Link layer
header, which includes physical device addressing information. Each frame processed
by the Link layer includes the source MAC address and the destination MAC address.
The Link layer then converts the frames into bits for transmission across the network
media.
The Link layer corresponds to the functions of the Physical and Data Link layers of the
OSI model.
The TCP/IP model focuses specifically on the functions in the Internet layer and the Transport layer. All other
functions of the traditional OSI model are encompassed in the first and fourth layers.
Network ports are logical connections, provided by the TCP or UDP protocols at the Transport layer, used by
protocols in the upper layers of the OSI model. The TCP/IP protocol stack uses port numbers to determine
what protocol incoming traffic should be directed to. Below are a few characteristics of ports:
 Ports allow a single host with a single IP address to run network services. Each port number
identifies a separate service.
 Each host can have over 65,000 ports per IP address.
 Port use is regulated by the Internet Corporation for Assigning Names and Numbers (ICANN).
ICANN specifies three port categories:
Categories
Characteristics
 Assigned for specific protocols and services
 Port numbers range from 0 to 1023
Well known
 ICANN can assign a specific port for a newly created network service
 Port numbers range from 1024 to 49151
Registered
 Assigned when a network service establishes contact and released when the
session ends
 Allows applications to 'listen' to the assigned port for other incoming requests
(traffic for a protocol can be received through a port other than the port which
the protocol is assigned, as long as the destination application or service is
'listening' for that type of traffic on that port)
 Port numbers range from 49,152 to 65,535
Dynamic
(private or
high)
The following table lists the well-known ports that correspond to common Internet services:
Protocol(s)
Port(s)
Service
TCP
20, 21
File Transfer Protocol (FTP)
TCP
UDP
22
Secure Shell (SSH)
TCP
UDP
23
Telnet
TCP
UDP
25
Simple Mail Transfer Protocol (SMTP)
TCP
UDP
53
UDP
67, 68
UDP
69
Trivial File Transfer Protocol (TFTP)
TCP
80
Hypertext Transfer Protocol (HTTP)
TCP
110
Post Office Protocol (POP3)
TCP
119
Network News Transport Protocol (NNTP)
UDP
123
Network Time Protocol (NTP)
TCP
UDP
143
Internet Message Access Protocol (IMAP4)
TCP
UDP
161, 162
Simple Network Management Protocol (SNMP)
TCP
UDP
389
Lightweight Directory Access Protocol (LDAP)
TCP
443
HTTP with Secure Sockets Layer (SSL)
Domain Name Server (DNS)
Dynamic Host Configuration Protocol (DHCP)
Encapsulation is the process of breaking a message into packets, adding control and other information, and
transmitting the message through the transmission media. You need to know the following four-step data
encapsulation process on the sending system using the TCP/IP model:
1. The Application layer prepares the data to be sent through the network.
2. The Transport layer breaks the data into pieces called segments, adding sequencing and control
information.
3. The Internet layer converts the segments into packets, adding logical network and device addresses.
4. The Link layer converts the packets into frames, adding physical device addressing information. It
also converts the frames into bits for transmission across the transmission media.
On the destination host, the process operates in reverse, with bits from the network medium being received
by the Link layer and being processed up the model to the destination application.
The following can help you remember the steps of the data encapsulation process:
1. Application layer: data
2. Transport layer: segments
3. Internet layer: packets containing logical addresses
4. Link layer: framing that adds physical addresses and bits that are transmitted on the network medium
The encapsulation process works in the same manner using the OSI model. As data travels through the OSI
model layers, it is broken into segments at the Transport layer. Logical addresses are added at the Network
layer, making each segment a packet. The Data Link layer creates frames from each packet with the physical
device address (MAC address). Frames are converted to bits at the Physical layer.
Encapsulation is the process of breaking a message into packets, adding control and other information, and
transmitting the message through the transmission media. You need to know the following four-step data
encapsulation process on the sending system using the TCP/IP model:
1. The Application layer prepares the data to be sent through the network.
2. The Transport layer breaks the data into pieces called segments, adding sequencing and control
information.
3. The Internet layer converts the segments into packets, adding logical network and device addresses.
4. The Link layer converts the packets into frames, adding physical device addressing information. It
also converts the frames into bits for transmission across the transmission media.
On the destination host, the process operates in reverse, with bits from the network medium being received
by the Link layer and being processed up the model to the destination application.
The following can help you remember the steps of the data encapsulation process:
1. Application layer: data
2. Transport layer: segments
3. Internet layer: packets containing logical addresses
4. Link layer: framing that adds physical addresses and bits that are transmitted on the network medium
The encapsulation process works in the same manner using the OSI model. As data travels through the OSI
model layers, it is broken into segments at the Transport layer. Logical addresses are added at the Network
layer, making each segment a packet. The Data Link layer creates frames from each packet with the physical
device address (MAC address). Frames are converted to bits at the Physical layer.
OSI model
The OSI model classifies and organizes the tasks that hosts perform to prepare data for transport across the
network. You should be familiar with the OSI model because it is the most widely used method for
understanding and talking about network communications. However, remember that it is only a theoretical
model that defines standards for programmers and network administrators, not a model of actual physical
layers.
Using the OSI model to discuss networking concepts:
 Provides a common language or reference point between network professionals.
 Divides networking tasks into logical layers for easier comprehension.




Allows specialization of features at different levels.
Aids in troubleshooting.
Promotes standards and interoperability between networks and devices.
Provides modularity in networking features (developers can change features without changing the
entire approach).
However, you must remember the following limitations of the OSI model:
 OSI layers are theoretical and do not actually perform real functions.
 Industry implementations rarely have a layer-to-layer correspondence with the OSI layers.
 Different protocols within the stack perform different functions that help send or receive the overall
message.
 A particular protocol implementation may not represent every OSI layer (or may spread across
multiple layers).
To help remember the layer names of the OSI model, try using one of the following mnemonic devices:
Layer
Name
Mnemonic
Mnemonic
(Bottom to top)
(Top to bottom)
Layer 7
Application
Away
All
Layer 6
Presentation
Pizza
People
Layer 5
Session
Sausage
Seem
Layer 4
Transport
Throw
To
Layer 3
Network
Not
Need
Layer 2
Data Link
Do
Data
Layer 1
Physical
Please
Processing
The following table summarizes basic characteristics of the lower OSI model layers:
Layer
Physical
Description
The Physical layer of the OSI model sets standards for sending and receiving
electrical signals between devices. It describes how digital data (bits) are
converted to electric pulses, radio waves, or pulses of lights.
Devices that operate at the physical layer send and receive a stream of bits.
The Media Access Control (MAC) layer defines specifications for controlling
access to the media. The MAC sublayer is responsible for:
Media
Access
Control
(MAC)




Adding frame start and stop information to the packet.
Adding Cyclical Redundancy Check (CRC) for error checking.
Converting frames into bits to be sent across the network.
Identifying network devices and network topologies in preparation for
media transmission.
 Defining an address (such as the MAC address) for each physical
device on the network.
 Controlling access to the transmission medium.
The Logical Link Control (LLC) layer provides an interface between the MAC
layer and upper-layer protocols. LLC protocols are defined by the IEEE 802.2
committee. The LLC sublayer is responsible for:
Data
Link
 Maintaining orderly delivery of frames through sequencing.
 Controlling the flow or rate of transmissions using the following:
Logical
Link
Control
(LLC)





Acknowledgements
Buffering
Windowing
Ensuring error-free reception of messages by retransmitting.
Converting data into an acceptable form for the upper layers.
Removing framing information from the packet and forwarding the
message to the Network layer.
Providing a way for upper layers of the OSI model to use any MAC
layer protocol.
Defining Service Access Points (SAPs) by tracking and managing
different protocols.
The Network layer describes how data is routed across networks and to the
destination. Network layer functions include:
Network
 Maintaining addresses of neighboring routers.
 Maintaining a list of known networks.
 Determining the next network point to which data should be sent.
Routers use a routing protocol to take into account various factors,
such as the number of hops in the path, link speed, and link reliability
to select the optimal path for data.
Packets forwarded from the Transport layer to the Network layer become
datagrams and network-specific (routing) information is added. Network layer
protocols then ensure that the data arrives at the intended destinations.
The Transport layer provides a transition between the upper and lower layers of
the OSI model, making the upper and lower layers transparent from each other.
 Upper layers format and process data without regard for delivery.
 Lower layers prepare the data for delivery by fragmenting and attaching
transport required information.
Transport layer uses the following:
 Port (or socket) numbers are used to identify distinct applications


Transport


running on the same system. This allows each host to provide multiple
services.
The Transport layer receives large packets of information from higher
layers and breaks them into smaller packets called segments.
Segmentation is necessary to enable the data to meet network size and
format restrictions.
The receiving Transport layer uses packet sequence numbers to
reassemble segments into the original message.
Connection-oriented protocols perform error detection and correction
and identify lost packets for retransmission. A connection-oriented
protocol is a good choice when:
Reliable, error-free communications are more important than
speed.
Larger chunks of data are being sent.
Connectionless services assume an existing link between devices and
allow transmission without extensive session establishment.
Connectionless communications use no error checking, session
establishment, or acknowledgements. Connectionless protocols allow
quick, efficient communication at the risk of data errors and packet
loss. Connectionless protocols are a good choice when:
Speed is important.
Smaller chunks of data are being sent.
The following table summarizes basic characteristics of the upper OSI model layers:
Layer
Application
Description
The Application layer integrates network functionality into the host operating system and
enables network services. The Application layer does not include specific applications
that provide services, but rather provides the capability for services to operate on the
network. These services include:
 File services (transferring, storing, and updating shared data)
 Print services (enabling network printers to be shared by multiple users)
 Message services (transferring data in many formats (text, audio, video) from one
location to another or from one user to another)
 Application services (sharing application processing throughout the network and
enabling specialized network servers to perform processing tasks)
 Database services (storing, retrieving, and coordinating database information
throughout the network)
The Application layer specifies many important network services that are used on the
Internet, which include:





HTTP
Telnet
FTP
TFTP
SNMP
Most Application layer protocols operate at multiple layers down to the Session layers and
even Transport layers. However, they are classified as Application layer protocols because
they start at the Application layer (the Application layer is the highest layer where they
operate).
The Presentation layer formats or "presents" data into a compatible form for receipt by
the Application layer or the destination system. Specifically, the Presentation layer
ensures:
 Formatting and translation of data between systems.
 Negotiation of data transfer syntax between systems by converting character sets
to the correct format.
 Compatibility with the host.
 Encapsulation of data into message envelopes through encryption and
compression.
Presentation
 Restoration of data through decryption and decompression.
The Presentation layer formats data for the Application layer. Therefore, it also sets
standards for multimedia and other file formats. These include standard file formats, such
as:




JPEG, BMP, TIFF, PICT
MPEG, WMV, AVI
ASCII, EBCDIC
MIDI, WAV
The Session layer's primary function is managing the sessions in which data is
transferred. Functions at this layer may include:
Session
 Establishment and maintenance of communication sessions between the network
hosts, ensuring that data is transported.
 Management of multiple sessions (each client connection is called a session). A
server can maintain thousands of sessions simultaneously.
 Assignment of the session ID number to each session, which is then used by the
Transport layer to properly route the messages.
 Dialog control that specifies how the network devices coordinate with each other
(simplex, half-duplex, and full-duplex).
 Termination of communication sessions between network hosts after completion
of the data transfer.
The Session layer protocols and interfaces coordinate requests and responses between
different hosts using the same application. These protocols and interfaces include:





Network File System (NFS)
Apple Session Protocol (ASP)
Structured Query Language (SQL)
Remote procedure call (RPC)
X Window
The following table compares the functions performed at each OSI model layer:
Layer
Description and Keywords
The Application layer:
 Provides an interface for a
Application
service to operate.
 Enables communication
partner identification.
Protocols





HTTP
Telnet
FTP
TFTP
SNMP




JPEG, BMP, TIFF, PICT
MPEG, WMV, AVI
ASCII, EBCDIC
MIDI, WAV
The Presentation layer:
 Defines data format (file
Presentation
formats).
 Enables encryption,
translation, and
compression.
 Defines data format and
exchange.
The Session layer:
Session
 Keeps data streams separate
(session identification).
 Network File System
(NFS)
 Apple Session Protocol
 Sets up, maintains, and tears
(ASP)
down communication
sessions.
The Transport layer:
 Provides reliable
Transport
(connection-oriented) and
unreliable (connectionless)
communications.
 Enables end-to-end flow
control.
 Defines port and socket
numbers.
 Uses segmentation,
sequencing, and
combination.
 TCP (connection-oriented)
 UDP (connectionless)
The Network layer:
 Defines logical addresses
Network
Logical
Link
Control
(LLC)
(host and network).
 Uses path determination
(identification and
selection).
 Routes packets.
The Data Link layer:
 Converts bits into bytes and

Data
Link
Media
Access
Control
(MAC)




Physical
 IP
 IPX
 AppleTalk
bytes into frames.
Uses MAC address, (also
called the burned in
address or hardware
address).
Defines the logical network
topology.
Specifies media access
methods.
Implements host-to-host
flow control.
Uses parity and CRC.
The Physical layer:
 LAN protocols: 802.2
(LLC), 802.3 (Ethernet),
802.5 (Token Ring),
802.11 (Wireless)
 WAN protocols: PPP,
Frame Relay, ISDN
 EIA/TIA 232 (serial
signaling)
 V.35 (modem signaling)
 Cat5
 RJ45
 Moves bits across the
media.
 Defines cables, connectors,
and pin positions.
 Specifies electrical signals
(voltage, bit
synchronization).
 Defines the physical
topology (network layout).
TCP/IP protocol suite facts
The following table lists several protocols in the TCP/IP protocol suite:
Description
OSI Model
Layer(s)
TCP/IP Model
Layer
File Transfer
Protocol (FTP)
File Transfer Protocol (FTP)
provides a generic method of
transferring files. It can include file
security through user names and
passwords, and it allows file transfer
between dissimilar computer
systems.
Application,
Presentation,
Session
Application/Process
Trivial File
Transfer
Protocol
(TFTP)
Trivial File Transfer Protocol
(TFTP) is similar to FTP. It lets you
transfer files between a host and an
FTP server. However, it provides no
user authentication and uses UDP
instead of TCP as the transport
protocol.
Application,
Presentation,
Session
Application/Process
Hypertext
Transfer
Protocol
(HTTP)
The Hypertext Transfer Protocol
(HTTP) is used by Web browsers
and Web servers to exchange files
(such as Web pages) through the
World Wide Web and intranets.
HTTP can be described as an
information requesting and
responding protocol. It is typically
used to request and send Web
documents but is also used as the
Application,
Presentation,
Session
Application/Process
Protocol
protocol for communication between
agents using different TCP/IP
protocols.
Simple Mail
Transfer
Protocol
(SMTP)
Simple Mail Transfer Protocol
(SMTP) is used to route electronic
mail through the internetwork. Email applications provide the
interface to communicate with
SMTP or mail servers.
Application,
Presentation,
Session
Application/Process
Simple
Network
Management
Protocol
(SNMP)
Simple Network Management
Protocol (SNMP) is a protocol
designed for managing complex
networks. SNMP lets network hosts
exchange configuration and status
information. This information can be
gathered by management software
and used to monitor and manage the
network.
Application,
Presentation,
Session
Application/Process
Telnet
Remote Terminal Emulation (Telnet)
allows an attached computer to act
as a dumb terminal, with data
processing taking place on the
TCP/IP host computer. It is still
widely used to provide connectivity
between dissimilar systems.
Application,
Presentation,
Session
Application/Process
Network File
System (NFS)
Network File System (NFS) was
initially developed by Sun
Microsystems. It consists of several
protocols that enable users on
various platforms to seamlessly
access files from remote file
systems.
Application,
Presentation,
Session
Application/Process
Voice over
Internet
Protocol (VoIP)
Voice over Internet Protocol (VoIP)
is a protocol optimized for the
transmission of voice through the
Internet or other packet switched
networks. Voice over IP protocols
carry telephony signals as digital
audio encapsulated in a data packet
stream over IP.
Application,
Presentation,
Session
Application/Process
Domain Name
System (DNS)
Domain Name System (DNS) is a
system that is distributed throughout
the internetwork to provide
address/name resolution. For
example, the name www.testout.com
would be identified with a specific
IP address.
Application,
Presentation,
Session
Application/Process
Transmission
Control
Protocol (TCP)
Transmission Control Protocol
(TCP) provides connection-oriented
services and performs segment
sequencing and service addressing. It
also performs important errorchecking functions.
Transport
Host-to-Host
(Transport)
User Datagram
Protocol (UDP)
User Datagram Protocol (UDP) is
considered a host-to-host protocol
like TCP but is not connectionoriented. Because of less overhead,
UDP transfers data faster but is not
as reliable.
Transport
Host-to-Host
(Transport)
Internet
Protocol (IP)
Internet Protocol (IP) is the main
TCP/IP protocol. It is a
connectionless protocol that makes
routing path decisions based on the
information it receives from ARP. It
also handles logical addressing
issues through the use of IP
addresses.
Network
Internet
Internet Control
Message
Protocol
(ICMP)
Internet Control Message Protocol
(ICMP) works closely with IP in
providing error and control
information that helps move data
packets through the internetwork.
Network
Internet
Internet Group
Membership
Protocol
(IGMP)
Internet Group Membership Protocol
(IGMP) is a protocol for defining
host groups. All group members can
receive broadcast messages intended
for the group (called multicasts).
Multicast groups can be composed
of devices within the same network
or across networks (connected with a
Network
Internet
router).
Address
Resolution
Protocol (ARP)
Address Resolution Protocol (ARP)
is used to get the MAC address of a
host from a known IP address. ARP
is used within a subnet to get the
MAC address of a device on the
same subnet as the requesting
device.
Network
Internet
Reverse
Address
Resolution
Protocol
(RARP)
and
Bootstrap
Protocol
(BOOTP)
Both BOOTP (Bootstrap Protocol)
and RARP (Reverse Address
Resolution Protocol) are used to
discover the IP address of a device
with a known MAC address.
BOOTP is an enhancement to RARP
and is more commonly implemented
than RARP. As its name implies,
BOOTP is used by computers as
they boot to receive an IP address
from a BOOTP server. The BOOTP
address request packet sent by the
host is answered by the server.
Network
Internet
Network
Internet
Network
Internet
Dynamic Host
Configuration
Protocol
(DHCP)
The Dynamic Host Configuration
Protocol (DHCP) simplifies address
administration. DHCP servers
maintain a list of available and
assigned addresses and communicate
configuration information to
requesting hosts. DHCP has the
following two components:
 A protocol for delivering IP
configuration parameters
from a DHCP server to a
host.
 A protocol specifying how
IP addresses are assigned.
Open Shortest
Path First
(OSPF)
Open Shortest Path First (OSPF) is a
route discovery protocol that uses
the link-state method. It is more
efficient than RIP in updating
routing tables, especially on large
networks.
Routing
Information
Protocol (RIP)
Routing Information Protocol (RIP)
is a route discovery protocol that
uses the distance-vector method. If
the network is large and complex,
OSPF should be used instead of RIP.
Network
Internet
The TCP/IP protocol suite was developed to work independently of the Physical layer
implementation. You can use a wide variety of architectures with the TCP/IP protocol suite.
During the IP-based communications between two network hosts, the following processes occur:
1. The data to be transferred is encapsulated on the sending host by moving from the top layer of the
TCP/IP or OSI model to the bottom.
2. The data is transmitted on the network medium.
3. If necessary, the data is transferred to various routers, which forward the data to the appropriate
network.
4. The data is delivered to the destination host.
5. The data received is de-encapsulated on the destination host by moving from the bottom layer of the
TCP/IP or OSI model to the top.
This process is detailed in the following table:
Process Step
Description
The data to be transferred is encapsulated on the sending host from the top layer of
the TCP/IP or OSI model to the bottom. The following events occur:
Source host
encapsulation
1. The Application layer prepares the data to be sent through the network by
encoding it using the appropriate Application layer protocol.
2. The Transport layer receives the stream of data from the Application layer
and breaks it into smaller chunks called segments. A Transport layer
header is applied to each segment that identifies the source port as well as
the destination port. Sequencing and control information is also added to
the header.
3. The Internet layer converts the segments into packets by adding an Internet
layer header, which specifies source and destination IP addresses for each
packet. IP addresses are 32-bit (4-byte) logical address that can be
assigned, unassigned, and reassigned as needed.
4. The Link layer converts the packets into frames by adding a Link layer
header, which specifies source and destination MAC addresses for each
frame. A MAC address is a 48-bit (6-byte) address that is physically
assigned in the firmware of all network interfaces that uniquely identify
each interface on the network. MAC addresses are displayed using
hexadecimal notation.
5. Each frame is converted into bits and transmitted across the network media.
If necessary, the data is transferred to various routers, which forward the data to the
appropriate network. The source and destination network addresses are used to
determine whether the hosts reside on the same network or on different networks:
 If they reside on the same network, the data can be sent directly to the
Network
transmission
destination host. The Address Resolution Protocol (ARP) is used to
determine the MAC address of the host with the destination IP address:
1. The sending host checks its ARP cache to see if it already has an
IP-to-MAC address mapping for the host. If so, it transmits the
frames to the destination host's MAC address. If not, it must use
the remaining steps to determine the appropriate MAC address.
2. The sending host sends out an ARP broadcast frame addressed to
all MAC addresses on the subnet asking for the hardware address
of the host with the destination IP address.
3. The host with the destination IP address responds to the ARP
broadcast with a unicast transmission containing its MAC address.
All other hosts ignore the broadcast.
4. The sending host caches the destination host's MAC address in its
ARP cache.
5. The source MAC address of the frames is set to the MAC address
of the sending system and the destination MAC address is set to







the MAC address of the receiving system.
6. The sending host transmits the frames to the destination host's
MAC address.
If they reside on different networks, the packets must be forwarded from
router to router until they reach the appropriate destination network and
host. The source IP address of each packet in the transmission is the IP
address of the sending system and the destination IP address is the IP
address of the receiving system. However, the frames can't be sent to
directly to the receiving system because it is not on the same network and
ARP can only be used on the local subnet. The following occurs in this
situation:
1. If it's not already cached, the source system uses ARP to
determine the MAC address of the first hop router interface
(usually the default gateway router) that is connected to the same
network segment as the source host.
2. The source MAC address of the frames is set to the MAC address
of the sending system, but the destination MAC address is set to
the MAC address of the router interface identified with ARP.
3. The frames are transmitted to the first router.
4. The router removes the frame header information and examines
the packets in the transmission for their source and destination IP
addresses. If the destination host is on a network that is directly
connected to the router, the router uses ARP to discover its MAC
address (if it's not already cached), re-encapsulates the packets in
new frames with the destination host's MAC address, and
transmits the frames directly to the destination host. If the
destination host is not on a directly-connected network, the
remaining steps occur.
5. The router uses its routing table to determine the next router the
packets should be sent to.
6. The router re-encapsulates the packets in the transmission in new
frames.
7. The source MAC address of the frames is set to the MAC address
of the local router interface and the destination MAC address is
set to the MAC address of the next hop router interface.
8. The router transmits the frames to the MAC address of the next
hop router interface.
The routing process repeats until the packets arrive at a router that is directly
connected to same network as the destination host.
The router receives the frames and removes the frame headers.
The router examines the packets. It recognizes that the destination host
resides on a network that is directly connected to the router.
If necessary, the router uses ARP to determine the MAC address of the
destination system.
The router re-encapsulates the packets in new frames. The source MAC
address of the frames is set to the MAC address of the router interface. The
destination MAC address is set to the MAC address of the destination host.
The frames are transmitted to the destination host.
The data received is de-encapsulated on the destination host by moving from the
bottom layer of the TCP/IP or OSI model to the top:
Destination host
de-encapsulation
1. The Link layer converts bits received on the network medium into frames
and passes them to the Internet layer.
2. The Internet layer extracts the packets from the frames and passes them to
the Transport layer.
3. The Transport layer receives packets and uses sequencing and error control
information to request retransmission of any missing or damaged packets.
4. The Transport layer uses sequencing information to convert the packets into
segments and passes them to the Application layer.
5. The Application layer converts the segments back into the original data
stream from the application on the source host using the appropriate
Application layer protocol.
Ethernet Architecture facts
The following table describes specifics of the Ethernet architecture:
Specification
Topology
Description
The physical topology is the mapping of the nodes of a network and the
physical connections between them, such as the layout of wiring, cables, the
locations of nodes, and the interconnections between the nodes and the cabling
or wiring system. The logical topology is the way messages are sent through
the network connections. Ethernet supports the following topologies:
Physical bus, logical bus
Physical star, logical bus
Physical star, logical star
Ethernet uses Carrier Sense, Multiple Access/Collision Detection (CSMA/CD)
to control access to the transmission medium. Devices use the following
process to send data:
Media access
1. Because all devices have equal access to the transmission media
(multiple access), a device with data to send first listens to the
transmission medium to determine if it is free (carrier sense).
2. If it is not free, the device waits a random time and listens again to the
transmission medium. When it is free, the device transmits its
message.
3. If two devices transmit at the same time, a collision occurs. The
sending devices detect the collision (collision detection) and sends a
jam signal.
4. Both devices wait a random length of time before attempting to resend
the original message (called a backoff).
Ethernet supports the following cable types:

Transmission
media


Unshielded twisted-pair cables (UTP) with RJ-45 connectors. This is
the most common transmission medium used for Ethernet. Each cable
consists of eight wires, twisted into four pairs. UTP cables are
classified by categories:
Cat3, rated up to 10 Mbps
Cat4, rated up to 16 Mbps
Cat5, rated up to 100 Mbps
Cat5e, rated up to 1,000 Mbps (gigabit)
Cat 6, rated up to 10,000 Mbps
Fiber optic, most commonly used in high-speed applications, such as
servers or streaming media. Fiber optic cables have ST, SC, LC, and
MT-RJ connectors.
Coaxial for older Ethernet implementations (often called thinnet or
thicknet networks). Coaxial cables have F-Type and BNC connectors.
The Ethernet frame size is 64 to 1518 bytes (this is the same for all Ethernet
standards). Four frame types are supported:


Frame type


Ethernet 802.3 is the original Ethernet frame type.
Ethernet 802.2 is the frame type that accommodates standards set by
the IEEE 802.2 committee related to the logical link control (LLC)
sublayer. It is a more current frame type than 802.3.
Ethernet II is a frame type that provides the ability to use TCP/IP as a
transport/network layer protocol. Other Ethernet frame types operate
strictly with IPX/SPX as a transport/network layer protocol.
Ethernet SNAP (Subnetwork Address Protocol) is an enhanced version
of Ethernet 802.2 that allows for greater compatibility with other
network architectures, such as Token Ring. This frame type also
supports TCP/IP.
The MAC address (also called the burned-in address) is the Data Link layer
physical device address. The MAC address is:
Physical
address


A 12-digit hexadecimal number (each number ranges from 0-9 or A-F).
Often written as 00-B0-D0-06-BC-AC or 00B0.D006.BCAC, although
dashes, periods, and colons can be used to divide the MAC address
parts.

Guaranteed unique through design. The first half (first 6 digits) of the
MAC address is assigned to each manufacturer. The manufacturer
determines the rest of the address, assigning a unique value that
identifies the host address. A manufacturer that uses all the addresses
in the original assignment can apply for a new MAC address
assignment.
Some network cards allow you to change (logically assigned address) the MAC
address through jumpers, switches, or software. However, there is little practical
reason for doing so.
With the original Ethernet standards, all devices shared the same cable. This caused two problems:
 Collisions would occur when two devices transmitted at the same time, requiring devices to be able
to detect and recover from collisions.
 Each device could either transmit data or receive data at any given time. This meant that the device
was either receiving data or listening for incoming data. Devices were not able to both send and
receive at the same time (much like using a one-lane road for traffic in two different directions).
These two problems were solved in the following ways:
 To allow simultaneous transmission, twisted pair cables are used. Twisted pair cables combine
multiple strands of wires into a single cable, allowing devices to use different wires to send and
receive data simultaneously.
 Collisions are eliminated by using switches. Switches use dedicated switch ports (a single device per
port) to give devices a dedicated communication path, making collisions impossible.
With these problems solved, you can turn off collision detection. Devices can transmit and receive data
simultaneously, and can begin transmitting data as soon as they have data to send. Devices with collision
detection turned on operate in half-duplex mode; devices with collision detection turned off operate in fullduplex mode. The following table describes half-duplex and full-duplex modes:
Mode
Description
Bandwidth
 Collision detection is turned on.
 The device can only send or receive
Halfduplex
at any given time.
 Devices connected to a hub must use
Up to the rated bandwidth (10 Mbps for
10BaseT, 100 Mbps for 100BaseT, etc.)
half-duplex communication.
Fullduplex
 Collision detection is turned off.
 The device can send and receive at
the same time.
Double the rated bandwidth (20 Mbps
for 10BaseT, 200 Mbps for 100BaseT,
etc.)
 NICs need to be full-duplex capable.
 A switch with dedicated switch ports
is required.
A frame is a unit of data that is ready to be sent on the network medium. Ethernet frames contain the
following components:
 The preamble is a set of alternating ones and zeros terminated by two ones (i.e., 11) that marks it as a
frame.
The destination address identifies the receiving host's MAC address.
The source address identifies the sending host's MAC address.
The data, or the information, that needs to be transmitted from one host to the other.
Optional bits to pad the frame. Ethernet frames are sized between 64 and 1518 bytes. If the frame is
smaller than 64 bytes, the sending NIC places "junk" data in the pad to make it the required 64
bytes.
 The CRC (cyclic redundancy check) is the result of a mathematical calculation performed on the
frame. The CRC helps verify that the frame contents have arrived uncorrupted.




Ethernet Standards
Ethernet standards are defined by the work of the IEEE 802.3 committee. The following table compares the
characteristics of various Ethernet implementations:
Category
Standard
Bandwidth
Cable Type
Maximum
Segment
Length
10BaseT
10 Mbps
(half duplex)
20 Mbps
(full duplex)
Twisted pair
(Cat3, 4, or 5)
100 meters
10BaseFL
10 Mbps
(full duplex)
Fiber optic
1,000 to 2,000
meters
100BaseTX
100 Mbps
(half duplex)
200 Mbps
(full duplex)
Twisted pair
(Cat5 or
higher) uses 2
pairs of wires
100 meters
100BaseFX
100 Mbps
(half duplex)
200 Mbps
Fiber optic
412 meters
(half-duplex
multi-mode
Ethernet
Fast
Ethernet
(full duplex)
Gigabit
Ethernet
cable)
2,000 meters
(full-duplex
single-mode
cable)
1000BaseT
Twisted pair
(Cat5e or
higher)
100 meters
1000BaseCX (short
copper)
Special copper
(150 ohm)
25 meters,
used within
wiring closets
1000BaseSX (short)
1,000 Mbps
(half duplex)
2,000 Mbps
(full duplex)
220 to 550
meters
depending on
cable quality
Fiber optic
1000BaseLX (long)
550 meters
(multi-mode
fiber)
5 kilometers
(single-mode
fiber)
10GBaseT
Twisted pair
(Cat6, 6a, or 7)
100 meters
10GBaseSR/10GBaseSW
Multimode
fiber optic
300 meters
10GBaseLR/10GBaseLW
Single-mode
fiber optic
10 kilometers
10GBaseER/10GBaseEW
Single-mode
fiber optic
40 kilometers
10 Gigabit
Ethernet
10 Gbps (full
duplex only)
You should also know the following facts about Ethernet:
 The maximum cable length for UTP Ethernet T implementations is 100 meters for all standards.
 Ethernet standards support a maximum of 1024 hosts on a single subnet.
 10GBase standards ending in W (i.e., 10GBaseSW) are used for SONET implementations.
WAN facts
The following table includes different WAN types and their description:
Method
Description
Point-topoint
A point-to-point connection is a single, pre-established path from the customer's network
through a carrier network (such as a telco) to a customer's remote network. A point-topoint line is usually leased from a carrier and thus is often called a leased line.
Circuit
switching
A circuit switching network allows data connections that can be initiated when needed and
terminated when communication is complete, working much like a telephone line for
voice communication. A circuit switched network uses a dedicated connection between
sites. It is ideal for transmitting data that must arrive quickly in the order it is sent, as is the
case with real-time audio and video.
Packet
switching
A packet switched network allows data to be broken up into packets and sent across the
shared resources. Packets are transmitted along the most efficient route to the destination.
Packet switching is ideal for transmitting data that can handle transmission delays, as is
often the case with Web pages and e-mail.
A typical WAN structure includes the following components:
Component
Description
Consumer Premises
Equipment (CPE)
The devices physically located on the subscriber's premises. CPE includes the
telephone wire, telephone, modem, and other equipment—both the devices the
subscriber owns and the ones leased from the WAN provider. The wiring
typically includes UTP cable with RJ-11 or RJ-45 connectors. CPE is
sometimes used synonymously with DTE.
Data Terminal
Equipment (DTE)
A device on the network side of a WAN link that sends and receives data. The
DTE resides on the subscriber's premises, and marks the point of entry
between the LAN and the WAN. DTEs are usually routers, but computers and
multiplexers can also act as DTEs. Broadly, DTEs are any equipment at the
customer's site and can include all computers. In a narrow sense, the DTE is
the device that communicates with the Data Communication Equipment
(DCE) at the other end.
Channel Service
Unit/Data Service Unit
(CSU/DSU)
The CSU/DSU is a device that connects a physical circuit installed by the
telco to some CPE device, adapting between the voltages, current, framing,
and connectors used in the circuit to the physical interface supported by the
DTE.
Demarcation point
(demarc)
The point where the telephone company's telephone wiring connects to the
subscriber's wiring. The demarc can also be called the network interface or
point of presence. Typically, the customer is responsible for all equipment on
one side of the demarc. The phone company is responsible for all equipment
on the other side of the demarc.
Local loop
The cable that extends from the demarc to the central telephone office. The
demarc media is owned and maintained by the telephone company. Typically,
it is UTP, but it can also be one or a combination of UTP, fiber optic, or other
media. Fiber optic cable to the demarc is rare.
Central Office (CO)
The switching facility closest to the subscriber, and the nearest point of
presence for the WAN provider. It provides WAN-cloud entry and exit points
for incoming and outgoing calls and acts as a switching point to forward data
to other central offices. A CO provides services, such as switching incoming
telephone signals to outgoing trunk lines. It also provides reliable DC power to
the local loop to establish an electric circuit. COs use long-distance, or toll,
carriers to provide connections to almost anywhere in the world. Longdistance carriers are usually owned and operated by companies such as AT&T
or MCI.
Data Communication
Equipment (DCE)
A device that communicates with both DTEs and the WAN cloud. DCEs are
typically routers at the service provider that relay messages between the
customer and the WAN cloud. In a strict sense, a DCE is any device that
supplies clocking signals to DTEs. Thus, a modem or CSU/DSU at the
customer site is often classified as a DCE. DCEs may be devices similar to
DTEs (such as routers), except that each device plays a different role.
WAN cloud
Packet-Switching
Exchange (PSE)
WAN Services
The hierarchy of trunks, switches, and central offices that make up the
network of telephone lines. It is represented as a cloud because the physical
structure varies, and different networks with common connection points may
overlap. Few people thoroughly understand where data goes as it is switched
through the "cloud." What is important is that data goes in, travels through the
line, and arrives at its destination.
A switch on a carrier's packet-switched network. PSEs are the intermediary
points in the WAN cloud.
Listed below are the most common WAN transmission media:
Carrier
Speed
Plain Old
Telephone
Service (POTS)
56 Kbps
T1 (a.k.a. DS1)
1.544
Mbps
Description
 Existing wires use only one twisted pair.
 Analog signals are used through the local loop.
 A modem is required to convert digital signals to analog.
 T-Carrier is a digital standard widely deployed in North
America.
 T1 lines usually run over two-pairs of unshielded twisted

T3 (a.k.a. DS3)
44.736
Mbps
E1
2.048
Mbps
E3
34.368
Mbps
J1
1.544
Mbps
J3
32.064
Mbps



pair (UTP) cabling, although they can also run over other
media such as coaxial, fiber-optic, and satellite.
A T1 line has 24 channels (also known as DS0s) that each
run at 64 Kbps.
T3 lines usually run over fiber-optic cable.
A T3 line has 672 channels that each run at 64 Kbps.
T1 and T3 connections require a CSU/DSU.
 E-Carrier is a digital standard very similar to T-Carrier, but
it is widely deployed in Europe.
 An E1 line has 32 channels (also known as DS0s) that run
at 64 Kbps.
 An E3 line transmits 16 E1 signals at the same time.
 E1 and E3 connections require a CSU/DSU.
 J-Carrier is a digital standard very similar to T-Carrier, but
it is widely deployed in Japan.
 A J1 line is virtually identical to a T1 line.
 A J3 line has 480 channels that run at 32 Mbps.
 J1 and J3 connections require a CSU/DSU.
WAN services can also use fiber optic, wireless, and other transmission media. However, the use of these media
to the local loop is not common at this time.
If your organization needs WAN connectivity, you can choose from the following service options:
Service
Bandwidth
(Max.)
Line
Type
Signaling
Method
Characteristics
Public Switched
Telephone
Network (PSTN)
56 Kbps
POTS
Analog
Dialup over regular
telephone lines
Leased lines
56 Kbps
POTS
Analog
Dedicated line with
consistent line quality
64 Kbps
POTS
Analog
Dedicated line
Variable packet sizes
(frames)
Ideal for low-quality lines
1.54 Mbps
POTS
T1
T3
Digital
Variable packet sizes
(frames)
Asynchronous
Transfer Mode
(ATM)
1.2 Gbps
Coaxial,
twisted
pair,
fiberoptic
Digital
Fixed-size cells (53-byte)
High-quality, high-speed
lines
Integrated
Services Digital
Network (ISDN)
144 Kbps
(BRI)
4 Mbps
(PRI)
Digital
Basic rate operates over
regular telephone lines and
is a dialup service
Primary rate operates over
T-carriers
Digital
Operates using digital
signals over regular
telephone lines
DSL comes in many
different flavors (such as
ADSL and HDSL)
X.25
Frame Relay
DSL
24 Mbps
POTS
T1
POTS
There is no clear distinction between WAN services, such as Frame Relay and ISDN. For example, you can
use Frame Relay protocol over ISDN lines. Once a device connects to the WAN cloud, internal protocols can
convert data traffic into the necessary formats and then convert the data again at the other end.
WAN connections are provided through several different connector types.
Type
Description
DB-60
WAN interface cards (WIC) with a single serial port use a DB60 connector. The connector has 4 rows of 15 pins each.
Connector
Serial WIC port
Smart Serial
WICs with two serial ports use the high-density Smart Serial
connector.
Connector
Smart Serial WIC ports
RJ-48
Integrated T1 CSU/DSU WIC ports use an RJ-48 connector.
The RJ-48 connector has the same size and shape as the RJ-45
used in Ethernet connections but has a different pinout.
Connector
Integrated T1 DSU/DSU WIC port
RJ-11
Connector
DSL WIC port
DSL (Digital Subscriber Line) WIC ports use an RJ-11
connector. The RJ-11 port connects to the phone line.