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1-Application
2-Presentation
3-Session
4-Transport
5-Network
6-Data Link
File Transfer
File Management
Message Handling (e-mail)
Formats data for network
Converts data to a language that the network can
handle.
May Encrypts/decrypts
(Codes and decodes graphics and file format
information)
(traffic cop)
Establish and maintain link (the session)
Maintain synchronization
Reestablish lost connections
Sets terms of communications
 Who talks first
 How long each can talk
(most important layer)
Flow Control – selects appropriate rate
Sequencing – reassembles packets in correct order
Sends ACK when data is correct
Sends requests when data is contains errors
Sending will retransmit in ACK not received
May break large packets into smaller ones
Transport layer services:
 (TCP in TCP/IP)
 (SPX (Sequence Packet Exchange - in
IPX/SPX)
Routs data - Selects Best path
Translates network to physical address
May segment/reassemble if necessary
Divides data into frames for transmission
Adds to frame:
 Destination address
 Source address
 Control Information
(data)
 Error Checking Information
7-Physical
OSI Model
Pneumonic
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Often
Terminal-toMainframe
connection
ISP-home
Routers
Bridges,
switches
Nic, cables
Open System Interconnect
Greek for Equal
All
People
Seem
To
Need
Data
Processing
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Richard L. Goldman
OSI Model
Application
Layer
Presentation
Layer
Session Layer
OSI
Transport Layer
Upper Layers
Gateways – Hardware or Software operate in these three layers
 The most common is software running on a computer.
Interfaces with network services ( applications)
Services Include:
 File
 Print
 Message
 Etc.
Formats the data
 Character sets are converted
 Data is encrypted
 Data may be compressed
 Usually handles the redirection of data streams
Most standard computer systems use the American Standard Code for Information
Interchange (ASCII). Mainframe computers (and some IBM networking systems) use the
Extended Binary Coded Decimal Interchange Code (EBCDIC). The two are totally
different. Protocols at the Presentation layer can translate between the two.
Maintains the session
Establishes, Synchronizes, Maintains and Ends a Session
Does:
 Security Authentication
 Connection ID Establishment
 Data Transfers (Checkpoints)
 Acknowledgment
 Connection Release
Middle Layers
Flow Control
 Handles Errors
 Acknowledges (ACK) receipt of data
 Resizes messages into smaller or larger segments
 (reassembles the messages back to original size)
 Handles logical address/name resolution
Establishes one of two types of Connection Services:
 Connection-oriented
o Uses acknowledgements and responses to establish and ensure virtual
connection is maintained
o TCP is a connection-oriented protocol (TCP/IP stack)
 Used by FTP and HTTP (File Transport Protocol) (HyperText
Transport Protocol)
o SPX is a connection-oriented protocol (IPX/SPX stack)
 Connectionless
o No error or flow control – less overhead – faster
o UDP is a connectionless protocol (User Datagram Protocol - part of TCP/IP
stack)
 Used by TFTP and NFS (Trivial File Transport Protocol and Network
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Richard L. Goldman
File System)
o IPX is a connectionless protocol
o NetBEUI is a connectionless protocol
Network Layer
Layer 3
Address/Name Resolution
 Translates logical address to logical name
 DNS (TCP/IP stack protocol) provides cross-platform Address/Name resolution.
(Domain Name Services)
 NDS Database – (Novel Directory Services) provides address/name resolution at the
Transport Layer in response to higher level NDS Directory Requester
Controls Logical Network Addressing and Routing
Logical Network Addressing
 Uses protocol centric addressing (i.e. TCP\IP or IPX)
 Duplicate addresses often render both users down
o With Win9X first user is OK second user will be off line
 Duplicate TCP/IP addresses are more common than IPX conflicts
o IPX uses the unique MAC address as part of its address
 IPX addresses have two portions
o Network portion
o Node portion
 For IPX (8:12 – XXXXXXXX:XXXXXXXXXXXX)
o Network portion is an 8 digit hex number – The IPX Network Address
 Randomly assigned by installation program or manually by SYSOP
o Node portion is the MAC address
 For TCP/IP
o A dotted decimal notation is used (XXX.XXX.XXX.XXX)
o 4 three digit decimal numbers (or four octets) (decimal #s 1-254)
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Routing
 Routers create an internetwork out of network segments.
 Each routers contain a Routing Table that permits the router to pass data with
specific addresses to a specific port
 Routing tables may be either Static or Dynamic
o Static Routing Tables are created and maintained manually by the
administrator. (Time-intensive, expensive) In NT use the ROUTE command
o Dynamic Routing uses discovery protocols to identify segments in the
internetwork and to update other routing tables – most popular routing
technology
 Older Route Discovery Protocols use RIP (Routing Information
Protocol) a Distance Vector Route Discovery method
 RIP for TCP/IP
 RIP for IPX
 Distance Vector Route Discovery method broadcast the routing table
every minute – receiving routers add a 1 to the hop count and then
rebroadcasts it. Creates high network overhead.
 Link State Route Discovery method is more efficient.
 Sends the table multicast (not broadcast) every five minutes
 Other routers only respond with changes to the table (a if
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
there is a change)
Current Link State Route Discovery protocols:
o OSPF for TCP/IP (Open Shortest Path First )
o NLSP for IPX (NetWare Link Services Protocol)
Routable Protocols:
 Protocol
Route Discovery Protocol
 IPX
RIP or NLSP
 TCP/IP
RIP or OSPF
 XNS
RIP
Nonroutable Protocols:
 NetBEUI
On a workstation a Default Gateway is the address that packets are sent to if the
address is not located on its segment
Network Layer Devices
 Router
 Connects segments into an internetwork
 Connects LAN to the Internet
 Facilitates communications between segments by selecting best route
 Can connect dissimilar lower level networks (Ethernet & Token Ring)
 Can provide firewall services and packet filtering
 Brouter
 Designed to bridge dissimilar networks – not very effective – not used much
 Layer 3 Switch
 Does everything that a Layer 2 Switch does plus basic routing for VPNs
OSI Model
Data Link Layer
Layer 2
Most routing protocols function at this layer
 Translates logical names into physical addresses
 Prioritizes data transfer (puts AV data ahead of e-mail)
 (Prioritization is know as Quality of Service (QoS)
 Routs data from source to destination
 Builds and tears down packets
Lower Layers
Controls:
 Controls logical network topology
o Ethernet
o Token Ring
o Etc.
 Method of media access
o Contention
o Polling
o Token Passing
 Controls physical addressing
 Controls transmission connection and synchronization
Sending Data:
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Richard L. Goldman





Accepts data frame from Network layer and reformats it to match the network
logical typology (Ethernet, Token-ring, etc).
Adds Data Link header and trailer to each new data frame
Manages the flow of data to the Physical layer
(The Physical layer will reject a data frame if the network is busy.)
Listens of an ACK for each data frame it sends and re-sends it if an ACK is not
received.
Receiving Data:
 Accept raw data frames from physical layer
 Checks destination address on each frame received
 Discards all data frames not addressed to it
 Performs an error check on accepted data frames
o Send ACK if the data frame is OK
o If it is bad, it does not send ACK and discards the data frame
 (The send site will re-sends a frame if the ACK is not received)
 Removes Data Link header and trailer from the data frame
 Repackages the data frame as required and passes it to Network layer
IEEE 802 divided the Data Link Layer into two Sublayers
(Institute of Electrical and Electronics Engineers)
 LLC - Logical Link Control
o Provides error correction and flow control
o 802 Specifications:
 802.1 Network management
 802.2 Logical Link Control
 MAC - Media Access Control (The Data Link layer physical address)
o Has direct communications with NIC
o Provides access control
o 802 Specifications:
 802.3 CSMA/CD
 802.4 Token Bus
 802.5 Token Ring
 802.12 Demand Priority
Physical Layer
Layer 1
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Data Link layer Devices
 Bridges
o Connects network segments
o Restricts local traffic to local segment
o Bridges that operate up to the LLC sublayer can connect some dissimilar
networks (ie Ethernet and Token Ring)
o Bridges that only operate up to the MAC layer can only connect identical
networks
 Switches (AKA – Layer 2 Switch or Switching Hub)
o Builds a MAC Address table and routs traffic to proper port
Network Hardware – The Physical Topology
Translate data received from the Data Link layer into signals that can be transmitted on the
medium being used. (called signal encoding)
Functional physical interface with the network
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

5-4-3 Rule
MAC Address
Media Access
CSMA/CD
Token Passing
CSMA/CA
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Transmission technique
Pin layout and connector type
Physical Layer Devices
 Cable media (also called bounded media)
 NICs
 Transceivers (may convert signal types – Example 15-pin DIN AUI to RJ-45
DIN – Detusche Industrie Norm (German National Standards body)
 Repeaters – Used to extend the length of a network – passes all data (and noise)
 Hubs – (also called a concentrator) a multiport repeater
o Active (Powered) amplifies signal – extends segment length
o Passive (Typically Unpowered) reduces signal strength and segment length
 MAUs – Multistation Access Unit (also MSAU)
o Used for Token Ring Networks
o Physical Star, Logical Ring
o Max of 33 MAUs can be chained
5 Network segment – Max
4 Repeaters – Max
3 Populated segments – Max
Assigned at the Factory (some can be changed in the field)
AKA:
 Hardware Address
 Ethernet Address
 Physical Address
 NIC Address
 Adapter Address
A 12 digit hexadecimal number (six-octet number)
 The first 3 octets identify Manufacturer
o The OUI – Organizationally Unique Identifer
 The last 3 octets are unique numbers for that manfacturer
o The Universal LAN MAC address
Carrier Sense Multiple Access with Collision Detection
 Used by Ethernet
Used by Token Ring
 The first computer turned on creates a token
 The token is then passed from computer to computer until it reaches a computer that
wants to transmit data.
 Only the computer that holds the token can transmit data.
 It transmits the data with the token.
 When the receiving computer receives the data it sends the token back to the sender.
 The sender may then send more data.
 When the computer finishes transmitting it passes the token on to the next computer.
 (Collisions are avoided)
Carrier Sense Multiple Access with Collision Avoidance
 Used by AppleTalk
 The sender transmits a request to send (RTS) packet and waits for a clear to send
(CTS) before sending.
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IEEE 802
Standard
802.1
802.2
802.3
802.4
802.5
802.6
802.7
802.8
802.9
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 802.1 LAN/MAN Management (and Media Access Control Bridges)
 802.2 Logical Link Control
 802.3 CSMA/CD (802.3u includes 100BaseT)
 802.4 Token Bus
 802.5 Token Ring
 802.6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)
 802.7 Broadband Local Area Networks
 802.8 Fiber-Optic LANs and MANs
 802.9 Integrated Services (IS) LAN Interface
 802.10 LAN/MAN Security
 802.11 Wireless LAN
 802.12 Demand Priority Access Method
LAN and MAN management
MAC Bridges
Spanning Tree algorithms
Logical Link Control (LLC)
(Upper half of the Data Link layer – link between MAC and Network layer)
CSMA/CD – Carrier Sense Multiple Access with Collision Detection
Ethernet (DIX)
802.3u – includes 100BaseT
Token Bus
Factory Automation – Not used for data communication
Resembles MAP – Manufacturing Automation Protocol (Developed by GM)
Token Ring
Developed by IBM 1985
Uses MAU
Physical star
Logical ring
4Mbps or 16Mbps
Can “talk” to IMB mainframe computers
Busy networks just get slow – do not crash like Ethernet
Uses twisted pair (can uses other cabling) rated by type (not Category).
Token Ring NICs are more expensive than Ethernet NICs
Signals are amplified and regenerated by each NIC or other Token Ring device
Distributed Queue Dual Bus (DQDB) Metropolitan Area Network
City size networks
Many-to-many computers
Broadband LANs
Uses FDM – Frequency Division Multiplexing
Fiber-optic LAN and MAN
Defines use of fiber optics on 802.3-802.6
Includes:
 FDDI – Fiber Distributed Data Interface
 10BaseFL
Integrated Services (IS) LAN Interface
ISDN spec - Integrated Switched Digital Network
ADSL and Cable Modems have eclipsed ISDN
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Richard L. Goldman
802.10
802.11
802.12
 (They are Cheaper, Easier to Setup, and have Better Performance than ISDN)
LAN/MAN Security
Spec for VPN Virtual Private Network
Using public (Internet) network for connections between LAN locations
Encrypts data for transmitting in the open
Cheaper than renting a dedicated line
Wireless LAN
Demand Priority Access
Developed by HP
Not supported by other manufactures (They use Fast Ethernet – with 802.3u)
Assigns higher bandwidth to frames identified with high priority
Important to real time audio and video transmission
AKA:
 100VG (Voice Grade)
 100VG-AnyLAN
 100Base VG
 AnyLAN
Use the OSI reference model to aid in topology decisions
Jun 7, 2001
Mike Mullins CCNA, MCP
Author's Bio | E-Mail | Archive
© 2001 TechRepublic, Inc.
Knowing the seven layers of the Open Systems Interconnection (OSI) reference model and understanding where
your network devices fit in that model can prove to be a valuable asset when it comes to adding devices to
different segments of your network and placing devices between networks. This knowledge enables you to take
control of your topology and keeps you from leaving your network design to your vendors. In this article, we’ll
explore the most common network devices and discuss where they should reside in a good network topology.
Let’s break down the devices by layer and by their function on a network. Figure A provides a good overview.
Figure A
Where different devices operate on the OSI reference model
Figure A
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Richard L. Goldman
Layer 1, The Physical Layer: Bits
The Physical Layer provides the mechanical and electrical connections to the network. In other words, it sends
bits down a wire.
Repeaters connect media segments and provide amplification and retransmission of signals with no filtering
and will propagate all errors. Use them only for connecting long segments of wire.
Hubs connect multiple hosts to one segment of wire. All hosts share the same bandwidth. This means that there
is one large collision domain. Use them at points where you would deploy a network sensor, so the device can
see all the traffic on that portion of the network.
Layer 2, The Data Link Layer: Frames
The Data Link Layer splits data into frames for sending on the physical layer and receives acknowledgement
frames. It performs error checking and retransmits frames not received correctly. It provides an error-free
virtual channel to the Network Layer. The Data Link Layer is split into an upper sublayer, Logical Link Control
(LLC), and a lower sublayer, Media Access Control (MAC).
Bridgesconnect different types of networks (token ring, Ethernet, etc.), filter network traffic based on MAC
address, and remove errors from the network. Use them to connect different types of internal networks.
Switches, also known as Multiport Bridges, transfer data between different ports based on the destination
addresses. Each segment or port connection is its own collision domain, but all ports are in the same broadcast
domain. Switches can be used to connect multiple ports to the same destination (i.e., multiple uplink ports), but
only one port can be active at a time. Historically, this is a hardware Layer 2 device and typically operates in
one of three modes:

Store and Forward: This mode copies the entire frame into memory, computes the Cyclic Redundancy
Check (CRC) for errors, and then looks up the destination MAC address and forwards the frame. This is
slow but offers the best solution for error correction without affecting the entire backbone in
retransmission.
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Richard L. Goldman


Cut-through: This mode reads the destination address of the frame and forwards the frame to the port
connected to that destination MAC address before the entire frame is seen. This is fast but provides very
little error correction and will propagate errors from one collision domain to the next.
Modified Cut-through: This mode reads the first 64 bytes of the frame and then forwards the frame to
a port based on MAC destination address. This is fast and efficient in error correction.
Use Layer 2 switches as your LAN subbackbone device with each host connected to a switched port. This
dedicates bandwidth and segments the lowest layer of your internal network. Switches are also pretty
inexpensive.
Layer 3, The Network Layer: Datagrams/Packets
The Network Layer determines the routing of packets of data from sender to receiver. Routes can be static or
dynamic. The Network Layer provides sequencing and flow control of data, selects routes, and provides quality
of service through error detection, recovery, and notification. It also segments collision and broadcast domains.
This is where a MAC or hardware address is translated into Internet Protocol (IP) addresses (or other routable
protocol addresses, such as IPX or AppleTalk).
Routers are basically software-based packet-forwarding engines. The Network Layer provides information to
these devices, allowing them to base their forwarding decisions on criteria such as IP, link, or network node
availability and performance. A router’s true advantage lies in its flexibility in network protocol.
Use routers for their intended purpose. They were born to bridge networks and separate broadcast domains. This
is your “gateway” device to the Internet or your linking device between LANs or WANs.
Switches (Layer 3)are nothing more than wire-speed routers. They come in two basic models.


Port switches decide which physical port network traffic needs to go to and direct the traffic
appropriately. Each lane is actually a backplane segment on the switch. Because the switching is
performed locally via logic circuits and at wire speed, port switches are easier and cheaper to implement
than frame switches but give many of the same benefits.
Frame switches examine each Ethernet packet, determine which segment it came from and where it is
going, and send it on its way. These are more expensive than port switches but add a significant
performance boost to your network. They are also known as Learning Switches.
Use these devices at the backbone of your network. They’re less expensive than routers and much faster. But
read on: There’s another switch that might be a better solution for your network.
Layer 4, The Transport Layer: Segments
The Transport Layer (sometimes referred to as the Host Layer) determines how to use the Network Layer to
provide a virtual point-to-point connection. It creates and dissolves connections between hosts. The most
popular types of transport connection are TCP and UDP. TCP is a point-to-point connection protocol that
delivers messages in the order in which they were sent and guarantees delivery. UDP is a connection-less
protocol with no guarantee of delivery. The Transport Layer is a legitimate end-to-end layer. In other words, a
program on the source machine carries on a conversation with a similar program on the destination machine.
Layer 4 Switches are sometimes called “session switches” because they track and maintain individual sessions
from start to finish. (This doesn’t make them Layer 5 Switches; they’re just “aware” of sessions.) Layer 4
Switches make forwarding decisions based on session and application-layer information and provide load
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Richard L. Goldman
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balancing across multiple servers. Layer 4 Switches determine (through different complex and weighted
algorithms) the best server of a cluster to process a service request and bind the session to that server’s IP
address until the session is terminated.
They also designate and prioritize traffic by application. Because they are “aware” at the session level, they
have the ability to prevent unauthorized access to servers. Layer 4 Switches’ true selling point is handling all
packet processing in hardware. They utilize custom application-specific integrated circuits (ASICs), making
them extremely fast!
Substitute Layer 4 Switches for your Layer 3 Switches only if you intend to operate high-speed intranet
application servers with multiple 100-Mbit or gigabit interfaces.
What you need and what you can afford
Do not let design and implementation of your network become dominated by one specific vendor. While no two
networks are exactly alike, a variety of vendors exist. So don’t settle for a device from one vendor based solely
on company name. Above Layer 2, most devices are very specific in the protocols and configurations they
support. Know the intended purpose of your network and let budget and function dictate design.
What kind of network design tips do you have?
Do you use the OSI reference model for topology planning and troubleshooting? We look forward to getting
your input and hearing your experiences regarding this topic. Join the discussion below or send the editor an email.
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Richard L. Goldman