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Chapter 4: Network Access
Chapter 4
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4.1
4.2
4.3
4.4
4.5
Physical Layer Protocols
Network Media
Data Link Layer Protocols
Media Access Control
Summary
Comparing the two models
 At the network access layer, the TCP/IP protocol suite does not
specify which protocols to use when transmitting over a physical
medium.
 Only describes the handoff from the internet layer to the physical
network protocols.
 OSI Layers 1 and 2 discuss the necessary procedures to access the
media and the physical means to send data over a network.
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Focus on Data Link Layer
IP
IP
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Reminder of encapsulation/decapsulation
IP
Header
Data Link
Header
IP Packet
Data Link
Trailer
Data Link
Header
IP Packet
Data Link
Trailer
Data Link
Header
IP Packet
Data Link
Trailer
Data Link
Header
IP
Header
TCP
Header
TCP
Header
HTTP
Header
Data Link
Trailer
Data Link
Header
HTTP
Header
Data
Data
Data Link
Trailer
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Getting it Connected
Connecting to the Network
 A physical connection can be a wired connection using a cable or a
wireless connection using radio waves.
Getting it Connected
Connecting to the Network
 Switches and wireless access points are often two separate
dedicated devices, connected to a router.
 Many homes use integrated service routers (ISRs),
Getting it Connected
Network Interface Cards
 Network Interface Cards (NICs) connect a device to the network.
 Ethernet NICs are used for a wired connection whereas WLAN
(Wireless Local Area Network) NICs are used for wireless.
Getting it Connected
Network Interface Cards
Connecting to the Wireless LAN with a Range Extender
 Wireless devices must share access to the airwaves connecting to
the wireless access point.
 Slower network performance may occur
 A wired device does not need to share its access
 Each wired device has a separate communications channel over
its own Ethernet cable.
The
Physical
Layer
 The OSI physical layer provides the means to transport the bits that
make up a data link layer frame across the network media.
Purpose of the Physical Layer
Physical Layer Media
 The physical layer produces the representation and groupings of bits
for each type of media as:
 Copper cable: The signals are patterns of electrical pulses.
 Fiber-optic cable: The signals are patterns of light.
 Wireless: The signals are patterns of microwave transmissions.
Purpose of the Physical Layer
Physical Layer Standards
 Upper OSI layers are performed in software designed by software
engineers and computer scientists.
 TCP/IP suite are defined by the Internet Engineering Task Force
(IETF) in RFCs
Purpose of the Physical Layer
Physical Layer Standards
Standard
Networking Standards
organization
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ISO 8877: Officially adopted the RJ connectors (e.g., RJ-11, RJ-45)
ISO 11801: Network cabling standard similar to EIA/TIA 568.
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TIA-568-C: Telecommunications cabling standards, used by nearly all
voice, video and data networks.
TIA-569-B: Commercial Building Standards for Telecommunications
Pathways and Spaces
TIA-598-C: Fiber optic color coding
TIA-942: Telecommunications Infrastructure Standard for Data Centers
ANSI
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568-C: RJ-45 pinouts. Co-developed with EIA/TIA
ITU-T
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G.992: ADSL
IEEE
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802.3: Ethernet
802.11: Wireless LAN (WLAN) & Mesh (Wi-Fi certification)
802.15: Bluetooth
ISO
EIA/TIA
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 Who maintaining physical layer standards?
 Different international and national organizations, regulatory
government organizations, and private companies
Fundamental Principles of Layer 1
Physical Layer Fundamental Principles
Physical
Components
Frame Encoding
Technique
Signalling
Method
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UTP
Coaxial
Connectors
NICs
Ports
Interfaces
• Manchester Encoding
• Non-Return to Zero (NRZ)
techniques
• 4B/5B codes are used with
Multi-Level Transition Level 3
(MLT-3) signaling
• 8B/10B
• PAM5
• Changes in the
electromagnetic field
• Intensity of the
electromagnetic field
• Phase of the
electromagnetic wave
Single-mode Fiber
Multimode Fiber
Connectors
NICs
Interfaces
Lasers and LEDs
Photoreceptors
• Pulses of light
• Wavelength multiplexing using
different colors
• A pulse equals 1.
• No pulse is 0.
Fiber Optic
cable
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Access Points
NICs
Radio
Antennae
• DSSS (direct-sequence spreadspectrum)
• OFDM (orthogonal frequency
division multiplexing)
• Radio waves
Wireless
media
Media
Copper
cable
Fundamental Principles of Layer 1
Physical Layer Fundamental Principles
 Encoding or line encoding - Method of converting a stream of data
bits into a predefined "codes”.
 Signaling - The physical layer must generate the electrical, optical,
or wireless signals that represent the "1" and "0" on the media.
Fundamental Principles of Layer 1
Encoding and Signaling
 http://www.flukenetworks.com/content/n
eal-allens-network-maintenance-andtroubleshooting-guide-revealed
Fundamental Principles of Layer 1
Bandwidth
 Bandwidth is the capacity of a medium to carry data.
 Typically measured in kilobits per second (kb/s) or megabits per
second (Mb/s).
Fundamental Principles of Layer 1
Throughput
 Throughput is the measure of the transfer of bits across the media
over a given period of time.
 Due to a number of factors, throughput usually does not match the
specified bandwidth in physical layer implementations.
 http://www.speedtest.net/
 http://ipv6-test.com/speedtest/
Fundamental Principles of Layer 1
Types of Physical Media
 Different types of interfaces and ports available on a 1941 router
Network Media
Copper Cabling
Copper Cabling
Characteristics of Copper Media
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1
4
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 Signal attenuation - the longer the signal travels, the more it
deteriorates - susceptible to interference
 Crosstalk - a disturbance caused by the electric or magnetic fields
of a signal on one wire to the signal in an adjacent wire.
Copper
Cabling
Copper
Media
 Counter the negative effects of different types of interference some cables
are wrapped in metallic shielding
 Counter the negative effects of crosstalk, some cables have opposing
circuit wire pairs twisted together which effectively cancels the crosstalk.
Copper Cabling
Unshielded Twisted-Pair (UTP) Cable
Copper Cabling
Shielded Twisted-Pair (STP) Cable
Braided or Foil Shield
Foil Shields
Copper Cabling
Coaxial Cable
Copper Cabling
Cooper Media Safety
UTP Cabling
Properties of UTP Cabling
UTP Cabling
UTP Cabling Standards
UTP Cabling
UTP Connectors
UTP Cabling
Types of UTP Cable
UTP Cabling
Testing UTP Cables
Fiber Optic Cabling
Properties of Fiber Optic Cabling
Fiber Optic Cabling
Properties of Fiber Optic Cabling
Fiber Optic Cabling
Fiber Media Cable Design
Fiber Optic Cabling
Types of Fiber Media
Fiber Optic Cabling
Network Fiber Connectors
Fiber Optic Cabling
Testing Fiber Cables
Fiber Optic Cabling
Fiber versus Copper
Implementation issues
Copper media
Fibre-optic
Bandwidth supported
10 Mbps – 10 Gbps
10 Mbps – 100 Gbps
Distance
Relatively short
(1 – 100 meters)
Relatively High
(1 – 100,000 meters)
Immunity to EMI and RFI
Low
High
(Completely immune)
Immunity to electrical hazards
Low
High
(Completely immune)
Media and connector costs
Lowest
Highest
Installation skills required
Lowest
Highest
Safety precautions
Lowest
Highest
Wireless Media
Properties of Wireless Media
Wireless Media
Types of Wireless Media
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IEEE 802.11 standards
Commonly referred to as Wi-Fi.
Uses CSMA/CA
Variations include:
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802.11a: 54 Mbps, 5 GHz
802.11b: 11 Mbps, 2.4 GHz
802.11g: 54 Mbps, 2.4 GHz
802.11n: 600 Mbps, 2.4 and 5 GHz
802.11ac: 1 Gbps, 5 GHz
802.11ad: 7 Gbps, 2.4 GHz, 5 GHz, and 60 GHz
• IEEE 802.15 standard
• Supports speeds up to 3 Mbps
• Provides device pairing over distances from 1 to
100 meters.
• IEEE 802.16 standard
• Provides speeds up to 1 Gbps
• Uses a point-to-multipoint topology to provide
wireless broadband access.
Wireless Media
802.11 Wi-Fi Standards
Standard
Maximum
Speed
Frequency
Backwards
compatible
802.11a
54 Mbps
5 GHz
No
802.11b
11 Mbps
2.4 GHz
No
802.11g
54 Mbps
2.4 GHz
802.11b
802.11n
600 Mbps
2.4 GHz or 5 GHz
802.11b/g
802.11ac
1.3 Gbps
(1300 Mbps)
2.4 GHz and 5.5
GHz
802.11b/g/n
802.11ad
7 Gbps
(7000 Mbps)
2.4 GHz, 5 GHz and
60 GHz
802.11b/g/n/ac
The Data
Link
Layer
 The OSI physical layer provides the means to transport the bits that
make up a data link layer frame across the network media.
Purpose of the Data Link Layer
The Data Link Layer
 The data link layer is responsible for the exchange of frames
between nodes over a physical network media.
Purpose of the Data Link Layer
Network
Data Link Sublayers
LLC Sublayer
Data Link
 Data Link layer has two sublayers (sometimes):
 Logical Link Control (LLC) – Software processes that provide services
to the Network layer protocols.
 Frame information that identifies the Network layer protocol.
 Multiple Layer 3 protocols, (ICMP, IPv4 and IPv6) can use the same
network interface and media.
 Media Access Control (MAC) - Media access processes performed by
the hardware.
 Provides Data Link layer addressing and framing of the data
according to the protocol in use.
802.15
Bluetooth
802.11
Wi-Fi
Physical
802.3
Ethernet
MAC Sublayer
Purpose of the Data Link Layer
Providing Access to Media
 At each hop along the path, a router:
 Accepts a frame from a medium
 De-encapsulates the frame
 Re-encapsulates the packet into a
new frame
 Forwards the new frame
appropriate to the medium of that
segment of the physical network
Data Link Layer
Layer 2 Frame Structure
 The data link layer prepares a packet for transport across the local
media by encapsulating it with a header and a trailer to create a
frame.
Topologies
Controlling Access to the Media
Media
Access
Control
 Media Access Control - Regulates the placement of data frames
onto the media.
 The method of media access control used depends on:
 Media sharing
 Do more than two nodes share the media?
 If so, how? (Switches, hubs, etc.)
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Serial vs multi-access
Multi-access
Point-to-Point
 Point-to-Point networks
 Only two nodes
 /30 subnets are common (later)
 Protocols: PPP, HDLC, Frame Relay
 Multi-access networks (LANs)
 Multiple nodes
 Subnets mask range depends upon the number of hosts (nodes)
 Protocols: Ethernet, 802.11 (wireless), Frame Relay Multipoint
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Topologies
Physical and Logical Topologies
Physical
Topology
 The physical topology
is an arrangement of
the nodes and the
physical connections
between them.
Layer 2
Switch
Multilayer
Switch
Serial
Connections
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Logical Topology
 A logical topology The way a network
transfers frames from
one node to the next.
 Defined by Data Link
layer protocols.
 Media Access
Control used.
 Type of network
framing
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Point-to-Point topology
11111111
 A point-to-point topology connects two nodes directly together.
 The media access control protocol can be very simple.
 Frames from one devices are for the device at the other end.
 Point-to-point topologies, with just two interconnected nodes, do not
require special addressing.
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Logical Point-to-Point Networks
 Point-to-point networks may include intermediate devices.
 No affect on logical topology.
 The logical connection (in some cases) may be a virtual circuit.
 A virtual circuit is a logical connection created within a network
between two network devices.
 The two nodes exchange the frames with each other.
 Data Link Destination address is the device at the other end of
the virtual circuit.
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Multi-access Topology
 A logical multi-access topology - Enables a number of nodes to
communicate by using the same shared media.
 “Data from only one node can be placed on the medium at any
one time.”
 (This is only true when using CSMA/CD (hubs), NOT true
with switches. Wireless uses CSMA/CA)
 Every node “may” see all the frames that are on the medium.
 Data Link Destination Address denote which device the frame is for.
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Multi-access Addressing
2222
4444
3333
6666
6666
5555
2222
 Multi-access networks require an address to specifically identify the
destination.
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LAN Topologies
Logical Topology for Shared Media
LAN Topologies
Contention-Based Access
Characteristics
Contention-Based Technologies
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Stations can transmit at any time
Collision exist
There are mechanisms to resolve
contention for the media
CSMA/CD for 802.3 Ethernet networks
CSMA/CA for 802.11 wireless networks
Media Access Control
 The media access control methods used by logical multi-access
topologies are typically:
 CSMA/CD - Hubs
 CSMA/CA - Wireless
 Token passing – Token Ring
 Later
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WAN Topologies
Half and Full Duplex
Duplex Transmissions
 Simplex Transmission: One way and one way only.
 One way street
 Half-duplex Transmission: Either way, but only one way at a time.
 Two way street, but only one way at a time (land slide).
 Ethernet hubs use half-duplex
 Full-duplex Transmission: Both ways at the same time.
 Two way street
 Ethernet switches use full-duplex
 Most serial links are full-duplex
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Data Link Frame Fields
 Data Link frame header fields may include:
 Start Frame field - Indicates the beginning of the frame
 Source and Destination address fields - Indicates the source
and destination nodes on the media
 Priority/Quality of Service field - Indicates a particular type of
communication service for processing
 Type field - Indicates the upper layer service contained in the
frame
 Logical connection control field - Used to establish a logical
connection between nodes
 Physical link control field - Used to establish the media link
 Flow control field - Used to start and stop traffic over the media
 Congestion control field - Indicates congestion in the media
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Framing- The Trailer
 The signals on the media could be subject to:
 Interference
 Distortion
 Loss
 This would change the bit values that those signals represent.
 The trailer is used to determine if the frame arrived without error.
 Error detection.
 The Frame Check Sequence (FCS) field is used to determine if
errors occurred in the transmission and reception of the frame.
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Cyclic Redundancy Check
 Cyclic redundancy check (CRC) is commonly used.
 Sending node includes a logical summary of the bits in the frame.
 Receiving node calculates its own logical summary, or CRC.
 Compares the two CRC values.
 Equal – Accepts the frame
 Different – Discards the frame
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Ethernet Protocol for LANs
 Ethernet is a family of networking technologies that are defined in
the IEEE 802.2 and 802.3 standards.
 Uses 48 bit addressing (Ethernet MAC addresses) for Source and
Destination
 More later!
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Point-to-Point Protocol for WANs
 Point-to-Point Protocol (PPP) is a protocol used to deliver frames
between two nodes.
 PPP can be used on various physical media, including:
 Twisted pair
 Fiber optic lines
 Satellite transmission
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Wireless Protocol for LANs
 802.11 is an extension of the IEEE 802 standards.
 It uses the same 48-bit addressing scheme as other 802 LANs.
 Contention-based system using a Carrier Sense Multiple
Access/Collision Avoidance (CSMA/CA)
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