cis81-E1-7-DataLinkLayer NOTES

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Chapter 7

Data Link Layer

CIS 81 Networking Fundamentals

Rick GrazianiCabrillo College

[email protected]

Last Updated: 4/27/2008



2

This Presentation

For a copy of this presentation and access to my web site for other

CCNA, CCNP, and Wireless resources please email me for a

username and password.

Email: [email protected]

Web Site: www.cabrillo.edu/~rgraziani



3

Data Link Layer

Ethernet,

PPP,

ISDN,

FrameRelay

The Data Link layer provides a means for exchanging data over a common

local media.



4

Application

Header + data

Generic Data Link Header



5

Node to node, not host to host


local media.



6

Focus on Transport Layer

IP

IP



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DataHTTP

Header

TCP

Header

IP

Header

Data Link

Header

Data Link

Trailer

IP PacketData Link

Header

Data Link

Trailer IP PacketData Link

Header

Data Link

Trailer

IP PacketData LinkHeader

Data LinkTrailer

IP PacketData LinkHeader

Data LinkTrailer

IP PacketData Link

Header

Data Link

Trailer IP PacketData Link

Header

Data Link

Trailer

DataHTTP

Header

TCP

Header

IP

Header

Data Link

Header

Data Link

Trailer

Reminder of encapsulation/decapsulation



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Data Link Frame


local media.

The Data Link layer performs two basic services:

1. Allows the upper layers to access the media using techniques such as

framing

2. Controls how data is placed onto the media and is received from the

media.

1.

2.



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Data Link Frame

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.

The Data Link layer frame includes:

Data ± Layer 3 (IP Packet or other Layer 3 information)

Header - Control information, such addressing

Trailer - Control information, such as error detection



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Data Link

Frame

Typical field types may include:

Start and stop indicator fields

Addressing fields (have to have origin and destination)Type field - The type of PDU contained in the frame

Quality - control fields

Data field -The frame payload (Network layer packet)

N ot all protocols include all of these fields.



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Data Link Layer

Data Link layer

Connects the Network Layer with the Physical Layer

Network Layer and above is software (IP, TCP, HTTP, etc.)

Physical layer is implemented in hardware (converting bits to atransmission signal)( copper,fiber,atmosphere)

Data Link layer is implemented in both:

Software

Hardware

Data Link Layer prepares Network Layer packets for transmissionacross some form of media, be it copper, fiber, or the atmosphere.



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Data Link Sublayers

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, (IP and IPX) can use the same

network interface and media.

MediaAccess 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.



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Data Link Standards

TCP/IP standards are defined by Internet Engineering Task Force

(IETF).

Data Link layer protocols are defined by:ISO - International Organization for Standardization

IEEE - Institute of Electrical and Electronics Engineers

ITU - International Telecommunication Union

ANSI - American National Standards Institute

LLC-Logical Link

C ontrol



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Media

AccessControl

MediaAccess 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? (S witches, hubs, etc.)



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Serial vs multi-access

Point-to-Point networks

Only two nodes

/30 subnets are common

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

Point-to-Point

Multi-access



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Duplex Transmissions

Simplex Transmission: One way and one way only.

One way street( simplex=one way one way only)

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( construction-one lane at a time on a two way street)

Full-duplex Transmission: Both ways at the same time.

Two way street( open two way street, normal traffic)

Ethernet switches use full-duplex

Most serial links are full-duplex



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Physical

Topology

The physical topology

is an arrangement of

the nodes and the

physical connections

between them.

P hysical=arrangment,physical connection

Hub

Switch

Serial

Connections

( serial is a point -to±point)



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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

(logical=traffic, actual movement)



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Point-to-Point topology

A point-to-point topology connects two nodes directly together.

The media access control protocol can be very simple.

F rames from one devices are for the device at the other end. ( no

collisions)

Point-to-point topologies, with just two interconnected nodes, do not

require special addressing. ( dosen¶t need a MAC address)

11111111



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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.

³Data from only one node can be placed on the medium at any

one time.´

(T his is only true when using CS MA/ C D ( hubs), NOT true

with switches or wireless)

Every node ³may´ see all the frames that are on the medium.

Data Link Destination ( MAC address) Address denote which device the

frame is for.

( broadcast traffic)





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Media Access Control

The media access control methods used by logical multi-access

topologies are typically:

CSMA/CD - HubsCSMA/CA - Wireless

Token passing ± Token Ring

Later



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Ring Topology

Token Passing media access control

Each node in turn receives a frame.

If data link destination address is not for that device, passes

frame to next node.



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Data Link Frame

No one frame structure meets the needs of all data transportationacross all types of media.

Depending on the:

Environment

Amount of control information needed

Topology



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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 sourceand 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

frameLogical 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( radio signals, electromagnetic 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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Bandwidth

LAN typically uses a high bandwidth technology

Supporting large numbers of hosts

WAN

High bandwidth technology is usually not cost-effective acrosslarge geographic areas (cities or multiple cities, for example).

The cost typically results in lower bandwidth capacity.

Note: T his is relative and the need for high bandwidth on WAN s is

increasing due to video, voice, and other applications.



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Ethernet Protocol for LANs

Ethernet is a family of networking technologies that are defined inthe IEEE 802.2 and 802.3 standards.

Uses 48 bit addressing (Ethernet MAC addresses) for Source and

Destination

More next week!



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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 MultipleAccess/Collision Avoidance (CSMA/CA)



Chapter 7

Data Link Layer

CIS 81 Networking Fundamentals

Rick GrazianiCabrillo College

[email protected]

Last Updated: 4/27/2008

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cis81-E1-7-DataLinkLayer NOTES