Access to Shared Media -

Media Access Control protocols

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Layer 1 involves ◦ Media

Physical , Wireless ◦ Digital data

bit streams that travel on media, ◦ Components that put signals on media, and

cables, connectors.. Have I missed anything?

◦ Topologies Shared access

Any examples ? Dedicated access

any example ?

Physical layer does not have any provision to:

Issue 1: Identify◦ Which computer does data belong to?

Issue 2: Data Format◦ How is data ORGANISED and Carried on the media

bits/bytes, Frames Issue 3: Access shared media

Answer: Assign these jobs to the layer above it – the Data Link Layer

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Data Link layer components Data Frames MAC address Media Access and protocols

◦ Ethernet◦ Ring Topology protocol

Token Ring FDDI

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Application

Presentation

Session

Transport

Network

Physical

Data Link

Computer One

Application

Presentation

Session

Transport

Network

Physical

Data Link

Computer Two

Underlying Physical Medium

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The OSI –Data Link Layer is divided into two separate sub-layers:◦ Media Access Control (MAC) – provides service to

the layer below - physical layer ◦ Logical Link Control (LLC) - provides services to

layer above - the network layer

LLC (Logical Link Control)LLC (Logical Link Control)

MAC (Media Access Control)MAC (Media Access Control)

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Every Ethernet NIC card has a unique address – called MAC address, which provides a way for computers to identify themselves.

They give hosts a permanent,

unique identification on a network. ◦ Example: 00-60-2F-3A-07-BC

How long is the MAC address

( in terms of bytes)?

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◦ Example of MAC address 00-60-2F-3A-07-BC

◦ 48 bits (6 bytes) in length.◦ The first 3 bytes, are

administered by the IEEE (Institution of Electrical and Electronics engineers) to help identify the manufacturer.

◦ The first three bytes are called the Organizational Unique Identifier (OUI).

◦ The remaining 3 bytes comprise the interface serial number and are unique to a card

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Data bits are generated by a computer that wishes to communicate

Stream of data bits or bit-stream is framed by layer 2 and transmitted on the cable (layer 1) by the Network Interface Card (NIC).

Framing provides structure to the bit-stream. Example data frame :

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Two ways to access shared media Random Access

◦ Any device on the network that has data to send can access the medium, if not used

Taking turns ◦ Each device on a network takes turns in a round-robin manner.

Multi-access

1111, 2222,… are MAC addresses When an Data frame is sent out all devices on the

network receive it. What do they do with it?

1111 2222 3333 nnnn Abbreviated MAC Addresses

11113333

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When information (frame) is transmitted, each NIC on the shared media copies part of the transmitted frame to see if the destination address matches the MAC address on its NIC.

If there is a match, the rest of the frame is copied, otherwise rest of the frame is ignored.

1111 2222 3333nnnn Abbreviated

MAC Addresses

11113333

Nope

Nope

Hey, that’s me!

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Data Frame

Issue: What happens when multiple computers want try to transmit at the same time? E.g., Computer 1 and 2 has data to send… Two or more computers accessing media at the same time causes data Collision Collision is detected by all computers, which then stop transmitting data Ethernet protocol CSMA/CD summarizes these steps..

I have data to send

I have data to send

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XCollision!

Listens (Carrier Sense) to the network’s shared media to see if any other users are “on the line” by trying to sense a neutral electrical signal or carrier.

If no transmission is sensed, then multiple access allows anyone onto the media without any further permission required.

If two PCs detect a neutral signal and access the shared media at the exact same time, a collision occurs and is detected.

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Which topology is this ? Taking turns MAC protocols need

a permission to transfer data on network

Permission is given in the form of a control token (T), which is passed from one computer to next in a round-robin fashion◦ Token is short, reserved frame

that cannot appear in data There is only one token, so only

one computer can transmit at a time

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When a computer wants to transmit, it waits for the token

After it acquires the token, it can send data for the preset time.

After preset time, computer transmits token on ring

Next computer ready to transmit receives token and then transmits

If no computer is ready to transmit data, token circulates around ring

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IBM Token Ring ◦ Now obsolete ◦ Data transfer rate upto16Mbps

Fibre Distributed Data Interconnect (FDDI)◦ Uses ring topology◦ Uses fibre optics cable between stations ◦ Transmits data at 100Mbps

Uses pairs of fibres to form two concentric rings to provide fault tolerance

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IEEE 802.x series standards are defined for Layer 2 (Data Link Layer) protocols for media access.

Two Popular standards for local area network are: ◦ Ethernet

IEEE 802.3 - Wired network IEEE 802.11 – Wireless

network◦ Token Ring

IEEE 802.5

Data Link Layer Components

Logical Link Control (LLC)

Ethernet Token Ring

Media Access Control (MAC) Protocols

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Let’s pause here for a moment and bring the following together:◦ What is the data entity at layer 2 called ?◦ W hat is role of MAC address in data

communication?◦ Which protocols of media access are probabilistic/

deterministic? CSMA/CD Token Ring