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RD-CSY1017 2
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
RD-CSY1017 3
Data Link layer components Data Frames MAC address Media Access and protocols
◦ Ethernet◦ Ring Topology protocol
Token Ring FDDI
RD-CSY1017 4
RD-CSY1017 5
Application
Presentation
Session
Transport
Network
Physical
Data Link
Computer One
Application
Presentation
Session
Transport
Network
Physical
Data Link
Computer Two
Underlying Physical Medium
RD-CSY1017 6
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)
RD-CSY1017 7
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)?
RD-CSY1017 8
◦ 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
RD-CSY1017 10
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 :
RD-CSY1017 11
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!
13RD-CSY1017
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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RD-CSY1017 19
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