Access to Shared Media - Media Access Control protocols RD-CSY10171

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-CSY10173

 Data Link layer components  Data Frames  MAC address  Media Access and protocols ◦ Ethernet ◦ Ring Topology protocol  Token Ring  FDDI RD-CSY10174

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-CSY10176  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) MAC (Media Access Control)

RD-CSY10177  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: F-3A-07-BC  How long is the MAC address ( in terms of bytes)?

RD-CSY10178 ◦ Example of MAC address  F-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-CSY10179

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-CSY  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? nnnn Abbreviated MAC Addresses RD-CSY1017

 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 nnnn Abbreviated MAC Addresses 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 14RD-CSY1017 X Collision!

 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. 15RD-CSY1017

 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 16RD-CSY1017

 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 17RD-CSY1017

 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 18RD-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 Wired network  IEEE – Wireless network ◦ Token Ring  IEEE  Data Link Layer Components Logical Link Control (LLC) Ethernet Token Ring Media Access Control (MAC) Protocols

RD-CSY  Let’s pause here for a moment and bring the following together: ◦ What is the data entity at layer 2 called ? ◦ What is role of MAC address in data communication? ◦ Which protocols of media access are probabilistic/ deterministic?  CSMA/CD  Token Ring