1 Data Link Layer Lecture 17 Imran Ahmed University of Management & Technology

2 Agenda Introduction & services Error detection and correction Multiple access protocols LAN addresses and ARP Ethernet Hubs, bridges and switches

3 Multiple Access Links & Protocols Two types of “links”: Point-to-point:- –PPP for dial-up access –Point-to-point link between Ethernet switch and host Broadcast:- –Traditional Ethernet – wireless LAN

4 Multiple Access Links & Protocols MAC protocols: –Channel partitioning Time division, Code division, Frequency division –Random partitioning ALOHA, S-ALOHA, CSMA, CSMA/CD –Taking turns Token passing

5 Agenda Introduction & services Error detection and correction Multiple access protocols LAN addresses and ARP Ethernet Hubs, bridges and switches

6 LAN Addresses & ARP LAN (or MAC or physical or Ethernet) address: It is used to get datagram from one interface to another physically-connected interface (same network). 48 – bit MAC address; burned in the adapter ROM.

7 LAN Addresses & ARP

8 IP Addressing Starting at A, given IP datagram addressed to B: Look up net. address of B, find B on same net. as A. Link layer send datagram to B inside link-layer frame

9 Address Resolution Protocol Each IP node on LAN has ARP table. ARP Table: IP/MAC address mappings for some LAN nodes. –TTL: time after which address mapping will be forgotten (typically 20 min.)

10 Agenda Introduction & services Error detection and correction Multiple access protocols LAN addresses and ARP Ethernet Hubs, bridges and switches

11 Project 802 In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication between equipment from a variety of manufacturers. Project 802 uses the first two layers and, to a lesser extent, the network layer.

12 OSI Model and Project 802

13 Project 802 The IEEE has subdivided the data link layer into two sub-layers: –Logical Link Control (LLC) –Medium Access Control (MAC)

14 IEEE Project 802 Each subdivision is identified by a number: –IEEE (internetworking) –IEEE (LLC) –IEEE (CSMA/CD or Ethernet) –IEEE (Token Bus) –IEEE (Token Ring)

15 Project 802

16 IEEE Project 802 IEEE 802.1:This section is devoted to internetworking issues in LANs and MANs. Logical Link Control:IEEE LLC is the upper sub-layer of the data link layer. It contains logical addresses, control information and data unit. Media Access Control:MAC is the lower sub-layer of the data link layer. It contains the flag, flow and error control specification necessary to move information from one place to another.

17 IEEE Project 802 Ethernet IEEE supports a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation, Intel Corporation and Xerox. This was called Ethernet. IEEE defines two categories: –Baseband –Broadband

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