Data Communication Networks: Understanding the Data Link Layer, Study notes of Electrical and Electronics Engineering

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CDA 4506

Design and Implementation of Data

Communication Networks

Lecture Set 2

Dr. R. Lent

Chapter 5 outline

n 5.1 Introduction and

services

n 5.2 Error detection and

correction

n 5.3 Multiple access

protocols

n 5.4 LAN addresses and

ARP

n 5.5 Ethernet

n 5.6 Hubs, bridges, and

switches

n 5.7 Wireless links and LANs

n 5.8 PPP

n 5.9 ATM

n 5.10 Frame Relay

Chapter 5: The Data Link Layer

Our goals:

understand principles behind data link layer services:

n error detection, correction n sharing a broadcast channel: multiple access n link layer addressing n reliable data transfer, flow control (Ch. 3) n instantiation and implementation of various link layer technologies

Layered Architecture

SMTP, FTP, HTTP, etc

TCP, UDP

IP, ICMP

Ethernet, WiFi, PPP, ATM, Frame Relay, X.

Application

Transport

Network

Link

Physical

Datagrams

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

2-PDU is often called a frame ( encapsulates datagram)

Link Layer Services

1. Framing, link access: n encapsulate datagram into frame, adding header, trailer n channel access if shared medium n ‘physical addresses’ used in frame headers to identify source, dest n different from IP address!
2. Reliable delivery between adjacent nodes n seldom used on low bit error link (fiber, some twisted pair) n wireless links: high error rates n Q: why both link-level and end-end reliability?

Link Layer Services (more)

3. Flow Control: n pacing between adjacent sending and receiving nodes 4. Error Detection : n errors caused by signal attenuation, noise. n receiver detects presence of errors: n signals sender for retransmission or drops frame

5. Error Correction: n receiver identifies and corrects bit error(s) without resorting to retransmission 6. Half-duplex or full-duplex n with half duplex, nodes at both ends of link can transmit, but not at same time

Chapter 5 outline

n 5.1 Introduction and services

n 5.2 Error detection and

correction

n 5.3Multiple access protocols

n 5.4 LAN addresses and ARP

n 5.5 Ethernet

n 5.6 Hubs, bridges, and

switches

n 5.7 Wireless links and LANs

n 5.8 PPP

n 5.9 ATM

n 5.10 Frame Relay

Error Detection

EDC = Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!
  • protocol may miss some errors, but rarely
  • larger EDC field yields better detection and correction

Internet checksum

Sender: n treat segment contents as sequence of 16-bit integers n checksum: addition (1’s complement sum) of segment contents n sender puts checksum value into UDP checksum field

Receiver: n compute checksum of received segment n check if computed checksum equals checksum field value: n NO - error detected n YES - no error detected. But maybe errors nonetheless? More later ….

Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only )

Checksumming: Cyclic Redundancy Check

n view data bits, D, as a binary number

n choose r+1 bit pattern (generator), G

n goal: choose r CRC bits, R, such that

n <D,R> exactly divisible by G (modulo 2) n receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! n can detect all burst errors less than r+1 bits

n widely used in practice (ATM, HDCL)

Chapter 5 outline

n 5.1 Introduction and services

n 5.2 Error detection and

correction

n 5.3Multiple access protocols

n 5.4 LAN addresses and ARP

n 5.5 Ethernet

n 5.6 Hubs, bridges, and

switches

n 5.7 Wireless links and LANs

n 5.8 PPP

n 5.9 ATM

n 5.10 Frame Relay

Multiple Access Links and Protocols

Two types of “links”: n point-to-point n PPP for dial-up access n point-to-point link between Ethernet switch and host n broadcast (shared wire or medium) n traditional Ethernet n upstream HFC n 802.11 wireless LAN

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1. When one node wants to transmit, it can send at rate R.
2. When M nodes want to transmit, each can send at average rate R/M
3. Fully decentralized:

n no special node to coordinate transmissions n no synchronization of clocks, slots

4. Simple

MAC Protocols: a taxonomy

Three broad classes:

1. Channel Partitioning

n divide channel into smaller “pieces” (time slots, frequency, code) n allocate piece to node for exclusive use

2. Random Access

n channel not divided, allow collisions n “recover” from collisions

3. “Taking turns”

n tightly coordinate shared access to avoid collisions