CSC4220/6220 Computer Networks

-1- Georgia State UniversitySensorweb Research Laboratory

CSC4220/6220 Computer Networks

Dr. WenZhan Song

Professor, Computer Science


Course roadmap Introduction Application Layer: WWW, FTP, email, DNS, multimedia Transport Layer: reliable end-end data transfer principles, UDP,

TCP Network Layer: IP addressing, routing and other issues Data Link Layer: framing, error control, flow control

Medium Access Control (MAC) Layer: multiple-access, channel allocation

Physical Layer: wired, wireless, satellite Other Topics: network security, social issues, hot topics, research

directions


Data Link Layer

Our goals: understand principles behind data link layer

services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done!

instantiation and implementation of various link layer technologies


Link Layer: Introduction

Some terminology: hosts and routers are nodes communication channels that

connect adjacent nodes along communication path are links

wired links wireless links LANs

layer-2 packet is a frame, encapsulates datagram

“link”

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


Data Link Layer Road Map

Data link layer design issues Framing Error Control

Example data link protocols HDLC and PPP

Multiple Access Protocols Static channel allocation Dynamic channel allocation

LAN technologies and their MAC protocols Ethernet WiFi and WiMax


Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Regulating data flow

Slow receivers not swamped by fast senders Sharing broadcast channels: multiple access

channels Determine who gets to use the channel when there is

competition for it


Functions of the Data Link Layer

Relationship between packets and frames.


Services Provided to Network Layer

Various services Unacknowledged connectionless service

Without ack, no logical connection Appropriate when error rate is low, real-time traffic (voice,

late data worse than bad data) Most LANs

Acknowledged connectionless service No logical connection, but individually acked Useful over unreliable channels (wireless)

Acknowledged connection-oriented service Guarantee frame received only once and in right order Three phases: connection established, frames transmitted,

connection released


Adaptors Communicating

link layer implemented in “adaptor” (called NIC)

Ethernet card, PCMCI card, 802.11 card

sending side: encapsulates datagram in

a frame adds error checking bits,

rdt, flow control, etc.

receiving side looks for errors, rdt, flow

control, etc extracts datagram, passes

to rcving node adapter is semi-

autonomous link & physical layers

sendingnode

frame

rcvingnode

datagram

frame

adapter adapter

link layer protocol


Framing

Encapsulate datagram into frame, adding header and trailer

Break the bit stream up into discrete frames and compute the checksum of each frame

Relationship between packets and frames


Framing – Character CountA character stream.

(a) Without errors. (b) With one error.


Framing (2) – Flag Bytes

(a) A frame delimited by flag bytes.(b) Four examples of byte sequences before and after

stuffing.

Actually, after insert ESC control byte, it also inverts the fifth bit of (or XOR 0x20 with) the following data byte. See wiki or textbook p491. For simplicity, many textbooks do not mention it.


Framing (3) – Bit Flags Delimit frame at both ends using 01111110 May close one frame and open another Receiver hunts for flag sequence to synchronize Bit stuffing used to avoid confusion with data

containing 01111110 0 inserted after every sequence of five 1s If receiver detects five 1s it checks next bit If 0, it is deleted If 1 and seventh bit is 0, accept as flag If sixth and seventh bits 1, sender is indicating abort


Framing (4) – Bit Flags

Bit stuffing(a) The original data.(b) The data as they appear on the line.(c) The data as they are stored in receiver’s memory

after destuffing.


Error Control Error Detection:

errors caused by signal attenuation, noise. receiver detects presence of errors:

error-detection codes signals sender for retransmission or drops frame

Error Correction: receiver identifies and corrects bit error(s) without

resorting to retransmission error-correction codes


Error Detection and Correction

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


Error-Detection & Correction Code

Single Bit Parity:Detect single bit errors

Only odd bit errors detection

Examine: Number of 1s in the code is even

Consider a code with only 4 valid codewords:

00000 0000000000 1111111111 0000011111 11111

They have Hamming diatance 5 with each other, hence can correct double errors.

Example: 00000 00111 must be 00000 11111

Rules To correct d errors, you need a distance 2d+1 code To detect d errors, you need a distance d+1 code

Hamming distance of x and y is XOR(x,y)


Error-Correction Code

Theoretical lower limit A code with m message bits and r check bits which will allow all

single errors to be corrected 2m legal messages – legal codewords For each legal message, there is n illegal codewords Total number of bit pattern is 2n

Therefore, (n+1)2m2n

n=m+r, (m+r+1) 2r => the limit can be achieved by Hamming code

Hamming Code It can correct any single bit error bits in power of 2 positions (1,2,4,8,) are check bits, the rest are

m data bits Check bit forces the parity of some collection of bit “1” to be

even To see which check bits the data bit in position k contributes to,

rewrite k as a sum of powers of 2. 11=1+2+8, 29=1+4+8+16

Hamming codeHamming code


Hamming Code Example

If a 12-bit hamming code 0x9B8 = 100110111000 arrives at receiver, is there any bit error? If so, which bit is wrong?

Check bit

Checked Data bits Bits Sequence

1 3 5 7 9 11 1 0 1 1 1 0 (Yes)

2 3 6 7 10 11 0 0 0 1 0 0 (No)

4 5 6 7 12 1 1 0 1 0 (No)

8 9 10 11 12 1 1 0 0 0 (Yes)

1 2 3=2+1 4 5=4+1 6=4+2 7=4+2+1 8 9=8+1 10=8+2 11=8+2+1

12=8+4

1 0 0 1 1 0 1 1 1 0 0 0

Hence bit 6 = 2+4 is wrong, so correct hamming code should be 0x9F8 = 100111111000


Correct burst errors

Use of Hamming code to correct k burst errors by send k ASCII character together, column by column. (Read Tanenbaum textbook page 195)


Error-Detection Codes

Note When adding numbers, a carryout from the most

significant bit needs to be added to the result

Example: add two 16-bit integers

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

wraparound

sumchecksum

Internet checksumInternet checksum


Error-Detection Codes

view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that

<D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G. If non-zero

remainder: error detected! can detect all burst errors less than r+1 bits, and odd

number of bit errors widely used in practice (ATM, HDCL)

Cyclic Redundancy Check


CRC Example

Want:D.2r XOR R = nG

equivalently:D.2r = nG XOR R

equivalently: if we divide D.2r by G,

want remainder R

R = remainder[ ]

If G has r+1 bit, R has r bit

D.2r

GOperation is XOR, not +, or -!!!

Notice: x^3+1 is a generator!

Left shift r bit

0000

0000


Another ExampleCalculation of the polynomial code checksum.

GCRC-32=100000100110000010001110110110111


CRC generator preferenceGenerator G(x) with degree k can detect: All single-bit errors, as long as the xk and x0 terms have

nonzero coefficients. All double-bit errors, as long as G(x) has a factor with at

least three terms. Any odd number of errors, as long as G(x) contains the

factor (x+1). Any “burst” error(i.e., sequence of consecutive errored bits)

with the length less than (including equal) k bits. Burst errors of larger than k bits can also be detected with

high probability 1-1/2k)

* Refer textbook page 196-200


Flow Control Ensuring the sending entity does not overwhelm

the receiving entity Preventing buffer overflow

Transmission time Time taken to emit all bits into medium

Propagation time Time for a bit to traverse the link


Model of Frame Transmission

X


Data Link Layer Road Map

Data link layer design issues Framing Error Control

Example data link protocols HDLC and PPP

Multiple Access Protocols Static channel allocation Dynamic channel allocation

LAN technologies and their MAC protocols Ethernet WiFi and WiMax


Point to Point Data Link Control

one sender, one receiver, one link: easier than broadcast link:

no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line

popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to

be considered “high layer” in protocol stack!


HDLC (High-Level Data Link Control)

Frame format for bit-oriented protocols.

Data link layer protocol used in IBM mainframe world, X.25 network, etc

Using slide window protocol (GBN, SR, or variations) for flow control and reliable data transfer


HDLC (High-Level Data Link Control)

Control field of (a) An information frame. (b) A supervisory frame

ACK or RECEIVE READY NAK or REJECT: follow GBN protocol RECEIVE NOT READY: ask sender to stop SELECTIVE REJECT: follow SR protocol

(c) An unnumbered frame used for control purpose or carry data when unreliable connectionless service

needed

ACKed/NAKed frame#


The Data Link Layer in the Internet

A home personal computer acting as an internet host.


PPP Design Requirements [RFC 1557]

packet framing: encapsulation of network-layer datagram in data link frame

carry network layer data of any network layer protocol (not just IP) at same time

ability to demultiplex upwards bit transparency: must carry any bit pattern in

the data field error detection (no correction) connection liveness: detect, signal link failure to

network layer network layer address negotiation: endpoint can

learn/configure each other’s network address


PPP non-requirements

no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g.,

polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!


PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible

multiple control fields Protocol: upper layer protocol to which frame

delivered (eg, PPP-LCP, IP, IPCP, etc)


PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error

detection

Not bit-oriented, e.g., bit-stuffing, it uses byte-stuffing!


Byte Stuffing

“data transparency” requirement: data field must be allowed to include flag pattern <01111110>

Q: is received <01111110> data or flag?

Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte

Receiver: two 01111110 bytes in a row: discard first byte,

continue data reception single 01111110: flag byte


Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data


PPP Data Control Protocol

Before exchanging network-layer data, data link peers must

LCP: configure PPP link (max. frame length, authentication)

NCP: learn/configure network

layer information for IP: carry IP Control

Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

LCP LCP configuratioconfiguratio

nn


Reading Assignment Chapter 5

5.2 – 5.6

CSC4220/6220 Computer Networks

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