Direct Link Networks: Building Blocks (2.1), Encoding (2.2), - - PowerPoint PPT Presentation

Direct Link Networks: Building Blocks (2.1), Encoding (2.2), Framing (2.3)

ECPE/CS 5516: Computer Networks

Originally by Scott F. Midkiff (ECpE) Modified by Marc Abrams (CS) Virginia Tech courses.cs.vt.edu/~cs5516

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 2

Lecture Topics

I Physical layer

G Examples of direct links (2.1) G Data encoding (2.2)

I Point-to-point protocols

G Framing (2.3)

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 3

2.1: Hardware Building Blocks

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 4

Links at Physical Layer

I Physical media for links:

G Twisted pair G Coaxial cable G Optical fiber G Radio waves G Infrared

I Media + Electronics + Optics =

media properties

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 5

Physical Layer Properties

I Bit encoding:

How is information -- 1’s and 0’s -- encoded?

I Full-duplex versus half-duplex operation

G

Full-duplex: data in both directions simultaneously

G

Half-duplex: data in one direction at a time

I Data rate:

How much info can be sent in unit of time?

I Extent:

What’s max link length for reliable operation?

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 6

Link Examples

Category 5 twisted pair 50-ohm coax (Thinwire) 75-ohm coax (Thickwire) Multimode fiber Single-mode fiber Service 10-1000 Mbps 10-100 Mbps 10-100 Mbps Bandwidth 100 Mbps 100-2400 Mbps 100 m 200 m 500 m Distances 2 km 40 km

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 7

Examples of Leased Links

ISDN (B-channel) T1 (DS1) T3 (DS3) STS-3 (OC-3) STS-12 (OC-12) STS-24 (OC-24) STS-48 (OC-48) Service 64 Kbps 1.544 Mbps 44.736 Mbps 1.244160 Gbps 2.488320 Gbps Bandwidth 155.251 Mbps 622.080 Mbps

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 8

Links to Homes

POTS ISDN xDSL CATV Service 28.8-56 Kbps 64-128 Kbps 16Kbps-55Mbps Bandwidth 20-40 Mbps

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 9

2.2: Encoding

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 10

Encoding

I Encoding determines how information is

represented by electrical, optical, or electromagnetic signal

Node Adaptor Node Adaptor signal information

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 11

Example of Encoding

I The most intuitive… Non-return to Zero (NRZ)… I Encode “1” as high voltage level, “0” as low I Others:

G Non-return to zero inverted (NRZI) G Manchester G Block codes, e.g. 4B/5B

1 1 1 1

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12

Physical Layer “Bit Pipe”

I Physical layer defines signal levels and timing

to deliver bit stream to Data Link layer

I Signal bandwidth determines data rate limit I Timing errors

G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift”

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12

Physical Layer “Bit Pipe”

I Physical layer defines signal levels and timing

to deliver bit stream to Data Link layer

I Signal bandwidth determines data rate limit I Timing errors

G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift”

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12

Physical Layer “Bit Pipe”

I Physical layer defines signal levels and timing

to deliver bit stream to Data Link layer

I Signal bandwidth determines data rate limit I Timing errors

G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift”

10111010001

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 12

Physical Layer “Bit Pipe”

I Physical layer defines signal levels and timing

to deliver bit stream to Data Link layer

I Signal bandwidth determines data rate limit I Timing errors

G Noise or distortion can lead to errors in timing G Sender and receiver clocks may differ -- “drift”

10111010001 ??11???00?1

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 13

Asynchronous vs. Synchronous Transmission

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 14

Asynchronous Transmission (1)

I Each transmission is synchronized

G A start bit begins, stop bit ends transmission G Line stays in an idle state until next start bit

I Samples timed from beginning of start bit I Used in applications where performance can be reduced

to reduce costs

G Modems G PC serial ports

1 1 1 1 T/2 T start stop idle start NRZ encoding:

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 15

Asynchronous Transmission (2)

I Physical layer can provide characters (n-bit

units) to Data Link layer

n bits n bits n bits idle idle

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 16

Asynchronous Transmission (3)

I Advantages

G Simple timing mechanism G Inherent character framing G Adapts to different data rates (idle serves as fill)

I Disadvantages

G Timing errors can occur if line is noisy

(e.g., missed start bit)

G Overhead for stop and start bits

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 17

Synchronous Transmission (1)

I Used for high data rates, including T1 and or

interoffice digital transmission lines

I Information is sent continuously

G Receiver & repeater maintain synchronization

between incoming signaling rate and local sample clock

G Idle or fill characters inserted if line is idle

I Signal transitions (high-to-low or

positive-to-negative) enable clock recovery, or synchronization

G Some minimum occurrence of signal transitions are

needed to maintain synchronization (Why?)

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 18

Why Frequent Signal Transitions Needed

I Long strings of 0’s or 1’s cause problems.

G Receiver synchronizes clock on 0-1 transitions.

No transitions = no synchronization

G Receiver averages signal it receives to determine hi

• vs. low signal.

No transitions = incorrect average.

G NRZ encoding is bad.

I Solutions…

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 19

Solutions: Ensuring Signal Transitions

I Manchester encoding:

G Sender XOrs local clock w/ NRZ encoded data,

producing hi/low transitions for every bit

G But doubles rate at which signal is transmitted

I Dedicated timing bits

G Use some bit transmissions just for timing G Example: Dataphone Digital Service

N Use every one bit out of eight to guarantee a

signal transition

G Example: Synchronous modems

N Periodically insert a SYNC character

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 20

Ensuring Signal Transitions (2)

I Bit insertion

G Use bit transmissions for timing only when needed

by inserting timing bits into data bit stream

G HDLC “bit stuffing”

N 01111110 indicates end of a data block (for

framing)

N Six consecutive 1’s must not be sent as data (may

be mistaken as end of a data block)

N Sender inserts a 0 after every string of five

consecutive 1’s; receiver must strip a 0 after five consecutive 1’s

G Disadvantage: extra bits = extra delay

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 21

Ensuring Signal Transitions (3)

I Data scrambling

G Similar to encryption/decryption G Prevents transmission of repetitive patterns G With high probability, prevents long strings of 0’s (or

1’s) that would not have signal transitions

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 22

Module at Node A Module at Node B “Link”

Point-to-Point Protocols and Links (1)

I Point-to-point protocols involve exactly two

peer entities or modules that are connected by some “link”

I Modules must interact to ensure proper transfer

• f information using link

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 23

Point-to-Point Protocols and Links (2)

I For example, link may be:

G Physical link (e.g. RS-232 is a point-to-point

protocol)

G Virtual bit pipe (e.g. at data link layer) G A connection or virtual connection (e.g. at transport

• r session layer)

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 24

Data Link Control -- DLC (1)

I For each point-to-point link in a network re are

two data link control (DLC) peer modules, one at each end

I DLC modules use a distributed algorithm to

transfer packets

G Received from and delivered network layer

I Usual objective is to deliver packets in order of

arrival (from network layer) without errors or repeated packets

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 25

Network DLC Physical Network DLC Physical

Data Link Control -- DLC (2)

I DLC modules must use unreliable “virtual bit

pipe” provided by physical layer

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 26

Data Link Control -- DLC (3)

I DLC must:

G Detect errors (using redundancy bits) G Request retransmission if data is lost (using

automatic repeat request -- ARQ)

G Perform framing (detect packet start and end) G Support initialization and disconnection operations

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 27

service data unit header trailer frame

Data Link Control -- DLC (4)

I These functions require that extra bits be added

to packet to be transmitted

G Header bits are added to front of each each packet G Trailer bits are added to rear of each packet G

header, packet from upper layer (service data unit), and trailer form a frame

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 28

2.3 Framing

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 29

service data unit header trailer frame

Frame Format

I Packet from upper layer is Service data unit

(SDU)

I Frame (header, network layer packet, and

trailer) is protocol data unit (PDU)

I Note that DLC does not care what is in network

layer packet and physical layer does not care what is in frame generated by data link layer

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 30

Framing (1)

I We have assumed that DLC knows where a

frame begins and ends

I This is not automatic due to …

G Continuous transmission of one frame after another G Idle fill characters in synchronous bit pipes G No transmission in asynchronous bit pipes

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 31

Framing (2)

I Framing is process of deciding start and end of

successive frames

G Byte-oriented or character-based framing G Bit-oriented framing -- flags G Length fields G Clock based framing

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 32

Character-Based Framing (1)

I Special characters are used to indicate idle fill

(ASCII SYN), start of text (STX), and end of text (ETX)

G SYN = 0001 0110

(16H)

G STX = 0000 0010

(02H)

I Data Link layer extracts frame boundaries

SYN SYN STX header packet ETX CRC SYN frame 00010110 00010110 00000010 ...

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 33

Character-Based Framing (2)

I If packet data is arbitrary it may contain control

characters, so transparent mode must be used based on DLE (data link escape) character

G DLE STX is start of transparent mode G DLE ETX ends text G DLE DLE needed if DLE is in packet data

I Problems:

G Excessive framing overhead G Error may cause premature ETX detection G Error may alter ETX so that end of frame is missed

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 34

Bit-Oriented Framing -- Flags (1)

I Character-based framing:

G Use full characters as “flags” G DLE STX and DLE ETX

I Bit-oriented framing:

G Use bit patterns as flags to reduce overhead of

character-based framing

G 01111110 = 0160 is usual flag –

six consecutive 1’s indicates end of frame to receiving DLC

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 35

Problem with 01111110 Flag

I So if we use 01111110 as flag… I … and the actual data contains 01111110…

What do we do?

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 36

Bit-Oriented Framing -- Flags (2)

I Sender uses bit stuffing to eliminate any string

• f six 1’s in data.

I Receiver strips off stuffed bits

G Sender inserts 0 after five consecutive 1’s in data G Example (one 0 is inserted):

N Original frame: 00111001111111011 N Transmitted:

001110011111011011

G Receiver strips off one 0 after five consecutive 1’s

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 37

Framing with Length Fields (1)

I There are two aspects to framing:

G Detecting end of idle fill -- can be done by

transmitting a special character (e.g. SYN) or a constant bit stream (e.g. all 1’s) that is interrupted when frame begins

G Detecting end of frame -- can be done by sending

length information (e.g. number of bytes in frame) in header SYN SYN length packet CRC SYN frame header

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 38

Framing with Length Fields (2)

I Brute force:

Need log2 Kmax bits for frame size Kmax

I More efficient encodings:

G If only certain frame sizes possible, fewer bits needed

N Ex: just 2 bits if all packets are of length a, b, c, or d

G If some lengths more likely than others, assign most likely

lengths shorter codes

I But brute force = faster, simpler implementations

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 39

Framing with Length Fields (3)

I Errors can corrupt length field, causing receiver

to look for CRC in wrong place!

G Frame accepted with probability 2-L

(L is length of CRC)

G If length field's corrupted,

how do you find start of next frame?

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 40

Framing with Length Fields (3)

I Possible (partial) solutions:

G Use extra CRC over a fixed length header, but

synchronization is still needed

G Embed length field in trailer of previous frame G Use a longer CRC to reduce probability of

acceptance

G Use fixed-length frames and pass job off to

a higher layer

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 41

Some Choices…

I Should frames be

G Large or small? G Fixed size or variable size?

I What are the pros/cons?

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 42

Maximum Frame Size (1)

I Large maximum frame size is good…

G Reduces transmission overhead for framing and

header information (not critical for high data rates)

G Reduces frame processing load (important for high

data rates)

I Short maximum frame size is good…

G Can reduce delay variance and packetization delay

(Think of trucks vs. cars on I81)

G Allows pipelining over multiple links to reduce

delay…

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 43

Maximum Frame Size (2)

D (short frames) D (long frames) Link 1 Link 3 Link 2 Pipelining over multiple links

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 44

Fixed Frame Lengths

I Could require all frames to be same length I Don't need bits for length field or framing flags.

G Do need padding if a packet doesn't completely fill

frame.

I Small fixed length frames…

G Reduces need for padding G Reduce latency for stream data, e.g. packetized voice

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 45

Example of Fixed Frame Lengths

I Asynchronous Transfer Mode (ATM)

G Frames (called cells) are fixed at 53 bytes

N 48-byte data field plus 5-byte header

G Simplifies high-speed switching G Supports voice and video that demand low latency G Reduces variance in delay

CS/ECPE 5516 (1/31/00) Direct Link Networks: 2.1, 2.2, 2.3 - 46

You should now be able to …

I Describe role of a point-to-point protocol and

direct links in a network

I Describe and compare different encoding

schemes

I Describe and compare basic framing

techniques, citing limitations