cmpe 80n spring 2003 week 4

1CMPE 80N - Introduction to Networks and the Internet

CMPE 80N

Spring 2003

Week 4

Introduction to Networks and the Internet


Announcements

• HTML tutorial on 04.29 (in class).– Links to on-line HTML tutorials/tools.


Layer 2: Data Link Layer


Data Link Layer

• So far, sending signals over transmission medium.

• Data link layer: responsible for error-free (reliable) communication between adjacent nodes.

• Functions: framing, error control, flow control, addressing, and medium access (in shared networks).


medium access control

logical link control

Medium Access Control Protocols

Coordinate competing requests

for medium.

PHYSICAL

LINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATION

Sharing of link and transportof data over the link


Medium Access Control

• Problem: – Computers in a shared network environment.– Only one computer can transmit at a time.

• If two computers try to use the same line at the same time, their messages get garbled.

• Collision!

– How can we organize the transmission so that all computers are given an opportunity to exchange messages?


Medium Access Control

• Control access to shared medium.• How?


The Multiplexing Problem

Analogy: a highway shared by many userstime

frequency

Shared channel

(how to divide resource among multiple recipients?)


Frequency-Division Multiplexing

Analogy: a highway has multiple lanestime

frequency

user 1

user 2

user 3user 4

guard-band


Time-Division Multiplexing

Requirement: precise time coordinationtime

frequency

user 1 user 2 user 3 user 4

guard-band

user 1 user 2


Frequency-Time-Division

time

frequency

time-slot (usually of the same size)


Centralized versus Distributed MAC

• Centralized approaches:– Controller grants access to medium.– Simple, greater control: priorities, qos.– But, single point of failure and performance

bottleneck.

• Decentralized schemes:– All stations collectively run MAC to decide when

to transmit.


Round-Robin MAC

• Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time).

• Centralized (e.g., polling) or distributed (e.g., token ring) control of who is next to transmit.

• When done, station relinquishes and right to transmit goes to next station.

• Efficient when many stations have data to transmit over extended period (stream).


Scheduled Access MAC

• Time divided into slots.• Station reserves slots in the future.• Multiple slots for extended transmissions.• Suited to stream traffic.


Contention-Based MAC

• No control.• Stations try to acquire the medium.• Distributed in nature.• Perform well for bursty traffic.• Can get very inefficient under heavy load.

• NOTE: round-robin and contention are the most common.


MAC Protocols

• Contention-based– ALOHA and Slotted ALOHA.

– CSMA.

– CSMA/CD.

• Round-robin : token-based protocols.– Token bus.

– Token ring.


Standardized MACsTopologies

Bus Ring

Round robin

Scheduled

Contention

Token bus(802.4)Polling (802.11)

DQDB(802.6)

CSMA/CD(802.3)CSMA/CA(802.11)

Token ring(802.5; FDDI)

Techniques


Contention-Based MACs


The ALOHA Protocol

• Developed @ U of Hawaii in early 70’s.• Packet radio networks.• “Free for all”: whenever station has a frame to

send, it does so.– Station listens for maximum RTT for an ACK.– If no ACK, re-sends frame for a number of times

and then gives up.– Receivers check FCS and destination address to

ACK.


Collisions

• Invalid frames may be caused by channel noise or

• Because other station(s) transmitted at the same time: collision.

• Collision happens even when the last bit of a frame overlaps with the first bit of the next frame.


ALOHA’s Performance 1

Timet0

t0+t t0+2t t0+3t

vulnerable


ALOHA’s Performance 2

• S = G e-2G, where S is the throughput (rate of successful transmissions) and G is the offered load.

• S = Smax = 1/2e = 0.184 for G=0.5.


Slotted Aloha

• Doubles performance of ALOHA.• Frames can only be transmitted at beginning

of slot: “discrete” ALOHA.• Vulnerable period is halved.• S = G e-G.

• S = Smax = 1/e = 0.368 for G = 1.


ALOHA Protocols

• Poor utilization.• Key property of LANs: propagation delay

between stations is small compared to frame transmission time.

• Consequence: stations can sense the medium before transmitting.


Carrier-Sense Multiple Access (CSMA)

• Station that wants to transmit first listens to check if another transmission is in progress (carrier sense).

• If medium is in use, station waits; else, it transmits.

• Collisions can still occur.• Transmitter waits for acknowledgement

(ACK); if no ACKs, retransmits.


CSMA/CD 1

• CSMA with collision detection.• Problem: when frames collide, medium is

unusable for duration of both (damaged) frames.

• For long frames (when compared to propagation time), considerable waste.

• What if station listens while transmitting?


CSMA/CD Protocol

1. If medium idle, transmit; otherwise 2.

2. If medium busy, wait until idle, then transmit.

3. If collision detected, abort transmission.

4. After aborting, wait random time, try again.


Ethernet


Ethernet • Most popular CSMA/CD protocol.• What if a computer transmits a very long

message?– It keeps the line busy for very long time, while

all other computers in the LAN must wait for the long message to end

• Rule #1 of resource sharing: All “messages” must be “small”, to allow other computers to access the line– For Ethernet, the maximum size of the

payload is 1,500 bytes


Ethernet (cont’d)• What is expected performance?

– When only one computer needs to transmit: it can immediately access the line.

– When many computers want access (high traffic):

• Average waiting time is high.• There is high probability of “collision”.

– For every collision, Xmission must start agan

– Fairness?.• Conclusion: expected delay depends on the

traffic on the LAN!


Ethernet Frame Format

Preamble DA SA Type Data CRC Postamble

Type: identifies upper layer protocol (for demux’ing)Data: 0-1500 bytes (min. is 46 bytes).DA and SA: destination and source addresses.

8 6 6 2 4 1


Round-Robin MACs

• Polling.• Token passing.


Token Passing

• Consider the following game: – A group of friends sitting in a circle– A ball is passed from friend to friend

• When somebody receives the ball, it passes to the friend to his/her left

– A person is allowed to talk only when s/he has the ball in his/her hands

• This guarantees that only one person talks at a time!


Token Passing (cont’d)

• Let’s make the game more difficult:– A person that receives the ball and has something to

say, rather than saying it, s/he writes it on a letter• Including the name of the addressee

– Before passing along the ball, s/he passes along the letter

– Everyone who receives the paper passes it to the person to his/her left

• If s/he is the recipient of the letter, s/he signs it after reading it

– Once the letter arrives back to the sender, s/he throws it away

• The ball is still circulating in the circle


Token Ring

• A Token Ring MAC works similarly:– A special pattern (3-bytes word) of bits called token

moves from one computer to the next.

– If a computer does not have a message to send, it just passes the token along.

– Otherwise, it “seizes the token” and transmits its message (including the address) instead.

– The message is passed from one computer to the next, until it arrives back to the sender, which “destroys” it (does not pass it along anymore).

– The addressee may “write” something on the message so that the sender knows it has been received correctly.

– Once the computer is done transmitting the message, it “releases” (transmits) the token.


Token Ring (cont’d)

• When station wants to transmit:– Waits for token.

– Seizes it.

– Transmits frame.

• When station seizes token and begins transmission, there’s no token on the ring; so nobody else can transmit.


Token Ring (cont’d)• What is expected performance of Token

Passing?– It is fair.

• Each computer is given in turn an opportunity to transmit, even when the traffic is high.

– However, even if only one computer needs to transmit a message, it has to wait that it receives the token.

• Again, long messages should not be allowed, because otherwise one computer may “hold the token” for too long.


Token Ring Frame Format

1SD AC FC DA SA Data FCS

1 1 2 or 6 2 or 6 4

SD: starting delimiter; indicates starting of frame.AC: access control; PPPTMRRR; PPP and RRR priority and reservation; M monitor bit; T token or data frame.FC: frame control; if LLC data or control.DA and SA: destination and source addresses.FCS: frame check sequence.

SD AC FC Token frame

ED: ending delimiter; contains the error detection bit E; containsframe continuation bit I (multiple frame transmissions).FS: frame status.

1 1

ED FS


Ethernet versus Token Ring

• Token ring:– Efficient at heavy traffic.

– Guaranteed delay.

– Fair.

– But, ring/token maintenance overhead.

– But, under light traffic?

• Ethernet is simple!


Wireless LANs

• IEEE 802.11.• Distributed access control mechanism (DCF)

based on CSMA with optional centralized control (PCF).

•

Physical Layer

DCF

PCFMAClayer

Contention-freeService (polling)

ContentionService(CSMA)


MAC in Wireless LANs

• Distributed coordination function (DCF) uses CSMA-based protocol (e.g., ad hoc networks).

• CD does not make sense in wireless.– Hard for transmitter to distinguish its own

transmission from incoming weak signals and noise.

• Point coordination function (PCF) uses polling to grant stations their turn to transmit (e.g., cellular networks).

cmpe 80n spring 2003 week 4

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