data and computer communications chapter 11 local area...

Data and Computer Communications

Chapter 11 – Local Area Network

Computer Engineering Kyungpook National University

LAN Topologies Refers to the way in

which the stations attached to the network are interconnected


Bus Topology

Used with multipoint medium

All stations share capacity of bus

Heard by all stations

Only one station can transmit at a time

Need to regulate transmission to avoid collisions

Terminator absorbs frames at end of medium


Star Topology

Each station connects to central node

Usually via two point to point links

One for transmission and one for reception

Central node can act as hub or frame switch

Central Hub,

switch,

or repeater

Figure 11.2 Star Topology


Hubs Each station connected to hub

by two lines

Physically a star, logically a bus

Operate in broadcast fashion Hub acts as a repeater

Transmission from any station received by hub and retransmitted on all outgoing lines

Only one station can transmit at a time (hub) If two stations transmit at the

same time, there will be a collision

Station Station Station Station

Station

HHUB

Figure 11.10 Two-Level Star Topology

IHUBIHUB

Two cables

(twisted pair or

optical fiber)

Transmit

Receive


Hub vs Switch

Hub

Frame handling done in software

Analyzes and forwards one

frame at a time

Uses store-and-forward operation

Switch

Performs frame forwarding in

hardware

Can handle multiple frames

at a time

Can be viewed as full-duplex hub


The important ones are marked with *. The ones marked with are

hibernating. The one marked with † gave up.

IEEE 802 Standards Working Groups


IEEE 802 Standards

Physical

Data Link

Medium

Network

Transport

Session

Presentation

Application

OSI ReferenceModel

Physical

Medium AccessControl

Medium

Logical Link Control

( ) ( ) ( )

UpperLayer

ProtocolsLLC ServiceAccess Point

(LSAP)

Scopeof

IEEE 802Standards

Figure 11.3 IEEE 802 Protocol Layers Compared to OSI Model

IEEE 802Reference

Model


LAN Protocol Architecture


Encoding/decoding of signals

Preamble generation/removal

Bit transmission/reception

Physical Layer


Govern access to transmission medium

Encapsulation & Decapsulation On transmit, assemble data into frame

On reception, disassemble frame

PDU is referred to as a MAC frame

Perform address recognition Physical address (MAC address)

Detects errors and discards frames LLC optionally retransmits unsuccessful frames

Media Access Control


Reliable transmission of link level PDUs between stations

Flow and error control

Logical Link Control


Types of MAC

Round robin

• Each station given turn to transmit data

Reservation

• Divide medium into slots

• Good for stream traffic

Contention

• All stations contend for time

• Good for bursty traffic

• Simple to implement

• Tends to collapse under heavy load


Aloha

When station has frame, it immediately transmits its frame completely

Then listens for a bit over max round trip time

If receive ACK, then fine

If not, retransmit after a random backoff time

Frame may be damaged by noise or by another station transmitting at the same time (collision)

If no ACK after repeated transmissions, give up

Maximum utilization of channel about 18%

ACK

ACK ACK

ACK


DATA

Frame generation

Send immediately

ACK

DATA

Frame generation

Send immediately

No ACK

DATA

Backoff

Aloha


Example

Backoff


Slotted Aloha

Time divided into discrete intervals (slot) 1 slot → 1 frame

transmission time

Transmission begins at slot boundary The sending station

waits until the beginning of the next slot

Need central clock (or other sync mechanism)

Frames either miss or overlap totally

Increased utilization to about 37%

DATA

Frame generation

Send at slot beginning

ACK

Frame generation No ACK

Backoff

DATA DATA


CSMA (Carrier Sense Multiple Access)

First listen for channel to determine if there is another transmission in progress

If idle, transmit and wait for ACK

If no ACK in a reasonable time, then collision is assumed and retransmit

If two stations start at the same instant, collision

Utilization far exceeds ALOHA


Collision

channel

propagation

delay


Example

Backoff


Types of CSMA


Nonpersistent CSMA

1. If channel idle, transmit immediately

2. If channel busy, random backoff & retry

Random backoff reduces probability of collisions

Capacity is wasted because the medium will generally remain idle following the end of a transmission even if there are one or more stations waiting to transmit

DATA

DATA

Frame generation

Backoff

Sense channel again

If idle, send immediately

Defer


1-persistent CSMA

1. If channel idle, transmit immediately;

2. If channel busy, listen until idle; then transmit immediately

Avoids idle channel time

If two or more stations waiting, a collision is guaranteed

1-persistent stations are selfish 1-persistent seems more

unstable because of greed of the stations

DATA

DATA

Frame generation

Defer

Send immediately



P-persistent CSMA 1. If channel idle, transmit with

probability p, and delay one time slot with probability (1–p)

2. If channel busy, listen until idle and repeat step 1

3. If transmission is delayed one time slot, repeat step 1

A compromise to try and reduce collisions and idle time

DATA

Frame generation slot

DATA Delay Delay Delay

DATA Delay Delay


Value of p?

Have n stations waiting to send, at end of transmission, expected number of sending stations is np If np > 1 on average there will be a collision

To avoid catastrophe np < 1

If heavy load expected, p must be small

Smaller p means stations wait longer


Which Persistence Algorithm?

IEEE 802.3 uses 1-persistent

Both nonpersistent and p-persistent have performance problems

• Because of greed of the stations

• Wasted time due to collisions is short

• With random backoff unlikely to collide on next attempt to send

1-persistent seems more unstable than p-persistent


CSMA/CD

Carrier sense multiple access with collision detection (CSMA/CD)

Simple rules for a polite human conversation

Listen before talking

If someone else begins talking at the same time as you, stop talking : CD (collision detection)

Most widely used LAN standard

Developed by

Xerox - original Ethernet

IEEE 802.3


CSMA/CD

If the medium is

idle, transmit;

otherwise, go to step 2

If the medium is busy,

continue to listen until the channel is idle, then transmit immediately

(1-persistent )

If a collision is detected, transmit a

brief jamming signal to

assure that all stations know that there has

been a collision and

cease transmission

After transmitting the jamming signal, wait a random amount of

time, referred to

as the backoff,

then attempt to transmit

again


CSMA/CD

DATA

Frame generation

Defer Send immediately as channel goes idle

DATA

Jam

If collision, stop & jam & backoff

Backoff

Contention window

slot

Retry

DATA



CSMA/CD


Example

At time t0, station A begins transmission

At t1, both B and C are ready to transmit.

B senses a transmission and so defers.

However, C begins its own transmission

since C is still unaware of A’s transmission

When A’s transmission reaches C, at t2,

C detects the collision and ceases

transmission

The effect of the collision propagates

back to A, where it is detected by A at t3,

at which time A ceases transmission.


CSMA/CD Operation

CSMA/CD can be in one of the three states:

contention, transmission, or idle


Collision – Worst Case

tt A begins transmission

ttt p B begins transmission just before the frame sent from A arrives at B

ptt B is aware of collision immediately after it starts transmission

ptt 2 A is aware of collision after 2Tp

A B

A B

A B

A B

pt:delay nPropagatio

Collisions can occur only during 2Tp after transmission After 2Tp , the sender is guaranteed successful transmission without collision


Slot time

Slot_Time = 2Tp (round trip propagation delay) + safety margin

Time domain during which there is a possibility of collision

For 10Mbps Ethernet (d=2.5km),

2Tp = 2*(d/V)=25us → Slot_Time = 51.2us

Slot_Size = Slot_Time * Data_Rate = 512 bits (64byets)

The sender should be able to cease its transmission before completing its frame when it is aware of collision. So,

Frame Transmission Time (TX = L/R) > Slot_Time

Frame Length (L) > Slot_Size


Comparison

When channel idle

When channel busy

When collision

Nonpersistent Transmit immediately

Backoff & retry

N/A

P-persistent

Transmit with probability p or delay one slot with probability (1-p)

Listen until idle and transmit with p or delay one slot with (1-p)

N/A

1-persistent

Transmit immediately

Listen until idle, and then transmit

N/A

CSMA/CD

Stop transmission and backoff & retry


Binary Exponential Backoff

CW = 2k, k = min[n, 10] for nth retransmission

On first 10 attempts, contention window (CW) is doubled

Mean backoff time doubled

Value then remains the same for 6 further attempts

After 16 unsuccessful attempts, station gives up and reports

error

1-persistent algorithm with binary exponential backoff is

efficient over wide range of loads

But, backoff algorithm has last-in, first-out effect

TimeSlotCWUNIFORMTimeBackoff _)1,0(_


Example

Data rate = 10Mbps, Propagation delay = 12.5us

Slot_Time = 2* Propagation delay + safety margin =51.2us

Slot_Size = 51.2us*10Mbps = 512bit = 64Byte

1. Assuming that station A and B collide, they independently choose random backoff delays by

Backoff_Time = UNIFORM{0,1} * Slot_Time

2. If they collide again at the 2nd trial, they again choose random backoff delays by

Backoff_Time = UNIFORM{0,1,2,3} * Slot_Time


Example

A1 C2

C1 D1 C2 B1 B2 C1

B1

A2

A2

D1

A2

D1 D1 A2

A1 B1 C1 D1 C2 B2 A2

D1’s Waiting time = 31 Backoff time

0 10 13 30 34 39 50 70

A B C 1 2

1 2

1 t=0

30

t=13

50

t=10

34

Frame generation time

B and C collide and backoff

2 D 1 t=39

0

1

0

1

2

1 1

6

4

A, C, and D collide and backoff

A and D collide and backoff

A and D again collide and backoff


Collision Detection


Summary

LAN Topologies

Hubs and Layer 2 switches

LAN protocol architecture

IEEE 802 reference model

Logical link control

Medium access control

CSMA

CSMA/CD

data and computer communications chapter 11 local area...

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