1 chapter 6 medium access control protocols and local area networks part i: medium access control...

1

Chapter 6 Medium Access Control

Protocols and Local Area Networks

Part I: Medium Access Control

Part II: Local Area Networks

2

Chapter Overview

Switched Networks Interconnection services by

means of transmission, multiplexing, and switching/routing

Broadcast Networks All information sent to all

users No routing Shared media

Radio: Cellular telephony, Wireless LANs

Copper & Optical: Ethernet LANs, Cable Modem Access

Local Area Networks High-speed, low-cost

communications between co-located computers

Typically based on broadcast networks

Simple & cheap Limited number of users

Medium Access Control To coordinate the access to

shared medium Information can get through

the broadcast network from source to destination

3



Part I: Medium Access ControlMultiple Access Communications

Random AccessScheduling

ChannelizationDelay Performance

4

Part II: Local Area NetworksOverview of LANs

LAN Standards: Ethernet LAN* LAN Standards: Token Ring and FDDI LAN

LAN Standards: 802.11 Wireless LAN* LAN Bridges



5

Chapter 6Medium Access Control


6.1 Multiple Access Communications

6

Multiple Access Communications

Shared media basis for broadcast networks Inexpensive: radio over air; copper or coaxial cable M users communicate each other by broadcasting into

medium

Key issue: How to share and access the medium?

12

3

4

5M

Shared multipleaccess medium

7

Medium sharing techniques

Static channelization

Dynamic medium access control

Scheduling Random access

Approaches to Media Sharing

Centralized FDMA/TDMA/CDMA Dedicated allocation

of channels to users Satellite

communication systems

Cellular communication systems

Centralized/Distributed Polling: take turns Request for slot in

transmission schedule Token ring Wireless LANs

Distributed Loose coordination Send, wait, retry if

necessary Aloha Ethernet

8

Satellite Channel:

Frequency Division Multiple Access (FDMA),

Time Division Multiple Access (TDMA)

uplink fin downlink fout

Satellite Communication Systems

9

Cellular Communication Systems

uplink f1 ; downlink f2 uplink f3 ; downlink f4

Multiple Access Methods:

1G: FDMA (Frequency Division Multiple Access)

2G: TDMA (Time Division Multiple Access)

3G: CDMA (Code Division Multiple Access)

10

Inbound line

Outbound lineHost

computer

Stations

Scheduling: Polling

1 2 3 M

Poll 1

Data from 1

Poll 2

Data from 2

Data to M

11

Ring networks

Scheduling: Token-Passing

token

Station that holds token transmits data into the ring

tokenData to M

12

Multi-tapped Bus

Random Access

Crash!!

Transmit when ready

Collision may occur; need transmission and re-transmission strategy

13

AdHoc: station-to-station Infrastructure: stations to base station (Access Point) Random access and polling

Wireless LAN

14

Selecting a Medium Access Control

Applications What type of traffic? Voice streams? Steady traffic, low delay/jitter Data? Short messages? Web page downloads? Enterprise or Consumer market? Reliability, cost

Scale How much traffic can be carried? How many users can be supported?

Current Examples: Design MAC to provide wireless DSL-equivalent access to

rural communities (Fixed Wireless Access) Design MAC to provide Wireless-LAN-equivalent access to

mobile users (user in car travelling at 130 km/hr)

15

Delay-Bandwidth Product

Delay-bandwidth product: key parameter Delay-bandwidth product affects the performance of the random

access protocol Difficulty of coordination commensurate with delay-bandwidth

product

Consider a simple example: two-station case Station with frame to send listens to the channel (medium) and

transmits if the medium is found idle Station continues monitoring the medium to avoid collision There is a capture effect, meaning that the station captures the

medium channel if there is no collision and its entire message can through the medium channel

16

Two stations are trying to share a common medium

Two-Station MAC Example

A transmits

at t = 0

Distance d meterstprop = d / v seconds

A B

A B

B does not transmit before t = tprop and A captures the channel

Case 1

B transmits before t = tprop and detectscollision soonthereafter

A B

A B

A detectscollision at

t = 2 tprop

Case 2

17

Efficiency of the Two-Station MAC

Each frame transmission requires 2tprop of quiet time to detect collision Approximately 2tprop is required to coordinate the success for each frame

transmitted R transmission bit rate L bits/frame

Normalized Delay-Bandwidth Product

Propagation delay

Time to transmit a frame

18

Typical MAC Efficiencies

CSMA-CD (Ethernet) protocol:

Token-ring network

a΄= latency of the ring (bits)/average frame length = sum of bit delays at every ring adapter + the delay-bandwidth product

Two-Station Example:

If a<<1, then efficiency close to 100%

As a approaches 1, the efficiency becomes low

19

Typical Delay-Bandwidth Products

Max size Ethernet frame: 1500 bytes = 12000 bits

Distance 10 Mbps 100 Mbps 1 Gbps Network Type

1 m 3.33 x 10-02 3.33 x 10-01 3.33 x 100 Desk area network

100 m 3.33 x 1001 3.33 x 1002 3.33 x 1003 Local area network

10 km 3.33 x 1002 3.33 x 1003 3.33 x 1004 Metropolitan area network

1000 km 3.33 x 1004 3.33 x 1005 3.33 x 1006 Wide area network

100000 km 3.33 x 1006 3.33 x 1007 3.33 x 1008 Global area network

20

MAC protocol features

Delay-bandwidth product Efficiency Transfer delay Fairness Reliability Capability to carry different types of traffic Quality of service Cost

21

MAC Delay Performance Frame transfer delay: from first bit of frame arrives at source

MAC to last bit of frame delivered at destination MAC Throughput: actual transfer rate through the shared medium

measured in frames/sec or bits/sec

ParametersR bits/sec: Transmission rate and L bits/frame: Frame length

X = L/R seconds/frame: frame transmission time

λ frames/second: average arrival rate

Load ρ = λ X= λ L/R < 1, rate at which “work” arrives

22

Load

Tra

nsfe

r d

ela

y

E[T]/X

ρρ max 1

1

Normalized Delay versus Load

E[T] = average frametransfer delay

X = average frametransmission time

At low arrival rate, only frame transmission time

At high arrival rates, increasingly longer waits to access channel

Max efficiency typically less than 100%

23

Dependence on Rtprop/LT

rans

fer

De

lay

Load

E[T]/X

ρmax 1

1

aa’

a’ > a

ρ’ max

24



6.2 Random Access

25

6.2.1 ALOHA Wireless link to provide data transfer between main

campus & remote campuses of University of Hawaii Simplest solution: just do it

A station transmits whenever it has data to transmit If more than one stations have data to transmit, they interfere

with each other (collide) and data are lost If ACK not received within timeout, then a station picks random

backoff time (to avoid repeated collision) Station retransmits frame after backoff time

tt0t0-X t0+X t0+X+2tprop

t0+X+2tprop + B

Vulnerableperiod

Time-outBackoff period B

First transmission Retransmission

Fig. 6.10: ALOHA random access scheme

Retransmission if necessary

26

ALOHA Model

Definitions and assumptions X: frame transmission time (assume constant) S: throughput (average number of successful frame transmissions per

X seconds) G: load (average number of transmission attempts per X sec.) Psuccess : probability a frame transmission is successful

XX

frame transmission

Prior interval

Any transmission that begins during vulnerable period leads to collision

Assume Poisson arrival of data in each station

Success if no arrivals during 2X seconds

27

Abramson’s Assumption

What is probability of no arrivals in vulnerable period? Abramson assumption: The backoff algorithm spreads the

retransmissions so that frame transmissions, new and repeated, are equally likely to occur at any time instant

This implies that the number of frames transmitted in a time interval has a Poisson distribution with the average number of arrivals of 2G/2X

G is the total arrival rate of frames (new arrivals + retransmission arrivals) per X seconds

!0

)2(

seconds] 2Xin arrivals 0[

20

G

success

eG

PP

28

Throughput of ALOHA

Collisions are means for coordinating access

Max throughput is max=1/2e (18.4%) at G=0.5 WHY ?

Bimodal behavior:Small G, S≈G

Large G, S↓0 Collisions can

snowball and drop throughput to zero

e/2 = 0.184

29

6.2.2 Slotted ALOHA Time is slotted in X seconds slots Stations are synchronized to frame times Stations having frame arrivals will transmit frames

immediately at the first slot after the frame arrival Backoff intervals in multiples of slots, depending on the

number of collisions

t(k+1)XkX t0 +X+2tprop+ B

Vulnerableperiod

Time-out

Collision Detection

Backoff period B

t0 +X+2tprop

Only frames that arrive during the vulnerable period will make collision

t0 t0 +XRetransmissionFirst transmission

Retransmission If necessary

30

Throughput of Slotted ALOHA

GG

success

GeeG

G

GPGPS

0

!0

)(

seconds] Xin arrivals no[

Ge-G

Ge-2G

G

S0.184

0.368=1/e

The maximum throughput happens at G=1The maximum throughput of ALOHA system is not sensitive to the reaction time because stations act independently

31

Reservation ALOHA

Reservation protocol allows a large number of stations with infrequent traffic to reserve slots to transmit their frames in future cycles

Each cycle has mini-slots allocated for making reservations

Stations use slotted ALOHA during mini-slots to request slots

cycle

X-second slotReservation mini-slots

. . .. . .

32

6.2.3 Carrier Sense Multiple Access (CSMA)

A

Station A begins transmission at t = 0

A

Station A captureschannel at t = tprop

A station senses the medium channel before it starts transmission to avoid too many collisions If sensing idle, start transmission If busy, either wait or schedule backoff (different options) ACK is sent by the receiver Vulnerable period is reduced to tprop When collisions occur they involve entire frame transmission times If tprop >X (or if a>1), no gain compared to ALOHA or slotted ALOHA

X

ta prop

33

CSMA differs when the channel is sensed busy 1-persistent CSMA (most greedy)

Start transmission as soon as the channel becomes idle again Low delay and low efficiency

Non-persistent CSMA (least greedy) Wait a backoff period, then sense the carrier again High delay and high efficiency

p-persistent CSMA (adjustable greedy) Wait till channel becomes idle, transmit with probability p; or wait

one propagation delay tprop and re-sense with probability 1-p Delay and efficiency can be balanced

CSMA Options

Sensing

34

0.53

0.45

0.16

S

G

a 0.01

a =0.1

a = 1

1-Persistent CSMA Throughput

Better than Aloha & slotted Aloha for small a

Worse than Aloha for a > 1

35

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.…

0.…

0.…

0.…

0.25 0.5 1 2 4 8 16 32 64

0.81

0.51

0.14

S

G

a = 0.01

Non-Persistent CSMA Throughput

a = 0.1

a = 1

Higher maximum throughput than 1-persistent for small a

Worse than Aloha for a > 1

36

6.2.4 CSMA with Collision Detection (CSMA/CD)

Monitor for collisions & abort transmission Stations with frames to send, first do carrier sensing After beginning transmissions, stations continue listening to the

medium to detect collisions If a collision is detected during transmission, a short jamming

signal is transmitted to ensure all other stations to know that collision has occurred before aborting the transmission, and then adopts a backoff algorithm to reschedule the future transmission

In CSMA, collisions result in wastage of X seconds spent transmitting an entire frame. But CSMA-CD reduces wastage to a time (2 propagation time) to detect collision and abort transmission

37

CSMA/CD reaction time

Station A takes about 2 tprop to find out if it has been successfully captured the channel.

Question: minimum frame size ?

A begins to transmit at

t = 0

A B

B begins to transmit at t = tprop- δ;B detectscollision at t = tprop

A B

A B

A detectscollision at t= 2 tprop- δ

38

CSMA-CD Model

Assumptions Collisions can be detected and resolved in 2tprop

Each contention interval takes 2tprop seconds, which is regarded as one minislot

Assume n stations are contending for the channel, and each may transmit with probability p in each contention time slot

Once the contention period is over (a station successfully occupies the channel), it takes X seconds for a frame to be transmitted

It takes tprop before the next contention period starts.

Busy Contention Busy(a)

Time

Idle Contention Busy

39

Contention Resolution

How long does it take to resolve contention? Contention is resolved (“success’) if exactly 1 station transmits in a

minislot:

By taking derivative of Psuccess we find max occurs at p=1/n

The average number of minislots that elapse until a station successfully captures the channel is 1/Pmax = e = 2.718 time slots

40

CSMA/CD Throughput

At maximum throughput, systems alternates between contention periods and frame transmission times

Time

Busy Contention Busy Contention Busy Contention Busy

where:R bits/sec, L bits/frame, X=L/R seconds/frame

a = tprop/X

meters/sec. speed of light in medium

d meters is diameter of system

2e+1 = 6.44

41

CSMA-CD Application: Ethernet

First Ethernet LAN standard used CSMA-CD 1-persistent Carrier Sensing R = 10 Mbps tprop = 51.2 microseconds

512 bits = 64 byte slot accommodates 2.5 km + 4 repeaters

Truncated Binary Exponential Backoff After nth collision, select backoff from {0, 1,…, 2k – 1},

where k=min(n, 10)

42

Maximum Achievable Throughputs for Random Access Schemes

ALOHA

Slotted ALOHA

1-P CSMA

Non-P CSMA

CSMA/CD

a

ρmax

For small a: CSMA-CD has best throughput For larger a: Aloha & slotted Aloha better throughput

43

Carrier Sensing and Priority Transmission Certain applications require faster response than others,

e.g. real-time services and ACK messages Impose different interframe times

High priority traffic sense channel for time Low priority traffic sense channel for time High priority traffic, if present, seizes channel first

This priority mechanism is used in IEEE 802.11 wireless LAN

Problems for Sections 6.1 and 6.2

6.1 (10%) 6.6 (10%) 6.8 (10%) 6.10 (20%) 6.13 (20%)

44

45

6.3 Scheduling Approaches to Medium Access Control



46

Scheduling for Medium Access Control

Schedule frame transmissions (assignment) to avoid collision in shared medium More efficient channel utilization Less variability in delay Fairness provisioning to stations Increased computational or procedural complexity

Two main approaches Reservation Polling

47

6.3.1 Reservation Systems

Time

Cycle n

Reservationinterval

Frame transmissions

r d d d r d d d

Cycle (n + 1)

r = 1 2 3 M

Transmissions organized into cycles Cycle: reservation interval + frame transmissions Reservation interval has a minislot for each station to request

reservations for frame transmissions

Fig. 6.19 Basic reservation system: each station has own minislot for making reservation

48

Reservation System Options Centralized or distributed system

Centralized systems: A central controller listens to reservation information, decides order of transmission, issues grants

Distributed systems: Each station determines its slot for transmission from the reservation information

Single or Multiple Frames Single frame reservation: Only one frame transmission can be

reserved within a reservation cycle Multiple frame reservation: More than one frame transmission

can be reserved within a frame Channelized or Random Access Reservations

Channelized (typically TDMA) reservation: Reservation messages from different stations are multiplexed without any risk of collision

Random access reservation: Each station transmits its reservation message randomly until the message goes through

49

tr 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3

(a)

tr 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3

8(b)

Example Initially stations 3 & 5 have reservations to transmit frames

Station 8 becomes active and makes reservation Cycle now also includes frame transmissions from station 8

50

Efficiency of Reservation Systems Assume minislot duration = vX TDM single frame reservation scheme

If propagation delay is negligible, a single frame transmission requires (1+v)X seconds

Link is fully loaded when all M stations transmit, maximum efficiency is:

TDM k frame reservation scheme If k consecutive frame transmissions can be reserved with a

reservation message and if there are M stations, as many as Mk frames can be transmitted in XM(k+v) seconds

Maximum efficiency is:

vv

1

1max XMMX

MX

kXMMkX

MkXvv

1

1max

51

Random Access Reservation Systems

Large number of light traffic stations Dedicating a minislot to each station is inefficient

Slotted ALOHA reservation scheme Stations use slotted Aloha on reservation minislots On average, each reservation takes at least “e” minislot attempts Effective time required for the reservation is 2.71vX The maximum achievable throughput ρmax is given by

If v = 5%, then we have ρmax = 0.88 1 1 + 2.71v

ρmax = =X(1+ev)

X

52

Example: GPRS

General Packet Radio Service (GPRS) in 2G GSM cellular telephone system Packet data service in GSM cellular telephone network GPRS devices, e.g. cell phones or laptops, send packet data over

radio and then to Internet Slotted Aloha MAC protocol is used for making reservations Single and multi-slot reservations can be supported

53

6.3.2 Polling Systems Centralized polling systems: A central controller transmits

polling messages to stations according to a certain order Distributed polling systems: A permit for frame

transmission is passed from station to station according to a certain order

A signaling procedure exists for setting up order

CentralController

Centralized Polling System

The central controller might poll the stations in a round-robin fashion, or according to some pre-determined order

The normal response mode of HDLC, discussed in Chapter 5, was developed for this type of system

54

Inbound line

Outbound lineHost

computer

Stations

Fig. 6.21(a) Polling by central controller over lines

55

6.3.2 Polling Systems Centralized polling systems over radio: A central controller accepts

requests from stations and issues grants to transmit Frequency Division Duplex (FDD): One frequency band for downlink (outbound)

and the other frequency band for uplink (inbound) Time-Division Duplex (TDD): Inbound and outbound transmissions share the

same frequency band but in different time slots

Distributed polling systems: Stations implement a decentralized algorithm to determine transmission order

All stations can receive the transmissions (broadcasting) After a station is done transmitting, it is responsible for sending a polling

message to the next station according to the polling list

CentralController

Fig. 6.21(b) Polling by central controller over radio

Fig. 6.21(c) Polling without central controller

57

Average Cycle Time Assume walk times all equal to t’

Exhaustive Service Duscipline: stations empty their buffers Cycle time = Mt’ + time to empty M station buffers Assume λ/M to be the frame arrival rate at a station NC average number of frames transmitted from a station Time to empty one station buffer, Tstation:

t1 32 4 5 1… M

t’ t’ t’ t’ t’ t’

Tc

,)(

))(()(M

TEXTE

MXNET c

ccstation

Average Cycle Time:

1

)E( )()()(tM

TTEtMTtMTE ccstationc

Xwhere

58

Polling System Options

Service Limits: How many blocks of information is a station allowed to transmit per poll? Exhaustive: until station’s data buffer is empty (including new

frame arrivals) Gated: all data in buffer when poll arrives Frame-Limited: one frame per poll Time-Limited: up to some maximum time

Priority mechanisms More bandwidth & lower delay for stations that appear multiple

times in the polling list Issue polls for stations with message of priority k or higher

59

Efficiency of Polling Systems Exhaustive Service: high throughput and large delay

Cycle time increases as traffic increases, so delays become very large Efficiency:

If walk time per cycle becomes negligible compared to cycle time the system throughput or say system efficiency can approach 100%

c

c

T

tMTEfficiency

Limited Service: relatively, low throughput, small delay The applications which cannot tolerate extremely long delays can adopt

limited service discipline The number of frames transmitted per station is limited This limits the cycle time and hence delay Efficiency of 100% is not possible; The efficiency of a single frame per

poll limited service polling system

60

6.3.3 Token-Passing Rings

Free Token = Poll

Listen mode

DelayInputfromring

Outputto

ring

Ready station looks for free token Flips bit to change free token to busyThe delay is due to processing of the input information

Transmit mode

Delay

To device From device Ready station inserts its frames into the ring

Reinserts free token when done

token

Frame Delimiter is TokenFree = 01111110Busy = 01111111

61

Three Approaches to Token Reinsertion

Ring latency: time required for bit to circulate ring (including interface delay)

Multi-token operation Free token transmitted immediately after the last bit

of data frame This allows several frames to be transmitted in

different parts of the ring Single-token operation

Free token inserted after the last bit of the busy token is received back and the last bit of the frame is transmitted

If frame is longer than ring latency, equivalent to multi-token operation

Simple error recovery by allowing only one token in the ring

Single-Frame operation Free token inserted after the transmitting station has

received the last bit of its frame This allows the transmitting station to check the

return frame for errors before the relinquishing the control of the token

Busy token

Free token

Frame

Idle Fill

Multi-token operation

Single-token operation

Single-frame operation

62

Token Ring Throughput Definition

: ring latency (time required for bit to circulate the ring)

X: maximum frame transmission time allowed per station Multi-token operation

Assume network is fully loaded, and all M stations transmit for X seconds upon the reception of a free token

This is a polling system with limited service time:

,R

Mb

: The total propagation delay around the ringb: the number of bit delays in an interface

63

Single-token operation Effective frame transmission time is maximum of X and ,

therefore

Token Ring Throughput

ρmax = = MX

M(X+ ) +

1 1+a΄(1 + 1/M)

ρmax = =

MX

M max{(X, }+

1

max{1, a΄} + a΄/M

Single-frame operation Effective frame transmission time is X+ ,therefore

64

Token Reinsertion Efficiency Comparison

Max

imum

th

roug

hput

a’

Multiple tokenoperation

0

0.2

0.4

0.6

0.8

1

1.2

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8

Single frameoperation

M = 50

M = 10

Single tokenoperation

M = 50M = 10

a’ <<1, any token reinsertion strategy acceptable a’ ≈1, single token reinsertion strategy acceptable a’ >1, multi-token reinsertion strategy necessary

65

Application Examples

Single-frame reinsertion IEEE 802.5 Token Ring LAN @ 4 Mbps

Single token reinsertion IBM Token Ring @ 4 Mbps

Multi-token reinsertion IEEE 802.5 and IBM Ring LANs @ 16 Mbps FDDI Ring @ 50 Mbps

All of these LANs incorporate token priority mechanisms

66

6.3.4 Comparison of Scheduling Approaches in Medium Access Control

Reservation On-demand transmission of bursty or steady streams Accommodates large number of low-traffic users with slotted

Aloha reservations Can support QoS provisioning Handles large delay-bandwidth product via delayed grants

Polling Generalization of time-division multiplexing Provides fairness through regular access opportunities Can provide bounds on access delay Performance deteriorates with large delay-bandwidth product

67

6.3.5 Comparison of Random Access and Scheduling Medium Access Controls

Aloha & Slotted Aloha Simple & quick transfer at very low load Accommodates large number of low-traffic bursty users Highly variable delay at moderate loads Efficiency does not depend on a

CSMA-CD Quick transfer and high efficiency for low delay-bandwidth

product Can accommodate large number of bursty users Variable and unpredictable delay

68



6.4 Channelization

69

Why Channelization?

Channelization Semi-static bandwidth allocation of portion of

shared medium to a given user during his data transfer period

Highly efficient for constant-bit rate traffic Preferred approach in

Cellular telephone networks Terrestrial & satellite broadcast radio & TV

70

Why not Channelization? Inflexible in allocation of bandwidth to users with different

requirements Inefficient for bursty traffic Does not scale well to large numbers of users

Average transfer delay increases with number of users M Dynamic MAC algorithms achieve better sharing and much better at

handling bursty traffic

Average delay for TDMA If we divide the transmission medium into M channels that can be allocated to the M users

71

Channelization Approaches

Frequency Division Multiple Access (FDMA) Frequency band allocated to users Broadcast radio & TV, analog cellular phone

Time Division Multiple Access (TDMA) Periodic time slots allocated to users Telephone backbone, 2G GSM digital cellular phone

Code Division Multiple Access (CDMA) Code allocated to users Cellular phones, 3G cellular

72

6.4.1 FDMA Divide the spectrum W into M frequency bands Each station transmits its information continuously on the assigned

bands during the data transfer phase

Each station transmits at most R/M bps Good for stream traffic; Used in connection-oriented systems Inefficient for bursty traffic

Frequency

Guard bands

Time

W

1

2

M

M–1

…

73

6.4.2 TDMA

Dedicate 1 slot per station in transmission cycles Stations transmit data burst at full channel bandwidth

Each station transmits at R bps 1/M of the time Excellent for stream traffic; Used in connection-oriented systems Inefficient for bursty traffic due to unused dedicated slots

1

Time

Guard time

One cycle

12 3 MW

Frequency

...

74

Guardbands

FDMA Frequency bands must be non-overlapping to prevent

interference Guardbands ensure separation; but a kind of overhead

TDMA Stations must be synchronized to common clock Time gaps between transmission bursts from different

stations to prevent collisions; form of overhead Must take into account propagation delays

75

6.4.3 CDMA Code Division Multiple Access

Channels determined by a code used in modulation and demodulation Stations transmit over entire frequency band all of the time!

Time

W

Frequency1

2

3

76

xBinaryinformation

R1 bpsW1 Hz

Unique user binary random

sequence

DigitalModulation

With carrier fc

Radio Antenna

Transmitter from one user

R >> R1bpsW >> W1 Hz

x

CDMA Spread Spectrum Signal

User information mapped into: +1 or -1 for T sec. Multiply user information by pseudorandom binary pattern of G “chips” of +1’s

and -1’s Resulting spread spectrum signal occupies G times more bandwidth: W = GW1

Modulate the spread signal by sinusoid at appropriate carrier fc

77

Signal and residualinterference

Correlate touser binary

random sequence

Signalsfrom all

transmittersDigital

demodulation

Binaryinformation

x x

CDMA Demodulation

Recover the spread spectrum signal Synchronize to and multiply spread signal by same pseudo-random

binary pattern used at the transmitter In absence of other transmitters and noise, we should recover the

original +1 or -1 of user information Other transmitters using different codes appear as residual noise

78

Channelization in Code Space

Each channel uses a different pseudorandom code Codes should have low cross-correlation

If they differ in approximately half the bits, the correlation between codes is close to zero and the effect at the output of each other’s receiver is small

As number of users increases, effect of other users on a given receiver increases as additive noise

CDMA has gradual increase in BER due to noise as number of users is increased

Interference between channels can be eliminated if codes are selected orthogonal and if receivers and transmitters are synchronized Shown in next example

79

Example: CDMA with 3 users Assume three users share same medium Users are synchronized & use different 4-bit orthogonal codes:

{-1,-1,-1,-1}, {-1, +1,-1,+1}, {-1,-1,+1,+1}, {-1,+1,+1,-1},

-1 +1 -1

User 1 x

+1 +1 -1

User 2 x

User 3 x

-1 -1 +1 SharedMedium

+

Receiver

80

Channel 1: 110 -> +1+1-1 -> (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1)Channel 2: 010 -> -1+1-1 -> (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1)Channel 3: 001 -> -1-1+1 -> (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1)Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)

Channel 1

Channel 2

Channel 3

Sum Signal

Sum signal is input to receiver

81

Example: Receiver for Station 2 Each receiver takes sum signal and multiplies by the

code sequence of desired transmitter Integrate over T seconds to smooth out noise

x

SharedMedium

+

Decoding signal from station 2

Integrate over T sec

a=(a1,a2,…an), b=(b1,b2,…bn)a ＊ b = 0 (if orthogonal)a ＊ a=b ＊ b = n r= -a+b (station A send 0, station B send 1)a ＊ r = -n+0 = -n (station A receive 0)

82

Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1) Channel 2 Sequence:(-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1)Correlator Output: (-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1)Integrated Output: -4, +4, -4Binary Output: 0, 1, 0

Sum Signal

Channel 2Sequence

CorrelatorOutput

IntegratorOutput

-4

+4

-4

Decoding at Receiver 2

X

=

6.5 Delay Performance of MAC and Channelization Schemes

We have already discussed the impact of delay-bandwidth product on the maximum throughput that can be achieved by an MAC protocol and by a channelization scheme

This section present performance models for assessing the frame (packet) transfer delay performance

But it is in the scope of queueing theory, which we shall not cover it because of time limitation

83

84



Part II: Local Area NetworksOverview of LANs

Ethernet*Token Ring and FDDI

802.11 Wireless LAN*LAN Bridges

85



6.6 LAN Protocols

86

Local Area Networks (LANs)

Local area means: Private ownership

freedom from regulatory constraints of WANs Short distance (~1km) between computers

low cost very high-speed, reliable, relatively error-free communication Unnecessary complex error control

Need a medium access control protocol Simply give each machine a unique address Broadcast all messages to all machines in the LAN

Most LAN standard has been developed by the IEEE 802 committee, which has been accredited by ANSI The set of standards include CSMA-CD (Ethernet) LAN and

token-passing ring LAN and place LAN within the data link layer of the OSI reference model

87

6.6.1 LAN Structure

RAM

RAMROM

Ethernet Processor

LAN structure can be arranged in a bus, ring, or star topology

Transmission Medium (Cabling System): twisted-pair cable, coaxial cable, or optical fiber, or even wireless

Network Interface Card (NIC): coordinate the transfer of information between the computer and the network, implement the MAC protocol

Unique MAC or “physical” address that is burned into the ROM

88

6.6.2 Medium Access Control Sublayer

In IEEE 802.1, Data Link Layer divided into:

1. Medium Access Control (MAC) Sublayer Execute the MAC protocol to access to the shared physical medium Provide for the connectionless frame transfer of datagrams Usually do not include procedures for error control Accept the block of information from the LLC sublayer or directly from

network layer and construct a PDU that includes source and destination MA addresses as well as a frame check sequence (FCS)

Machines identified by MAC/physical address Broadcast frames with MAC addresses

2. Logical Link Control (LLC) Sublayer Between Network layer & MAC sublayer

89

IEEE 802 LAN Standards

Data linklayer

802.3Ethernet

802.5Token Ring

802.2 Logical link control

Physicallayer

MAC

LLC

802.11Wireless

LAN

Network layer Network layer

Physicallayer

OSIIEEE 802

Various physical layers

OtherLANs

90

6.6.3 The Logical Link Control Layer

PHY

MAC

PHY

MAC

PHY

MAC

Unreliable Datagram Service

PHY

MAC

PHY

MAC

PHY

MAC

Reliable frame service

LLCLLC LLC

A C

A C

IEEE 802.2: LLC enhances service provided by MAC

91

Logical Link Control Services

The LLC builds on the MAC datagram service to provide three HDLC services Type 1: Unacknowledged connectionless service: the most

common one Unnumbered frame mode of HDLC

Type 2: Reliable connection-oriented service Asynchronous balanced mode of HDLC

Type 3: Acknowledged connectionless service

Additional addressing (Source + Destination) A workstation has a single MAC physical address Can simultaneously handle several logical connections for

data exchange in the workstation, distinguished by their SAP (service access points), over the same physical connection.

Source SAP Address Information

1

Control

1 or 2 bytes

Destination SAP Address Source SAP Address

I/G

7 bits1

C/R

7 bits1

I/G = Individual or group addressC/R = Command or response frame

DestinationSAP Address

1 byte

Figure 6.49LLC PDU Structure

LLC PDU Structure

93

IEEE MAC frames

IP

LLC Header

Data

MAC Header

FCS

LLC PDU

IP Packet

94



6.7 Ethernet and IEEE 802.3 LAN Standard

95

A bit of history…

1970 ALOHA radio network deployed in Hawaiian islands 1973 Robert Metcalf and his colleagues invented Ethernet LAN

protocol for connecting workstations 1979 DIX (DEC, Intel, and Xerox) Ethernet Standard for 10 Mbps LAN

based on coaxial cable transmission 1985 IEEE 802.3 LAN Standard (10 Mbps) 1995 Fast Ethernet (100 Mbps) 1998 Gigabit Ethernet 2002 10 Gigabit Ethernet Ethernet is the dominant LAN standard

Metcalf’s Sketch

Problem 6.15 Problem 6.26 Problem 6.31 Problem 6.36 Problem 6.38

96

97

6.7.1 IEEE 802.3 Ethernet Protocol

CSMA/CD for MAC protocol (1-persistent) Reschedule retransmission attempts after collision Truncated binary exponential backoff algorithm

for retransmission n, backoff r mini-slots, where 0 < r < 2k, and k = min (n,10)

Maximum retransmission times: 16 Mini-slot Time: the critical system parameter (see p. 428)

The basis for contention resolution that is required for a station to detect collision, or say to seize control of the channel

The time duration that is at least as big as two propagation delays

98

The Original IEEE 802.3

Transmission Rate: 10 Mbps Maximum Distance: 2500 meters + 4 repeaters Minislot time: 51.2 sec = 512 bits/10 Mbps

51.2 sec x (~3) x105 km/sec =15.0 km, 2 ways for 3 spans

Min Frame Length: 512 bits = 64 bytes

Each x10 increase in bit rate, must be accompanied by x10 decrease in distance

99

6.7.2 IEEE 802.3 MAC Frame Structure

Preamble SDDestination

addressSource address

Length Information Pad FCS

7-octet 1 6 6-octet 2 4

64 - 1518 bytesSynch Startframe

0 Single address

1 Group address

• Preamble for clock synchronization• Start frame delimiter for frame synchronization • The First bit of destination address

• single address• group address• broadcast = 111...111

• The second bit of destination addresses• local or global

• Global addresses• first 24 bits assigned to vendor, called OUI• next 24 bits assigned by vendor• Cisco 00-00-0C• 3Com 02-60-8C

• Pad to ensure the frame length is always at least 64 bytes• FCS is the CCITT 32-bit CRC check covering address, length, information, and pad

0 Local address

1 Global address

802.3 MAC Frame

100

6.7.3 IEEE 802.3 Physical Layer

Fig. 6.55(a) transceiversFig. 6.55(b)

10base5 10base2 10baseT 10baseF

MediumThick coax

10 mm

Thin coax

5 mmTwisted pair Optical fiber

Max. Segment Length 500 m200 m

185 m100 m 2 km

Topology Bus Bus StarPoint-to-point link

Table 6.2 IEEE 802.3 10 Mbps medium alternatives

Thick Coax: Stiff, hard to work withBaseband transmission: Manchester code T-shaped BNC junctions

Attach to NIC

Hubs & Switches!

101

Star Topology of 10 Base-T

Fig. 6.56(a)

The 1st approach using twisted-pair cabling

Single collision domain

Ethernet Hub• Cheap• Easy to work with• Reliable • Star-topology • CSMA-CD

The hub monitors all transmissions from the stations When there is only one transmission, the hub repeats the transmission on the other lines If there is a collision, the hub sends a jamming signal to all the stations The stations are said to be in the same collision domain, a domain where a collision will occur if two or more stations transmit simultaneously The CSMA-CD MAC procedure is applied Notice that the “a“ will become smaller

Ethernet Hub

102

High-Speed backplane or interconnection fabric

Ethernet Switch• Cheap• Bridging increases scalability• Separate collision domains• Full duplex operation

Ethernet Switch

It is a transparent bridging In the switch, the input port buffers incoming transmission frames The incoming frames are examined and transferred to appropriate outgoing port Collision will not occur if only one station is attached to the line; otherwise there is a collision domain if several stations share an input using another hubThe switching LANs provide a means of interconnecting a large numbers of stations without reaching the limit

Fig. 6.56(b)

The 2nd approach using twisted-pair cabling

103

6.7.4 Fast Ethernet

100baseT4 100baseTX 100baseFX

MediumTwisted pair category 3

UTP 4 pairs

Twisted pair category 5

UTP two pairs

Optical fiber multimode

Two strands

Max. Segment Length

100 m 100 m 2 km

Topology Star Star Star

Table 6.4 IEEE 802.3 100 Mbps Ethernet medium alternatives

Same frame format, same interfaces, same protocols to preserve compatibility with 10 Mbps Ethernet

But just to define a set of physical layers that are entirely based on hub (star) topology only involving twisted pair and optical fiber

Category 3 twisted pair (ordinary telephone grade) requires 4 pairs, and the transmission is divided among three of the twisted pairs

Category 5 twisted pair requires 2 pairs (most popular), one for transmission and one for reception

Most prevalent LAN today Two modes of operation: hub and switch

104

6.7.5 Gigabit EthernetTable 6.3 IEEE 802.3 1 Gbps Fast Ethernet medium alternatives

1000baseSX 1000baseLX 1000baseCX 1000baseT

Medium

Optical fiber

multimode

Two strands

Optical fiber

single mode

Two strands

Shielded copper cable

Twisted pair category 5

UTP

Max. Segment Length

550 m 5 km 25 m 100 m

Topology Star Star Star Star

Slot time increased to 512 bytes Small frames need to be extended to 512 B Frame bursting to allow stations to transmit a burst of small frames Frame structure is preserved but operates primarily for the switch mode and

the CSMA-CD is essentially abandoned Extensive deployment in backbone of enterprise data networks and in server

farms

105

6.7.6 10 Gigabit Ethernet

Table 6.5 IEEE 802.3 10 Gbps Ethernet medium alternatives

10GbaseSR 10GBaseLR 10GbaseEW 10GbaseLX4

Medium

Two optical fibersMultimode at 850 nm

64B66B code

Two optical fibersSingle-mode at 1310 nm

64B66B

Two optical fibers Single-mode at 1550 nm

SONET compatibility

Two optical fibers multimode/single-mode with four wavelengths at 1310 nm band8B10B code

Max. Segment Length

300 m 10 km 40 km 300 m – 10 km

Frame structure preserved CSMA-CD protocol officially abandoned LAN PHY for local network applications WAN PHY for wide area interconnection using SONET OC-192c Extensive deployment in metro networks anticipated

106

Server

100 Mbps links

10 Mbps links

ServerServer

Server

100 Mbps links

10 Mbps links

Server

100 Mbps links

10 Mbps links

Server

Gigabit Ethernet links

Gigabit Ethernet links

Server farm

Department A Department B Department C

Hub Hub Hub

Ethernet switch

Ethernet switch

Ethernet switch

Switch/router Switch/router

Typical Ethernet Deployment

107



6.10 Wireless LAN and IEEE 802.11 Standard

108

Wireless Data Communications

Wireless communications compelling Easy, low-cost deployment User mobility & roaming: Access information anywhere Supports personal devices

PDAs, laptops, data-cell-phones Supports communicating devices

Cameras, location devices, wireless identification Signal strength varies in space & time because of fading effect Signal can be captured by snoopers, not secure Spectrum is limited & usually regulated

Wireless LANs Ad hoc LAN Infrastructure LAN with wireless access point (AP)

109

B D

CA

6.10.1 Ad Hoc and Infrastructure Networks

Temporary association of a group of stations Within range of each other Need to exchange information E.g. Presentation in meeting, or distributed computer

game, or both

Fig. 6.65 Ad Hoc network 隨意網路

110

A2 B2

B1A1

AP1

AP2

Distribution SystemServer

Gateway tothe InternetPortal

Portal

BSS A BSS B

Infrastructure Network

Permanent Access Points provide access to Internet

111

A transmits data frame to BBut signal cannot reach C

(a)

Data Frame Data Frame

A

B C

C transmits data frame & collides with A at B

(b)

C senses medium, station A is hidden from C

Data Frame

B

CA

Hidden Terminal Problem

New MAC: CSMA with Collision Avoidance (CSMA-CA)

112

RTS

A requests to send

B

C

(a)

CTS CTS

A

B

C

B announces A ok to send

(b)

Data Frame

A sends

B

C remains quiet

(c)

CSMA with Collision Avoidance

113

IEEE 802.11 Wireless LAN

Stimulated by availability of unlicensed spectrum U.S. Industrial, Scientific, Medical (ISM) bands 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz

Targeted wireless LANs @ 20 Mbps MAC for high speed wireless LAN Ad Hoc & Infrastructure networks Variety of physical layers

114

802.11 Architecture

Basic Service Set (BSS) Group of stations that coordinate their access using a given

instance of MAC Located in a Basic Service Area (BSA) Stations in BSS can communicate with each other Uncorrelated BSSs may be colocated

Extended Service Set (ESS) Multiple BSSs interconnected by Distribution System (DS) Each BSS is like a cell and stations in BSS communicate with an

Access Point (AP) Portals attached to DS provide access to Internet

115

A2 B2

B1A1

AP1

AP2

Distribution SystemServer

Gateway tothe InternetPortal

Portal

BSS A BSS B

Infrastructure Network

116

Distribution Services

Stations within BSS can communicate directly with each other

DS provides distribution services: Transfer MAC SDUs between APs in ESS Transfer MSDUs between portals & BSSs in ESS Transfer MSDUs between stations in same BSS

Multicast, broadcast, or stations’s preference

ESS looks like single BSS to LLC layer

117

Infrastructure Services

Select AP and establish association with AP Then can send/receive frames via AP & DS

Reassociation service to move from one AP to another AP

Dissociation service to terminate association Authentication service to establish identity of other

stations Privacy service to keep contents secret

6.10.2 Frame Structure and Addressing

Management frames Station association & disassociation with AP Timing & synchronization Authentication & deauthentication

Control frames Handshaking ACKs during data transfer

Data frames Data transfer

Address2

FrameControl

Duration/ID

Address1

Address3

Sequencecontrol

Address4

Framebody

CRC

2 2 6 6 6 2 6 0-2312 4MAC header (bytes)

Frame Structure

MAC Header: 30 bytes Frame Body: 0-2312 bytes CRC: CCITT-32 4 bytes CRC over MAC

header & frame body

Address2

FrameControl

Duration/ID

Address1

Address3

Sequencecontrol

Address4

Framebody

CRC

Protocolversion

Type SubtypeToDS

FromDS

Morefrag

RetryPwrmgt

Moredata

WEP Rsvd

2 2 6 6 6 2 6 0-2312 4

2 2

MAC header (bytes)

4 1 1 1 1 1 1 1 1

Frame Control (1)

Protocol version = 0 Type: Management (00), Control (01), Data (10) Subtype within frame type Type=00, subtype=association; Type=01, subtype=ACK MoreFrag=1 if another fragment of MSDU to follow

ToDS

FromDS

Address1

Address2

Address3

Address4

0 0Destinationaddress

Sourceaddress

BSSID N/A

0 1Destinationaddress

BSSIDSourceaddress

N/A

1 0 BSSIDSourceaddress

Destinationaddress

N/A

1 1Receiveraddress

Transmitteraddress

Destinationaddress

Sourceaddress

Meaning

Data frame from station to station within a BSS

Data frame exiting the DS

Data frame destined for the DS

WDS frame being distributed from AP to AP

Address2

FrameControl

Duration/ID

Address1

Address3

Sequencecontrol

Address4

Framebody

CRC

Protocolversion

Type SubtypeToDS

FromDS

Morefrag

RetryPwrmgt

Moredata

WEP Rsvd

2 2 6 6 6 2 6 0-2312 4

2 2 4 1 1 1 1 1 1 1 1

To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS

Frame Control (2)

Address2

FrameControl

Duration/ID

Address1

Address3

Sequencecontrol

Address4

Framebody

CRC

Protocolversion

Type SubtypeToDS

FromDS

Morefrag

RetryPwrmgt

Moredata

WEP Rsvd

2 2 6 6 6 2 6 0-2312 4

2 2

MAC header (bytes)

4 1 1 1 1 1 1 1 1

Frame Control (3)

Retry=1 if mgmt/control frame is a retransmission Power Management used to put station in/out of sleep mode More Data =1 to tell station in power-save mode more data buffered

for it at AP WEP=1 if frame body encrypted

123q

6.10.3 IEEE 802.11 MAC

MAC sublayer responsibilities Channel access PDU addressing, formatting, error checking Fragmentation & reassembly of MAC SDUs

MAC security service options Authentication and privacy mechanisms

MAC management services Roaming within ESS

Power management

124

MAC Services The 802.11 MAC protocol defines the coordination functions that determines when a

station is allowed to transmit and receive. Distributed Coordination Function (DCF): the basic access method, supports for

asynchronous data transfer of MSDUs on a best-effort basis and in a contention mode exclusively

Point Coordination Function (PCF): optional, supports connection-oriented, time-bounded transfer of MSDUs. Under this function, the medium can alternate between contention period (CP), where the stations use the contention mode, and contention-free period (CFP), where the station access is controlled by AP, thereby more efficient.

Physical

Distribution coordination function(CSMA-CA)

Point coordinationfunction

Contention-free service Contention service

MAC

Distributed Coordination Function (DCF)

DCF is the basic access method Asynchronous best-effort data transfer All stations contend for access to medium Based on CSMA-CA protocol. Since collision detection is not possible here, a

random delay time for station access after sensing the channel busy is designed This approach reduces the likelihood of simultaneous attempts

Ready stations wait for completion of transmission (channel busy) Moreover, All stations are obliged to remain quite for a certain minimum period

after a transmission has been completed. Called the Interframe Space (IFS) The length of IFS is endowed with the priority of frame transmission. There are

SIFS, PIFS, and CIFS (see page 455)

DIFS

DIFS

PIFS

SIFS

Contentionwindow

Next frame

Defer access Wait for reattempt time

Time

Busy medium

Priorities through Interframe Spacing

High-Priority frames wait Short IFS (SIFS) Typically to complete exchange in progress ACKs, CTS, data frames of segmented MSDU, etc.

PCF IFS (PIFS) to initiate Contention-Free Periods DCF IFS (DIFS) to transmit data & MPDUs

DIFS

DIFS

PIFS

SIFS

Contention Window (CW):The idle period after DIFS period

Next frame

Defer access Backoff time; Wait for reattempt

Time

Busy medium

Fig. 6.69 Basic CSMA-CA operation

Contention Window

Basic CSMA-CA Operation (Fig. 6.69)

A ready station is allowed to transmit an initial MPDU under the DCF method if channel is still idle after a DIFS period or greater

If the station detects the medium busy, then the station must calculate a random backoff time to schedule a reattempt Backoff period is an integer number of idle contention time slots

The station monitors the medium and decrements backoff timer each time an idle contention slot transpires The station is allowed to transmit when its backoff timer expires

during the contention period If another station transmits during the contention period before the given

station, then the backoff procedure is suspended and resumed the next time the contention period takes places

A station that successfully completes a frame transmission is not allowed to transmit immediately. It must first perform a backoff procedure

Fairness of the CSMA-CA protocol

RTS20 bytes

CTS14 bytes CTS

Data Frame2300 bytes

A requests to send

BC

A

A sends

B

B

C

C knows that A has been given permission to send and remains quiet

B announces A ok to send

(a)

(b)

(c)

ACK

B(d)

ACK

B responses with ACKIf no error

C

C

A

A

A

Fig. 6.70 CSMA-CA Protocol

Carrier Sensing in 802.11 Carrier sensing at both air interface and MAC sublayer Physical Carrier Sensing at air interface

Detects the presence of other stations by analyzing all detected frames Detects activity in the channel via relative signal strength from other

sources

Virtual Carrier Sensing at MAC sublayer Source station informs other stations in the BSS of how long (in sec)

will be utilized for the successful transmission of an MPDU The source station sets the Duration field in the MAC header of data

frame or in RTS & CTS control frames Stations detecting the duration field in a transmitted MSDU adjust their

network allocation vector (NAV), which indicates the amount of time that must elapse until the current transmission is complete and the channel can be samples again for idle status

Channel is marked busy if either the physical or virtual carrier sensing mechanism indicates that the channel is busy

Data FrameDIFS

SIFS

Defer AccessWait for Reattempt Time

ACK

DIFS

NAV

Source

Destination

Other

Transmission of MPDU without RTS/CTS

NAV=Data Frame+SIFS+ACK

The duration field in the data frame lets all other stations know how long the medium will be busy

Data

SIFS

Defer access

Ack

DIFSNAV (RTS)

Source

Destination

Other

RTSDIFS

SIFSCTS

SIFS

NAV (CTS)

NAV (Data)

Transmission of MPDU with RTS/CTS

Collision Avoidance of CSMA-CA

Collision Avoidance If a station with a frame to transmit initially senses the channel to be busy, it

waits until channel becomes idle for DIFS period and then computes a random backoff time (in units of time slot. For IEEE 802.11, time is slotted that corresponds to Slot_Time.)

Stations decrement their backoff timer until the medium becomes busy again or the timer reaches zero; the station freezes its timer when the channel is busy

The station transmits its frame when the backoff timer expires. If two or more stations decrement to zero at the same time, collision will occur and each station will generate a new backoff time in the range interval that is twice longer than the previous one, which grows exponentially

Receiving stations will send ACK if no error by CRC The sending station interprets no reception of ACK as a loss of frame The sending station executes the backoff procedure and then reschedule a

retransmission attempt Receiving stations use sequence numbers to identify duplicate frames

Point Coordination Function PCF is an optional capability to provide connection-

oriented, contention-free services by enabling polled station to transmit without contending for the channel

Point coordinator (PC) in AP performs the PCF function, and the stations within the BSS that are capable of operating in the CFP are known as the CF-aware stations

Polling table are maintained in the PC of AP The PCF coexist with the DCF and logically sits on the top

of the DCF The CFP repetition interval (CFP_Rate) determines the frequency

with which the PCF occurs The CFP repetition interval is initiated by beacon frame transmitted

by PC in AP Within the repetition interval, a portion of time is allocated to

contention-free traffic and the remainder for contention-based traffic The maximum size of CFP is CFP_Max_Duration

CF End

NAV

PIFS

B D1 + Poll

SIFS

U 1 + ACK

D2+Ack+Poll

SIFS SIFS

U 2 + ACK

SIFS SIFS

CFP repetition interval

Contention period

CFP_Max_duration

Reset NAV

D1, D2 = frame sent by point coordinatorU1, U2 = frame sent by polled stationTBTT = target beacon transmission timeB = beacon frame

TBTT

PCF Frame Transfer

Contention-free period

1.The CFP repetition interval is initiated by beacon frame transmitted by PC in AP2. Within the repetition interval, a portion of timeis allocated to contention-free traffic and the remainder for contention-based traffic3. The maximum size of CFP is CFP_Max_Duration

4.During the CFP, stations may transmit only to respond a poll from the PCOr to transmit acknowledgement

Physicallayer

LLC

Physical layerconvergenceprocedure

Physical mediumdependent

MAClayer

PLCPpreamble

LLC PDU

MAC SDUMACheader

CRC

PLCPheader PLCP PDU

6.10.4 Physical Layers

The IEEE 802.11 LAN has several physical layer defined to operate with its MAC layer

The physical layer convergence procedure (PLCP) sublayer maps the MPDU into a format suitable for transmission and reception over a given physical medium

The physical medium dependent (PMD) sublayer is concerned with the characteristic and methods for transmitting over the wireless medium

PLCP Preamble and PLCP Header

The PLCP frame consists of three parts The first part is a preamble that provides synchronization and

start-of-frame information The second part is a PLCP header that provides transmission bit

rate and other initialization information as well as frame-length information and a CRC

The third part consists of the MPDU possibly modified (scrambled) to meet the requirement of the transmission system

The specific structure of PLCP depends on the particular physical layer definition

136

137

IEEE 802.11 Physical Layer Options

Frequency Band

Bit Rate Modulation Scheme

802.11 2.4 GHz 1-2 Mbps Frequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum

802.11b 2.4 GHz 11 Mbps Complementary Code Keying & QPSK

802.11g 2.4 GHz 54 Mbps Orthogonal Frequency Division Multiplexing

& CCK for backward compatibility with 802.11b

802.11a 5-6 GHz 54 Mbps Orthogonal Frequency Division Multiplexing

138



6.11 LAN Bridges and

Ethernet Switches

139

Hub

Station Station Station

Two TwistedPairs

Hubs, Bridges, Routers & Gateway

Hub: Active central element in a star topology Twisted Pair: inexpensive, easy to install Simple repeater in Ethernet LANs “Intelligent hub”: fault isolation, net configuration, statistics Requirements that arise:

Hub


Two TwistedPairs

User community grows, need to interconnect hubs

?

Hubs are for different types of LANs

Hub

140

Hub


Two TwistedPairs

Hubs, Bridges, Routers & Gateway Interconnecting Hubs

Repeater: Signal regeneration All traffic appears in both LANs

Bridge: interconnect at MAC or data link layer MAC address filtering

Routers: interconnect at network layer Internet routing

Gateway: interconnect at higher layer Protocol conversion and security functions

Hub


Two TwistedPairs

?

HigherScalability

141

Operation at data link level implies capability to work with multiple network layers

However, must deal with Difference in MAC formats Difference in data rates; buffering; timers Difference in maximum frame length

PHY

MAC

LLC

Network Network

PHY

MAC

LLC

802.3 802.3 802.5 802.5

802.3

802.3

802.3 802.5

802.5

802.5

CSMA/CD Token Ring

General Bridge Issues

142

Bridge

Network

Physical

Network

LLC

PhysicalPhysicalPhysical

LLC

MAC MACMAC MAC

Bridges of Same Type

Common case involves LANs of same type Bridging is done at MAC level

143

Interconnection of IEEE LANs with complete transparency

Use table lookup, and discard frame, if source &

destination in same LAN forward frame, if source &

destination in different LAN use flooding, if destination

unknown Use backward learning to build

table observe source address of

arriving LANs handle topology changes by

removing old entries

Transparent Bridges

Bridge

S1 S2

S4

S3

S5 S6

LAN1

LAN2

144

B1

S1 S2

B2

S3 S4 S5

Port 1 Port 2 Port 1 Port 2

LAN1 LAN2 LAN3

Address Port Address Port

Forwarding Table Forwarding Table

145

B1

S1 S2

B2

S3 S4 S5


LAN1 LAN2 LAN3

Address Port

S1 1

Address Port

S1 1

S1→S5

S1 to S5 S1 to S5 S1 to S5 S1 to S5

146

B1

S1 S2

B2

S3 S4 S5


LAN1 LAN2 LAN3

Address Port

S1 1S3 1

Address Port

S1 1S3 2

S3→S2

S3S2S3S2 S3S2

S3S2 S3S2

147

B1

S1 S2

B2

S3 S4 S5


LAN1 LAN2 LAN3

S4 S3

Address Port

S1 1S3 2

S4 2

Address Port

S1 1S3 1

S4 2

S4S3

S4S3S4S3

S4S3

148

B1

S1 S2

B2

S3 S4 S5


LAN1 LAN2 LAN3

Address Port

S1 1S3 2

S4 2

S2 1

Address Port

S1 1S3 1

S4 2

S2S1

S2S1

S2S1

149

Adaptive Learning

In a static network, tables eventually store all addresses & learning stops

In practice, stations are added & moved all the time Introduce timer (minutes) to age each entry &

force it to be relearned periodically If frame arrives on port that differs from frame

address & port in table, update immediately

150

Avoiding LoopsLAN1

LAN2

LAN3

B1 B2

B3

B4

B5

LAN4

(1)

(2)

(1)

151

Spanning Tree Algorithm1. Select a root bridge among all the bridges.

• root bridge = the lowest bridge ID.

2. Determine the root port for each bridge except the root bridge

• root port = port with the least-cost path to the root bridge

3. Select a designated bridge for each LAN• designated bridge = bridge has least-cost path from the LAN

to the root bridge. • designated port connects the LAN and the designated bridge

4. All root ports and all designated ports are placed into a “forwarding” state. These are the only ports that are allowed to forward frames. The other ports are placed into a “blocking” state.

152

LAN1

LAN2

LAN3

B1 B2

B3

B4

B5

LAN4

(1)

(2)

(1)

(1)

(1)

(1)

(2)

(2)

(2)

(2)

(3)

153

LAN1

LAN2

LAN3

B1 B2

B3

B4

B5

LAN4

(1)

(2)

(1)

(1)

(1)

(1)

(2)

(2)

(2)

(2)

(3)

Bridge 1 selected as root bridge

154

LAN1

LAN2

LAN3

B1 B2

B3

B4

B5

LAN4

(1)

(2)

(1)

(1)

(1)

(1)

(2)

(2)

(2)

(2)

(3)

Root port selected for every bridge except root port

R

R

R

R

155

LAN1

LAN2

LAN3

B1 B2

B3

B4

B5

LAN4

(1)

(2)

(1)

(1)

(1)

(1)

(2)

(2)

(2)

(2)

(3)

Select designated bridge for each LAN

R

R

R

R

D

D

D D

156

LAN1

LAN2

LAN3

B1 B2

B3

B4

B5

LAN4

(1)

(2)

(1)

(1)

(1)

(1)

(2)

(2)

(2)

(2)

(3)

All root ports & designated ports put in forwarding state

R

R

R

R

D

D

D DLAN1

LAN3 LAN4LAN2

B3B1 B3

B2,B4 and B5 are disabled !

157

Source Routing Bridges

To interconnect IEEE 802.5 token rings Each source station determines route to

destination Routing information inserted in frame

Routingcontrol

Route 1designator

Route 2designator

Route mdesignator

Destinationaddress

Sourceaddress

Routinginformation

Data FCS

2 bytes 2 bytes 2 bytes 2 bytes

158

Route Discovery

To discover route to a destination each station broadcasts a single-route broadcast frame

Frame visits every LAN once & eventually reaches destination

Destination sends all-routes broadcast frame which generates all routes back to source

Source collects routes & picks best

159

B4

B6

B3 B7LAN 1

B1

B2

S1S2

S3

B5

LAN 2 LAN 4

LAN 3 LAN 5

Find routes from S1 to S3

LAN1 B1

B3

B4

LAN3 B6 LAN5

LAN4

LAN2

160

B4

B6

B3 B7LAN 1

B1

B2

S1S2

S3

B5

LAN 2 LAN 4

LAN 3 LAN 5

LAN5

B6

B7

LAN3

LAN4

B2

B3

B5

LAN1 B1 LAN2B3

B4 LAN4 B5B7

LAN2B1

B4

LAN1 B2

LAN4 B5B7

LAN4 B4

B7

LAN2 B1B3

LAN1 B2

B4

B5

LAN2B1

B3 LAN3B2B5B6

LAN1 B1

LAN1

B2 LAN3B3B5B6

LAN3 B3

B2

B6

LAN1

LAN2

B1 LAN2B3B4

B1B4

LAN1 B2

161

Physical

partition

Logical partition

Bridge

or

switch

VLAN 1 VLAN 2 VLAN 3

S17

2 3 4 5 61

8

9 Floor n – 1

Floor n

Floor n + 1

S2

S3

S4

S5

S6

S7

S8

S9

Virtual LAN

162

Logical partition

Bridge

or

switch

VLAN 1 VLAN 2 VLAN 3

S17

2 3 4 5 61

8

9 Floor n – 1

Floor n

Floor n + 1

S2

S3

S4

S5

S6

S7

S8

S9

Per-Port VLANs

Bridge only forwards frames to outgoing ports associated with same VLAN

163

Tagged VLANs

More flexible than Port-based VLANs Insert VLAN tag after source MAC address in

each frame VLAN protocol ID + tag

VLAN-aware bridge forwards frames to outgoing ports according to VLAN ID

VLAN ID can be associated with a port statically through configuration or dynamically through bridge learning

IEEE 802.1q

164

Home Work

6.1 (10%), 6.6 (10%), 6.8 (10%) 6.10 (20%), 6.13 (20%)

1 chapter 6 medium access control protocols and local area networks part i: medium access control...

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