The Medium Access Sublayer -...

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Chapter 4

The Medium Access

Sublayer

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Chapter 4 The Medium Access Layer

• 4.1 The Channel Allocation problem

- Static and dynamic channel allocation in LANs & MANs

• 4.2 Multiple Access Protocols

- ALOHA, CSMA, CSMA/CD, Collision-free protocols,

Limited-contention protocols, Wireless LAN protocols

• 4.3 Ethernet

- Cabling, MAC sublayer protocol, Backoff algorithm,

Performance, Gigabit Ethernet, 802.2 Logical Link Control

• 4.4 Wireless LANs

- 802.11 protocol stack, physical layer, MAC sublayer

protocol, frame structure

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• 4.5 Broadband Wireless

- Comparison of 802.11 with 802.16, protocol stack,

frame structure

• 4.6 Data Link Layer Switching

- Bridges from 802.x to 802.y, Local internetworking,

Spanning tree bridges, Remote bridges

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4.1 The Channel Allocation problem

4.1.1 Static channel Allocation in LANs and WANs

Frequency Division Multiplexing (FDM)

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Poisson process and the M/M/1 queue will be discussed in the

next chapter.

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4.1.2 Dynamic Channel Allocation in LANs and WANs

Assumptions

1. Station Model. The model consists of N

independent stations, each generates the

frame with probability in an interval . Once a frame is generated, the station is

blocked.

2. Single Channel Assumption. A single channel

is available for all communication.

3. Collision Assumption(observable). If two

frames are transmitted simultaneously, they

are destroyed and must be retransmitted again

later. There are no other errors.

t t

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4a. Continuous Time. Frame transmission can

begin at any instant.

4b. Slotted Time. Time is divided into slots. Frame

transmission always begin at the start of a slot.

5a. Carrier Sense. Stations can tell if the channel

is in use before trying to use it.

5b. No Carrier Sense. Stations cannot sense the

channel before trying to use it.

4.2 Multiple Access Protocols

• ALOHA

• Carrier Sense Multiple Access

• Collision-free protocols

• Limited-contention protocols

• Wireless LAN protocols

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10

4.2 Multiple Access Protocols

4.2.1 ALOHA

4-1.

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4-2.

13

4-3.

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4.2.2 Carrier Sense Multiple Access Protocols

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P-persistent (slotted case)

1. When a station has data to send, it first listens to the

channel to see if any one else is transmitting.

2. If the channel is idle, the station transmits the frame with

probab P,with probability q=1-P, it defers until the next slot

and goes to 1.

3. If the channel is busy, it waits until the next slot and goes to

step 1.

4. If a collision occurs (no acknowledgement), the station

waits a ran amount of time and goes Step1.

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4-4.

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4-5.

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4.2.3 Collision-Free Protocols

4-6.

Collision-Free Protocols (2)

Token ring.

Station

Direction of

transmission

Token

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25

4-7.

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4.2.4 Limited-contention Protocols

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Limited-Contention protocols

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4.8

4.8.

4.8.

33

4-9

The Adaptive Tree Walk protocol

(Method for testing soldiers for syphilis)

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i

i

i

i2

iq 2

qi2

Wireless LAN Protocols (1)

A wireless LAN. (a) A and C are hidden terminals

when transmitting to B.

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A wireless LAN. (b) B and C are exposed terminals

when transmitting to A and D. 37


The MACA protocol. (a) A sending an RTS to B. (b) B

responding with a CTS to A. 38

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4.3 Ethernet

• Physical Layer

• The Ethernet MAC Sublayer Protocol

• The Binary Exponential Backoff Algorithm

• Ethernet Performance

• Switched Ethernet

• Fast Ethernet

• Gigabit Ethernet

• IEEE 802.2: Logical Link Control

• Retrospective on Ethernet

Classic Ethernet Physical Layer

Architecture of classic Ethernet 40

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Ethernet Cabling

The most common kinds of Ethernet cabling.

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Ethernet Cabling (2)

Three kinds of Ethernet cabling.

(a) 10Base5, (b) 10Base2, (c) 10Base-T.

Three kinds of Ethernet cabling.

(a) 10Base5, (b) 10Base2, (c) 10Base-T

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Cable topologies. (a) Linear, (b) Spine, (c)

Tree, (d) Segmented.

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(a) Binary encoding, (b) Manchester encoding,

(c) Differential Manchester encoding.

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Ethernet MAC Sublayer Protocol

Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

SOF: Start of frame

DIX (DEC, Intel, Xerox)

7 1

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Ethernet MAC Sublayer

Protocol (2)

Collision detection can take as long as 2 .

The Binary Exponential Back off Algorithm

1. Any station experiences collisions, it has to choose a

random number between 0 and ,and that number

of slots is skipped ( ≤10 )

2. After ten collisions have been reached the randomization

interval is frozen at a maximum of 1023 slots

3. After 16 collisions the controller has to stop transmission

and reports failure back the the computer, Further

recovery is up to higher layers.

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12

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Ethernet performance Assumptions

a. Heavy and constant load, that is,

stations always ready to transmit

b. Each station transmits during a

contention slot with probability

The probability A that some station acquires

the channel in that slot is

A is maximized when ,

with as

p

k

1)1( kpkpA

kp 1

eA 1 k

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The probability that the contention interval has exactly slots in

it is

so the mean number of slots per contention is given by

Since each slot has a duration , the mean contention interval

Assuming optimal , i.e.

as

If the mean frame takes seconds to transmit then

channel efficiency =

where B: bandwidth

F: frame length

L: cable length

C: propagation speed

j,)1( 1 jAA

AAjA

j

j 1)1(

0

1

2A

w 2

pk

p 1e

A1

k eA

1 4.52 ew

p

Ap

p

2

CFBLee

CL

BF

BF

21

1

)(2

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Ethernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

Switched Ethernet (1)

(a) Hub. (b) Switch. 51

Switched Ethernet (2)

An Ethernet switch.

Switch

Twisted pair

Switch ports

Hub

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Fast Ethernet

The original fast Ethernet cabling.

The reasons for fast Ethernet

1. The need to be backward compatible with existing Ethernet LANs

2. The fear that a new protocol might have unforeseen problems

3. The desire to get the job done before the technology changed

All fast Ethernet systems use hubs and switches

100 Base-T4 uses 8B/6T coding and 100 Base-TX uses 4B/5B coding

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Gigabit Ethernet

(a) A two-station Ethernet. (b) A multistation Ethernet.

• All configurations of gigabit Ethernet are point-to-point

• Gigabit Ethernet supports full-duplex mode (with switch) and half-duplex-mode (with hub)

• CSMA/CD protocol is required for half- duplex mode operation (maximum distance is 25 meters)

• When carrier extension (512 bytes frame) and frame bursting are used

the distance can be 200meters

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Gigabit Ethernet (2)

Gigabit Ethernet cabling.

•Gigabit Ethernet supports both copper and fiber cabling

•Two wavelengths are permitted = 0.85μm and 1.3μm

•Three fiber core diameters are permitted = 10, 50, and 62.5μm

10 Gigabit Ethernet

Gigabit Ethernet cabling 56

Wireless Lans

• 802.11 architecture and protocol stack

• 802.11 physical layer

• 802.11 MAC sublayer protocol

• 802.11 frame structure

• Services

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802.11 Architecture and Protocol Stack (1)

802.11 architecture – infrastructure mode

Access

Point

Client

To Network

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802.11 architecture – ad-hoc mode 59


Part of the 802.11 protocol stack. 60

The 802.11 MAC Sublayer Protocol (1)

Sending a frame with CSMA/CA. 61


The hidden terminal problem. 62


The exposed terminal problem. 63


The use of virtual channel sensing using

CSMA/CA. 64


Interframe spacing in 802.11 65

802.11 Frame Structure

Format of the 802.11 data frame 66

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Version: Protocol version

Type: data, control, or management

Subtype: RTS, CTS, ack, …

To DS and from DS: to or from inter cell distribution

system (e.g. Ethernet)

MF: more fragments

Retry: retransmission

Power management: put the receiver into sleep state or

take it out

More: additional frames coming

Protected: encrypted for Security

O: processed strictly in order

Duration: time length of the frame and ack

Addresses1.2.3 and 4: Source, destination, the source and

destination base stations for intercell traffic

Sequence: Sequence No.

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802.11 Services

• Association: To connect to a base station

• Reassociation: To change its preferred base

station

• Disassociation: To disconnect from a base station

• Distribution: How to route frames (local or intercell)

• Integration: Translation from 802.11 to other

protocol frame format

Distribution Services

(managing cell membership, and interacting

with station outside the cell)

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802.11 Services

• Authentication

• Deauthentication (leave the network)

• Privacy: managing the encryption

and decryption

• Data Delivery: transmit and receive data

Intracell Services

Data Link Layer Switching

• Uses of bridges

• Learning bridges

• Spanning tree bridges

• Repeaters, hubs, bridges, switches,

routers, and gateways

• Virtual LANs

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Bridges from 802.x to 802.y

The IEEE 802 frame formats. The drawing is not to scale.

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Bridges from 802.x to 802.y

Operation of a LAN bridge from 802.11 to 802.3.

Learning Bridges (1)

Bridge connecting two multidrop LANs 73


Bridges (and a hub) connecting seven point-to-

point stations. 74


Protocol processing at a bridge. 75

Spanning Tree Bridges (1)

Bridges with two parallel links

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Spanning Tree Bridges (2)

A spanning tree connecting five bridges. The dotted

lines are links that are not part of the spanning tree. 77

Poem by Radia Perlman (1985)

Algorithm for Spanning Tree (1)

I think that I shall never see

A graph more lovely than a tree.

A tree whose crucial property

Is loop-free connectivity.

A tree which must be sure to span.

So packets can reach every LAN.

. . .

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Poem by Radia Perlman (1985)

Algorithm for Spanning Tree (2)

. . .

First the Root must be selected

By ID it is elected.

Least cost paths from Root are traced

In the tree these paths are placed.

A mesh is made by folks like me

Then bridges find a spanning tree.

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Repeaters, Hubs, Bridges, Switches,

Routers, and Gateways

(a) Which device is in which layer.

(b) Frames, packets, and headers. 80

Virtual LANs (1)

A building with centralized wiring using hubs and a

switch. 81

Virtual LANs (2)

Two VLANs, gray and white, on a bridged LAN. 82

The IEEE 802.1Q Standard (1)

Bridged LAN that is only partly VLAN-aware. The shaded

symbols are VLAN aware. The empty ones are not. 83

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The IEEE 802.1Q Standard (2)

The 802.3 (legacy) and 802.1Q Ethernet frame formats.

Priority: This field makes it possible to distinguish hard real-time traffic from soft

real-time traffic from time-insensitive traffic.

CFI (Canonical Format Indicator): To indicate that the payload contains 802.5.

VLAN Identifier: To indicate which VLAN the frame belong to.

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Summary

Channel allocation methods and systems for a common channel.

Homework due 2012.04.24

1. State the CSMA/CD protocol.

2. State the Binary Exponential Backoff (Traucated) algorithm.

3. Measurements of a slotted ALOHA channel with an infinite

number of users show that 15% of the slots are idle.

(a) What is the channel load, G?

(b) What is the throughput?

(c) Is the channel underloaded or overloaded?

4. State the hidden terminal problem and the exposed terminal

problem in a wireless LAN.

5. The station source address of each Ethernet is globally unique

assigned to ensure that no two stations anywhere in the world

have the same address. What is the maximum number of

addresses we may have?

6. Why do the Ethernet frames have to be at least 64 bytes

long?

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