EEC4113Data Communication &

Multimedia SystemChapter 6: Media Access Control

of Data Link Sub-Layer

by Muhazam Mustapha, October 2011

Learning Outcome

• By the end of this chapter, students are expected to understand and able to explain the various protocols and technologies in MAC sub-layer

Chapter Content

• MAC Sub-Layer Issues

• ALOHA Protocols

• CSMA Protocols

• Collision-Free Protocols

• Topology

• IEEE 802.3 Ethernet

• IEEE 802.11/15/16 Wireless Ethernet

• IEEE 802.5 Token Ring

Media Access ControlSub-Layer

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Media Access Control

• Media Access Control is a sub-layer of data link layer in OSI’s 7 layer model

• Provides access to the shared networking medium in LAN or MAN

• The currently most popular technology that provides MAC is the Ethernet technology

• Others are FDDI (Fiber Distributed Data Interface), ARCNET and Token ring

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Ethernet

• A family of frame-based technology defining standards for wiring and signaling

• Standardized in IEEE 802.3 document

• Combination of twisted wire pair and optical fiber

• Characterized by the used of 8P8C connector

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Shared Network Medium• In shared environment, packets sent by one

sender will be received by all nodes, but only the packet addressee will process it, the rest will discard

sender

recipient

packet sent out

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Multiple Access Protocol

• Since the network medium is shared, there is a need to resolve competition between the nodes

• Two general schemes:– Static

• Frequency / Time Division Multiplexing(digital communication)

– Dynamic• ALOHA, Carrier Sense Multiple Access

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Channel Allocation Problem

• In shared medium, a user will first listen to the channel for its availability, then sends its frame

• COLLISION occurs when more than one user start using the medium at the same time

• At collision incidence, both user release the medium and wait for random time before re-sending

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ALOHA Protocols

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ALOHA Protocols

• Created in 1970s in the University of Hawaii by Norman Abramson

• First ingenious method to resolve channel allocation problem

• It was best for wireless communication and the concept is still in used by modern protocol like Wi-fi

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Pure ALOHA

• Basic ideas:1. Anyone is allowed to transmit their data

whenever they have something to transmit, without checking the channel availability first

2. After sending, the sender will listen to its own frequency to tell whether its frame has been destroyed due to collision or not• This is possible due to feedback property of

broadcasting channel, or• The sender will require an acknowledgement

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Pure ALOHA

• Basic ideas:3. If there is no feedback, then there is collision

4. If collision occurs, the sender will wait for a random amount of time, then re-send – this called backoff

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Pure ALOHA

A

B

C

D

E

User

Time

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Slotted ALOHA

• Time is divided into slots, and users can only transmit at start of slot

• Resulting advantage: Efficiency is doubled (see graph)

• Disadvantages:– Requires synchronization clock– Still poor at high loads (see graph)

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Pure vs Slotted ALOHA

0.10

0.20

0.30

0.40

S (

thro

ughp

ut p

er fr

ame

time)

0.5 1.0 1.5 2.0 2.5 3.0

G (attempts per packet time)

Slotted ALOHA: S = Ge-G

Pure ALOHA: S = Ge-2G

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Carrier Sense Multiple Access Protocols

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Carrier Sensing Protocols

• Network communication can be improved greatly if the nodes can sense the existence of any transmission signal inside the transmission medium

• Implemented in Carrier Sense Multiple Access (CSMA) and a few of its variations

• Improvement is due to the fact that collisions is reduced since hosts will only send data if medium is not in use

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CSMA

• A host that needs to transmit data will first listen into communication medium and decide whether another host is using the medium or not

• The host will only transmit its data if no one is using the medium

• After finish sending the data frame, there will be an interframe gap of 9.6μs idle before any host can take the medium

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Persistent and Non-persistent CSMA

• CSMA is called persistent if:– when sensing that a medium is being used,

the host waits and will definitely transmit once the current transmission ends

• may cause collision if more than one host was waiting

• And non-persistent if:– the host waits for a random duration and re-

sends only if no one using it• results in less collisionCO1

CSMA/CD (Collision Detection)

• The system will be having 3 states: transmission, contention and idle

• Transmission state is the state where one host sends data.

• After that host finishes, more than one of other hosts might be sending at the same time – a collision

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CSMA/CD (Collision Detection)

• On sensing a collision, all hosts involve would release the medium and they send a jamming signal to tell others that there is collision happened– so that everyone releases the medium

• Then they will wait for a random duration and re-try

• The above two steps is the contention state

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CSMA/CD (Collision Detection)

• Once one of the competing host gains control the system is in transmission state again

• Idle state is just the state that no one is using the medium

collisions

transmission

contention

transmission

contention

transmission transmission

idle

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CSMA/CA (Collision Avoidance)

• CSMA/CD is a persistence variation of CSMA – it handles collision when it happens

• CSMA/CA is a non-persistence variation CSMA

• CSMA/CA avoids collision by– not sending jamming signal– instead, just wait for a random duration then

re-sends if no one is usingCO1

Collision-Free Protocol

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Collision-Free Protocols

• In collision free protocols, instead of sensing the medium, the hosts will tell if they want to transmit

• There is a special frame called contention frame whose content is contributed by all hosts

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Collision-Free Protocols

• Contention frame is slotted and the hosts will take turns at a very precise timing to write information into the frame

• A host sets a binary 1 at bit location reserved for it in contention frame if it wants to use the medium

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Collision-Free Protocols

• Once all hosts write the binary bits according to its intention, the actual transmission will be granted to the requesting hosts in sequence.

• Once all transmissions finish, the hosts will then re-fill the contention frame

• This protocol is called basic bit-map protocol

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Collision-Free Protocols

8 contention slots

0 1 2 3 4 5 6 7

1 73

0 1 2 3 4 5 6 7

51

0 1 2 3 4 5 6 7

211 111 1

8 contention slots

8 contention slots

frames frames frames

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Ethernet Topology

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Topology

BusLinear Bus – 2 ends

Distributed Bus – more than 2 ends

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Topology

Star

Ring

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Topology

MeshTree

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Bus Topology

• Use of multipoint medium

• All stations attach directly to transmission medium (bus) through appropriate hardware interfacing known as tap

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Bus Topology• A transmission from any station

propagates the length of the medium in both directions & can be received by all other stations

• At each end of the bus is a terminator, which absorbs any signal, removing it from the bus

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Tree Topology

• Use of multipoint medium

• Transmission medium is a branching cable with no closed loops

• Tree layout begins at a point known as the headend

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Tree Topology• One or more cables start at the headend,

and each of these may have branches

• The branches in turn may have additional branches to allow quite complex layouts

• A transmission from any station propagates throughout the medium & can be received by all other stations

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Ring Topology

• Repeaters joined by point-to-point links in closed loop– Receive data on one link and retransmit on

another– Links are unidirectional– Stations attached to repeaters

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Ring Topology• Data in frames

– Circulate past all stations– Destination recognizes address and copies

frame– Frame circulates back to source where it is

removed

• Medium access control determines when station can insert frame

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Star Topology

• Each station connected directly to central node– Usually via two point-to-point links

• Two alternatives operation of central node:– Broadcast : Physical star, logical bus– Frame-switching device : Only one station can

transmit at a timeCO1

Star Topology• Broadcast

– A transmission of a frame from one station to the central node is retransmitted on all of the outgoing links

– Central node is referred as hub

• Frame-switching device– Incoming frame is buffered in the node &

retransmitted on an outgoing link to the destination station

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IEEE 802.3 Standard ofEthernet

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IEEE 802.3 Standard• Defines Ethernet as CSMA/CD protocol on

bus or ring topology

• Also defines the minimum frame length

• Also defines the cabling hardware

• Frame format:

PreambleS O F

Destination address

Source address

Length Data Pad Checksum

Bytes 7 1 6 6 0-460-1500 4

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Frame Fields• Preamble: 7 bytes of alternating 1-s and 0-

s for synchronization

• Start of Frame (SOF): Sequence of 10101011

• Destination Address: 6 bytes of MAC address

• Source Address: 6 bytes of address

• Length: Total size of data and pad

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Frame Fields• Data: Packet from upper layer

• Pad: Series of 0-s to make up a minimum total size of 46 bytes of data and pad – so that the min frame size is 64 bits

• Checksum: 32 bit CRC

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MAC Address• Identifying each individual network card

uniquely

• 46 bits address in 48 bits string

• Binary 0 in MSB indicates ordinary address

• Binary 1 in MSB indicates the 46 bits address is a group address (for multicast)

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MAC Address• If all address bit are 1-s then it is a

broadcast (all nodes are getting the message)

• If two MSB are 0-s then the 46 bits address is a combination of source and destination MAC address

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MAC Address• Examples of possible MAC addresses

include: – 00-0C-F1-56-98-AD– 00-11-F5-4B-20-56

• The first three bytes of this address identify the manufacture of this network device– 00-0C-F1 for Intel– Assigned by the IEEE and the database is

available online at IEEE OUI and Company_id Assignments website

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Need for Frame Minimum Size• In CSMA/CD, if there is a collision, the first

node to detect it will send a jamming signal

• We need to calculate the maximum delay after a node sends a message until the first jamming signal is heard by all nodes

• Then from there we can calculate what is the minimum frame size so that NO nodes will finish transmitting before it hears the jamming signal

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Need for Frame Minimum Size• The diagram below shows that there is a

maximum of 2td delay before the first jamming signal is heard by every node(td = propagation delay)

A BFrame sent at t = 0s

A BAt t ≈ td s, the frame almost reach the receiver

A B

At t ≈ td s, suddenly the receiver sends out frame

collision

Jamming signal sent out

A B

Jamming signal finishes propagating at t = 2td s

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Need for Frame Minimum Size(compare this calculation to link utilization calculation)

• Hence max delay is a function of bit rate, max distance allowed and velocity of propagation

• Given:– Ethernet bit rate: 10 Mbps (802.3 Standard for

10Base5 and 10Base-T)– Max distance: 500m (802.3 Standard)– Velocity of propagation: 2 × 108 ms−1

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Need for Frame Minimum Size(compare this calculation to link utilization calculation)

• Hence:– td = 2.5μs, hence 2td = 5μs

– Bit duration = 0.1μs

– No. bits traveling in 2td time = 50

• Adding some gap for error, the best min frame size chosen is 64 bits

• 802.3 Std sets 512 bits as min, because it allows max distance of 2.5km with 4 passive repeaters

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Ethernet Physical Standard• 10Base5: 10 Mbps, Baseband

transmission, 500m cable length

• 10Base2: 10 Mbps, Baseband transmission, 200m cable length

• 10Base-T: 10 Mbps, Baseband transmission, 500m UTP cable

• 100Base-TX: 100 Mbps, Baseband transmission, 200m UTP cable

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Ethernet Physical Standard• Wiring:

– Unshielded Twisted Pair (UTP)– Bundle of eight wires (only uses four)– Terminates in RJ-45 connector

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Ethernet Physical Standard• Hubs (10Base-T):

– A kind of passive repeater

– Used to connects nodes in bus topology

– Max length of UTP: 100m

– Max no. hubs in series: 4

– Hence, max distance between farthest nodes: 500m

100m

100m

100m

100m

100m

500m, 4 hubs

10Base-T hubs

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Repeaters• Regenerates signal

• Used to extend the network coverage

• Hubs are repeaters

• There will be a limit to the length of the farthest node due to physical signal limitation

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Bridges• Used to join LANs

• Results in local internet

• May filter the data traffic

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Switches• More intelligent kind of bridges

• Must be arranged in hierarchical arrangement – only one path from one switch to another

• Due to its intelligent close to a small node, there is no limit in number of switches in a LAN – as opposed to hubs

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Hubs vs Bridges vs Switches• Hub

– Has many ports– Redistributes data to all nodes– It depends on the receiver to process the data– Almost no intelligence– Used to extend connection within standard

limit

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Hubs vs Bridges vs Switches• Bridge

– Only two ports– Transfers data from one end to the other only

if the receiver address is at the other end– Have intelligence to interpret MAC addresses– Used to join two separate LANs

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Hubs vs Bridges vs Switches• Switch

– More than two ports– Have intelligence to interpret MAC addresses– Transfers data from one end to another only if

the receiver address is at that end– Extends LAN unlimitedly, but must conform to

hierarchical (tree) structure– Router:

• Switch that works on IP address instead of MAC• For internet instead of LAN• Smart enough to do protocol conversionCO1

Hubs vs Bridges vs Switches

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IEEE 802.11/15/16Standards of

Wireless Ethernet

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IEEE 802.11• Uses CSMA/CA instead of CSMA/CD

• Could not detect collision due to hidden nodes (target nodes beyond signal range)

• Sender listen to the medium (air) to see whether it is busy or not

• After the medium is free for a period of DIFS (Distributed Inter-Frame Space ~ 128μs), the sender sends RTS (request to send) signal to tell its intention, and others will make way

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IEEE 802.11a• Frequency = 5 GHz

• Maximum Speed = 54 Mbps

• Range = about 35 meters (varies)

• Encoding Scheme = Orthogonal FDM (closely located frequencies but far enough not to interfere each other)

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IEEE 802.11b• Frequency = 2.4 GHz

• Maximum Speed = 11 Mbps

• Range = about 38 meters (varies)

• Encoding Scheme = DSSS

• Modulation Technique = BPSK(1 Mbps), QPSK(2 Mbps), CCK(5.5 Mbps,11Mbps)

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IEEE 802.11g• Frequency = 2.4 GHz

• Maximum Speed = 54 Mbps

• Range = about 38 meters (varies)

• Encoding Scheme = OFDM

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IEEE 802.11n• Frequency = 5 GHz, 2.4 GHz

• Modulation = OFDM

• Maximum Speed = 150 Mbps

• Range = about 70 meters (varies)

• Encoding Scheme = OFDM

• Addition of MIMO (Multiple Input Multiple Output)– sender and receiver have 2 antennas to send

and receive 2 signals (one is modified redundancy) to improve performanceCO1

IEEE 802.16 – WiMAX• WiMAX is 802.11/Wi-Fi networks with

coverage and cellular networks quality of service

• Stands for "Worldwide Interoperability for Microwave Access"

• Most of WiMAX physical layer definitions and topology follows those of 802.11

• Provider in Malaysia: P1 & Yes

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IEEE 802.16 – WiMAX• Consists of two standards – Fixed &

Mobile

• Fixed WiMAX (IEEE 802.16d)– Speed = up to 70 Mbps– Range = up to 50 km

• Mobile WiMAX (IEEE 802.16e)– Speed = up to 30 Mbps– Range = up to 15 km

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IEEE 802.15 – Bluetooth• Open proprietary standard created by

Ericsson

• Not a direct descendent if 802.11

• Designed for communication between electronics devices as alternative to cabled RS-232

• Consisting of 1 master devices and up to 8 slaves

• Logo:CO1

IEEE 802.15 – Bluetooth• Frequency = 2.4-2.8 GHz

• Speed = 1 Mbps

• Range = 10 meters

• Encoding Scheme = FHSS with 79 channels at 1600 hops per second

• Most common uses: Mobile phone headset, wireless mouse & keyboard

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Token Based Protocol

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IEEE 802.5 Token Ring• The network is arranged in ring topology

• There is a special frame to be passed around the nodes named TOKEN

• Whoever is having the token can transmit data into transmission medium, otherwise it passes the token to the next node

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IEEE 802.5 Token Ring

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IEEE 802.4 Token Bus• The network is arranged in bus topology

• Just as token ring, there is a special frame TOKEN used

• Whoever is having the token can transmit data into transmission medium, otherwise it passes the token to the next node

• The use of this type of protocol is shown by the presence of coaxial cable connector on the network card instead of 8P8C

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FDDI• Fiber Distributed Data Interface

• Data rate = 100Mbps

• Used as a backbone

• With multi-mode fiber any given ring segment can be up to 200 km in length

• A total of 500 stations can be connected with a maximum separation of 2 km

• Two complete rings to overcome failures

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FDDI

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FDDI Interface in High Speed LANs

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