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Comparison of MAC protocols in wired systems and wireless systems

Mridula Sharma

19.07.2010


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Outline

• Introduction

• Multiple Access Protocols

• Wired Network

• Wireless Networks

• Conclusion

• References


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Introduction• Large class of networks is built on top of broadcast channels

• Problem: if you’re sharing a channel, then two stations may decide to start frame transmission at the same time• Frame collision, which means rubbish on the wire

• Solution: Allocate the channel to one of the competing stations

• Allocation can be of two types: • Static: FDM and TDM• Dynamic


Protocol to determine who goes next on a multi-access channel belongs to the Medium Access Control (MAC) sublayer

• Especially important for LANs• Provides addressing and channel access control mechanisms

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Introduction


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Introduction

Functions of MAC layer


•On transmission, assemble data into a frame with address and error detection fields

•On reception, disassemble frame, and perform address recognition and error detection

•Govern access to a LAN transmission medium

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Multiple Access Protocols

• ALOHA

• Carrier Sense Multiple Access (CSMA) Protocols

• Collision Free Protocols

• Limited Contention Protocols

• Wavelength Division Multiple Access Protocols

• Wireless LAN Protocols


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ALOHA

• Radio-based communication network

• Developed in 1970s at the University of Hawaii

• Applicable to any system with competing uncoordinated users

• Two versions

• Pure ALOHA

• Slotted ALOHA


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Pure ALOHA

• Let users transmit whenever they have data to be sent !!!

• If a collision occurs, finish your current transmission and retry later

• Performance is maximized if all frames have the same size

• Random waiting time required else collisions occur


Fig. : Frames transmitted at completely random intervals

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Pure ALOHA- Efficiency

• Probability that k frames are generated during a given frame time with G attempts per packet time:

•Let P0 be the probability that frame does not suffer from collision• Throughput , S= G.P0

• Vulnerable period = 2Tframe

• Probability that a frame will not be damaged during two frame timeslong is:


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Slotted ALOHA


• Frame transmission can only start at fixed times• Since the vulnerable period is now halved,

Fig. : Throughput versus offered traffic for ALOHA systems

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Carrier Sense Multiple Access (CSMA)


CSMA protocols do better than ALOHA: you monitor the channelbefore and/or during transmission

• 1-persistent

• Nonpersistent

• p-Persistent.

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Fig. : Comparison of channel utilization versus load for various random access protocols

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CSMA with Collision Detection (CSMA/CD)


• Sense the channel, but immediately stop transmission when you detect a collision• Saves Channel Bandwidth and time• Ethernet works like this!!!

Fig. : CSMA/CD can be in one of the three states: Contention, Transmission or Idle.

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Collision Free Protocols


• Bit-Map protocol

• Binary Countdown Protocol

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Limited Contention Protocols


• Contention systems are good when there’s not much going on

• Collision-free systems are good when there’s generally a lot of traffic

What we really want is the contention strategy during light loads, andcollision-free strategy during rush hours !!!

Solution: Limited Contention Protocols

•Dynamically regulate the number of competing stations during a contention period.

• If there’s not much traffic, the first station will be immediatelyallowed to transmit a frame.

• With a lot of traffic, the strategy reduces to the bit-map protocol.

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Limited-Contention Protocols

The Adaptive Tree Walk Protocol


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Wavelength Division Multiple Access Protocols


• If you have a lot of bandwidth, just divide the channel into sub-channels, and dynamically allocate the sub-channels !!!

• used in fiber optics

Fig. : Wavelength Division Multiple Access

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Ethernet


• CSMA/CD based• sense the channel, wait until idle, and backoff if collision

• Exponential Backoff

•Near implementation of IEEE 802.3 protocol

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802.3 Frame Layout


• Preamble: Seven times 10101010 is used to synchronize the receiver’s clock with that of the sender

• Start: Just a delimiter to tell that the real info is now coming

• Address: Generally 48-bit fields. Leftmost bit indicates ordinary or group addresses (multicast / broadcast). Second bit indicates global or local address

• Length: Ranges from 0-1500. Frames should always be at least64 bytes

• Pad: used to fill out the frame to the minimum size

• Checksum: Calculated over the data field. CRC-based

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Wireless Networks


• Often, there are a number of base stations (a.k.a. access points) connected through guided media

• Nodes can also group together to form an ad hoc network

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Wireless LAN Requirements


• Throughput

• Number of Nodes

• Connection to backbone LAN

• Service Area

• Battery Power Consumption

• Transmission Robustness and Security

• Collocated Network Operation

• License-free Operation

• Handoff/ roaming

• Dynamic Configuration

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Problems


• There can be subtle interference:

• Issue (a)How can C be prevented from trying to transmit something to B ?

• in that case it will ruin any receipt by B• hidden station problem

• Issue (b)How can we tell C that it is allowed to transmit to D, because this will not interfere with the communication from B to A ?

• exposed station problem

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Multiple Access with Collision Avoidance (MACA)


• A first sends a Request To Send (RTS)

• B answers with a Clear To Send (CTS)

• C hears only RTS and can freely transmit, knowing it will not interfere with A’s transmission- solves exposed station problem

• D hears only the CTS and keeps still for otherwise it wouldinterfere with B’s reception- solves hidden station problem

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MACA for Wireless (MACAW)


• Gets rid of Ethernet like unfairness associated with binary exponential

backoff algorithms !!!

• Acknowledges the importance of link layer acknowledgements

• Makes the protocol from RTS-CTS-Data to RTS-CTS-Data-ACK

• Significantly complex protocol

• Performance loss with lightly loaded channel

• Better throughput and fairer allocation in presence of high loads

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Wireless LANs MAC Sublayer


Problem: How do we solve the hidden / exposed station problem ?

• One way or the other, stations should not be allowed to continuously interfere with each other’s transmissions

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Wireless LANs MAC Sublayer


• IEEE 802.11 provides two methods to deal with this problem:

• Distributed coordination: let the stations figure it out by using acollision avoidance protocol (CSMA/CA)

• Point coordination: there’s a central base station that controls whogoes first- mostly used

• Collision detection is hard to implement on the wireless medium• radios cannot transmit and receive on the same frequency (half-duplex channels)

• Solution: Use collision avoidance protocol• Two method like operations

•Ethernet-like•MACAW

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CSMA/CA Ethernet-like


• Similar to non-persistent ALOHA• there random time was used before re-sensing the channel when busy• exponential backoff is used

• Nowadays most MAC are reprogrammable by users• vulnerable to cheating !!!

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Fig. : IEEE 802.11 Medium Access Control Logic

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CSMA/CA- MACAW


• MACAW: send RTS/CTS packets to see whether you should defertransmission to avoid interference with another transmission

• Evolution of MACA

• Network Allocation Vector (NAV): it’s a virtual channel that a stationassigns to itself telling it to shut up!!!

• Note: hidden station problem is solved but NOT exposed station problem!

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CSMA/CA- point Coordination


• In PCF the base-station polls the other stations, asking them if they have anything to send

• It sends a beacon frame once every 10 or 100 ms

• Sleep state to save battery

• When base station transmits, there can be no hidden terminals

• PCF and DCF can co-exist together•By carefully defining the inter-frame time interval• First the base station can poll the other stations• If nobody replies, any station can acquire the channel

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Fig. : IEEE 802.11 Protocol Architecture

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802.11 MAC Frame Layout


• Type: Data, control, or management frame• Subtype: Are we dealing with RTS, CTS, an ACK, etc• DS: Is the frame entering/leaving the current cell?• MF: Frames are allowed to be fragmented to increase reliability. This bit tells whether more fragments are on their way• Retry: Is this a retransmission?• O: Stick to ordered delivery of frames

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802.11 MAC Frame Layout


• Duration: Tells how long the transmission of this frame will take, allowing other stations to set their NAV accordingly• Addresses: Source/destination in a cell; and those of base stations outside the cell when dealing with inter-cell traffic• Sequence: Sequence number of this frame. 4 bits are used toidentify a fragment of a frame

Note: 802.11 supports encryption at MAC level, Ethernet does not !!!

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Broadband Wireless


Goal: Use wireless connection between buildingsi.e. avoiding the use of the local loop

• More bandwidth is needed: 10-to-66 GHz frequency range• Full-duplex communication possible• Much of the mobility stuff from 802.11 is not needed• Can be focused in directional beams (IEEE 802.11 is omnidirectional)

WiMAX (World Interoperability for Microwave Access) is a family ofIEEE 802.16 aiming at replacing ADSL and cable modems

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Broadband Wireless


• MAC: the base station controls the systems by scheduling downstream and upstream channels

• it is connection-oriented (QoS needed by phone-companies)• Time Division Duplexing (TDD)

• Downstream traffic is mapped onto time slots by the base station• Upstream traffic is more complex

• The base-station pre-allocates it (constant bit rate service)• The base-station periodically polls stations (variable bit rate service)• No polling but subscribers have to contend (best-effort service)

Fig. : TDD frame contatinig time slots

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Broadband Wireless- 802.16 Frame Structure


Fig. : (a) a generic frame (b) a bandwidth request frame

• Connection-oriented Service

• Checksumming optional

• Encryption is critical for the system• Managed at MAC level

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Wireless Sensor Networks

Zigbee


• IEEE standard 802.15.4

• Name of a specification for a suite of high level communication protocols using small, low-power digital radios

• MAC layer: CSMA/CA

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Conclusion


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Fig: Channel allocation methods and systems for a common channel

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Conclusion


•Simplest allocation schemes: FDM and TDM•Poor choices for large, variable or bursty traffic•Alternative: ALOHA (Dynamic Allocation)

•Carrier Sensing in LANs and MANs led to variety of protocols•Binaray Countdown

•Eliminates contention completely•Tree Walk

•Reduces contention by dividing stations dynamically

•Ethernet•Dominant in Wired LANs•Uses CSMA/CD

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Conclusion


•Wireless LANs•CSMA does not work here•MACA and MACAW•FHSS and DSSS•IEEE 802.11 suggests CSMA/CA

•Broadband Wireless•802.16, TDD

•Wireless Sensor Networks•Zigbee

•CSMA/CA

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References

1. Andrew S. Tanenbaum, Computer Networks, Fourth Edition

2. William Stallings, Data and Computer Communications, Eighth Edition

3. Holger Karl, Andreas Willig, Protocols and Architectures for Wireless Sensor Networks

4. Paolo Costa, Compter Networks- Medium Access Control Sublayer

5. Alaa Muqattash, Marwan Krunz, CDMA-Based MAC Protocol for Wireless Ad Hoc Networks

6. Sushant Jain, Ratul Mahajan, Wireless LAN MAC protocols


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Thank You!!!

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