communication sem.ppt

8/13/2019 communication sem.ppt

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Multiple ACCESS protocol


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pes of Multipleaccess protocol


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Transmission Technology

Transmission technology can be

categorized into two categories :

Point-to point networks

Broadcast networks


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Two types of transmissiontechnology

Point-to-Point Link Broadcast Link


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Point-to-pointnetworksPoint-to-point networks are those in which when a message issent from one computer to another, it usually has to be sent via

other computers in the network.A point-to-point network consists

of many connections between individual pairs of computers.


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BroadcastnetworksBroadcast networks have a single communication channel that is shared by all themachines on the network. Apacketsent by one computer is received by all theother computers on the network. The packets that are sent contain the address ofthe receiving computer; each computer checks this field to see if it matches itsown address. If it does not then it is usually ignored; if it does then it is read.

Broadcast channels are sometimes known as multi-access channel.

One Shared

channel


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Need of protocols in

Broadcast channelIssues in multi-access channel :

WHO is going to use the channel ?

WHEN the channel is going to be used ? For HOW much time the channel is used ?

Due to shared channel and unregulated traffic over the

network ..collisions and data loss occur.Some protocol must

be followed for regulated and safe transmission over the

network.


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Multiple ACCESS protocol

(Medium access protocol) is a sub layer of Data linklayer. MAC is the bottom part of the Data link layer. The protocols usedto determine who goes next on a multi-access channel belongs to thislayer. Some of the algorithms for allocating multi-access channel are as

follows:

Aloha protocol Carrier Sense Multiple Access Protocols(CSMA) Collision-free protocols :

Limited contention protocol Wireless LAN protocol Digital Cellular radio


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PURE ALOHAThe basic idea on which pure aloha is based upon is : It let userstransmit whenever they have data to be sent.So there will be collisions ,of course, and the colliding frames will bedamaged. However due to feed back property of broadcasting asender can always find out whether its frame was destroyed by

listening to the channel. All the receivers acknowledge the packetswhich means that the packet is received and if there is noacknowledgement from the receiver then the transmission isassumed to be unsuccessful and the station then retransmits the

packet after random amount of time.

The systems in which multiple users share a common channel in away that can lead to conflict are widely known as contentionsystems


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Pure Aloha


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If the first bit of a new frame overlaps with just the last bit of a frame

almost finished, both the frames will be totally destroyed. It does not

distinguish between a total loss or a near miss.


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Slotted Aloha In 1987, Roberts publish a method called slotted aloha which doubled

the capacity of pure aloha.

Divide time up into discrete intervals, each corresponding to onepacket.

The stations can only transmit data in one of the time slots only.

The vulnerable period is now reduced in half. If the frames collide theywill overlap completely instead of partially.


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Slotted ALOHA

Offered Load vs. Throughput

Max at G= 1, S = 1/e, only about 0.368

(37%)


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Slotted ALOHAAssumptions

All frames same size

Time divided into equal slots(time to transmit a frame)

Nodes are synchronized

Nodes begin to transmit

frames only at start of slots If two or more nodes

transmit, all nodes detectcollision

Operation

When node gets fresh data,transmits in next slot

No collision: success!

Collision: node retransmitsframe in each subsequent slot

with probability puntilsuccess

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Pros and Cons of Slotted Aloha

Pros

Single active node cancontinuously transmit atfull rate of channel

Highly decentralized: onlyneed slot synchronization

Simple

Cons

Wasted slots: Idle

Collisions

Nodes should detectcollision in less than time totransmit packet

Clock synchronization18


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SummaryAloha Protocol:

Whenever a station has data, it transmits immediately

Receivers ACK all packets

No ACK = collision. Wait a random time and retransmit


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The Slotted Aloha Protocol

Slotted Aloha - Aloha with an additional constraint

Time is divided into discrete time intervals (=slot) A station can transmit only at the beginning of a frame


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Definition

Figure 8 12


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Figure 8-12

p-Persistent

Approach

Attempts to minimize the idle time, while at the

same time minimize the chance of simultaneous transmissions


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ALOHA Network application

28

h h i l


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Ethernet Physical Layer

standards10Base5 10 Mbps, Baseband transmission, 500m cable length

10Base2

10 Mbps, Baseband transmission, ~200m cable length

10Base-T

10 Mbps, Baseband transmission, UTP cable

100Base-TX

100 Mbps, Baseband transmission, UTP cable


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Fast Ethernet


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MAP application

Comparison of MAC protocols in wired systems and wireless systems


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Wireless Sensor Networks


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


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 completelyTree Walk

Reduces contention by dividing stations dynamically

Ethernet

Dominant in Wired LANs

Uses CSMA/CD


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References1. Andrew S. Tanenbaum, Computer Networks, Fourth Edition

1. William Stallings, Data and Computer Communications, Eighth

Edition

1. Holger Karl, Andreas Willig, Protocols and Architectures for

Wireless Sensor Networks

1. http://en.wikipedia.org

2. Alaa Muqattash, Marwan Krunz, CDMA-Based MAC Protocol for

Wireless Ad Hoc Networks

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