wireless mac protocols1

8/10/2019 Wireless MAC Protocols1

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Wireless MAC Protocols

M.L.Dahanukar College(M.Sc.IT[Part 1]) Page 1

ABSTRACT

Medium access control protocols define rules for orderly access to the shared

medium and play a crucial role in the efficient and fair sharing of scarce wireless

bandwidth. The nature of the wireless channel brings new issues like location-dependent carrier sensing, time varying channel and burst errors. Low power

requirements and half duplex operation of the wireless systems add to the

challenge.

Wireless MAC protocols have been heavily researched and a plethora of protocols

have been proposed. Protocols have been devised for different types of

architectures, different applications and different media. This survey discusses the

challenges in the design of wireless MAC protocols, classifies them based onarchitecture and mode of operation, and describes their relative performance and

application domains in which they are best deployed.

The role of a MAC protocol is explored and the major design choices and

constraints are examined, discussing their impact on system complexity and cost.

We then identify the fundamental channel access techniques that are used almost

universally in a vast majority of wireless networks.

An overview of MAC protocol research that spans cellular telephony, wireless

ATM and ad hoc networks is then presented with a qualitative discussion of

relative characteristics and performance. We will provide insights into the

strengths and weaknesses of each protocol, revealing which protocols are best

suited for specific architectures and applications.


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INTRODUCTION:

The ability to communicate with anyone on the planet from anywhere on the planet

has been mankind's dream for a long time. Wireless is the only medium that can

enable such untethered communication. With the recent advances in wireless

technologies, it is now possible to build high-speed wireless systems that are cheap

as well as easy to install and operate.

However, the wireless medium is a broadcast medium, and therefore multiple

devices can access the medium at the same time. Multiple simultaneous

transmissions can result in garbled data, making communication impossible.

A medium access control (MAC) protocol moderates access to the shared medium

by defining rules that allow these devices to communicate with each other in an

orderly and efficient manner. MAC protocols therefore play a crucial role inenabling this paradigm by ensuring efficient and fair sharing of the scare wireless

bandwidth.

Wireless MAC protocols have been studied extensively since the 1970s. The

initial protocols were developed for data and satellite communications.

We are now witnessing a convergence of the telephone, cable and data networks

into a single unified network that supports multimedia and real-time applications

like voice and video in addition to data.


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GENERAL CONCEPTS:

In the broadest terms, a wireless network consists of nodes that communicate by

exchangingpackets via radio waves. These packets can take one of two forms. A

unicast packet contains information that is addressed to a specific node, while amulticast packet distributes the information to a group of nodes. The MAC

protocol simply determines when a node is allowed to transmit its packets, and

typically controls all access to the physical layer. Fig. 1.1 depicts the relative

position of the MAC protocol within a simplified protocol stack.

The specific functions associated with a MAC protocol vary according to the

system requirements and application. For example, wireless broadband networks

carry data streams with stringent quality of service (QoS) requirements. This

requires a complex MAC protocol that can adaptively manage the bandwidth

resources in order to meet these demands. Design and complexity are also affected

by the network architecture, communication model, and duplexing mechanism

employed.


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Network Architecture:

The architecture determines how the structure of the network is realized, and where

the network intelligence resides. A centralized network architecture features aspecialized node, i.e., the base station, that coordinates and controls all

transmissions within its coverage area, or cell.

Cell boundaries are defined by the ability of nodes to receive transmissions from

the base station. To increase network coverage, several base stations are

interconnected by land lines that eventually tie into an existing network, such as

the public switched telephone network (PTSN) or a local area network (LAN).

Thus each base station also plays the role of an intermediary between the wired

and wireless domains. Fig. 1.2 illustrates a simple two cell centralized network.


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CLASSIFICATION OF MAC PROTOCOLS:

Wireless MAC protocols can be broadly classified into two categories,

distributed and centralized, according to the type of network architecture for

which they are designed.

Protocols can be further classified based on the mode of operation into

random access protocols, guaranteed access protocols, and hybrid access

protocols.

In a random access protocol, nodes contend for access to the medium. When

only one node makes a transmission attempt, the packet is delivered

successfully. When multiple nodes make a transmission attempt, a collision


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results. Nodes resolve the collisions in an orderly manner according to rules

defined by the contention resolution algorithm (CRA).

In a guaranteed access protocol, nodes access the medium in an orderly

manner, usually in a round-robin fashion. There are two ways to implement

these protocols. One is to use a master-slave configuration, where the master

polls each node and the node sends data in response to the poll. These

protocols are called polling protocols. The second is to operate in a

distributed manner by exchanging tokens. Only the station with the token

can transmit data. Each station, after transmitting data, passes the token to

the next station. These protocols are called token-passing protocols.

Hybrid access protocols blend the best qualities of the above two protocols

to derive more efficient MAC protocols. Most hybrid access protocols are

based on request-grant mechanisms. Each node sends a request to the basestation indicating how much time or bandwidth is required to send the data

currently resident in its buffer. The request is sent using a random access

protocol. The base station then allocates an upstream time slot for the actual

data transmission and sends a grant to the node indicating that time slot.

Depending on the intelligence at the BS, the hybrid access protocols can be

further classified into Random Reservation Access (RRA) protocols and

Demand Assignment (DA) protocols.

In an RRA protocol, the BS has implicit rules for reserving upstreambandwidth. An example of a rule is: A successful request results in a

periodic reservation of an upstream slot. On the other hand, in a DA protocol

the BS controls upstream data transmissions according to their QoS

requirements.

Hybrid access protocols and polling protocols by their mode of operation

require a central node. Therefore they fall into the category of centralized

MAC protocols.

Random access protocols can operate in either architecture. Token passingprotocols could be used as distributed protocols but are not because of

robustness considerations. Due to the time varying nature of the wireless

channel, token loss would be common and token recovery is a huge

overhead. As a result, all proposed distributed MAC protocols are random

access protocols.


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1] Frequency Division Multiple Access (FDMA):

FDMA divides the entire channel bandwidth into M equal subchannels that are

sufficiently separated (via guard bands) to prevent co-channel interference, Fig.

1.4.

Ignoring the small amount of frequency lost to the guard bands, the capacity of

each subchannel is C/M , where C is the capacity associated with the entire channel

bandwidth. Each source node can then be assigned one (or more) of these

subchannels for its own exclusive use.

To receive packets from a particular source node, a destination node must belistening on the proper subchannel. The main advantage of FDMA is the ability to

accommodate M simultaneous packet transmissions (one on each subchannel)

without collision.

However, this comes at the price of increased packet transmission times that

results in longer packet delays. For example, the transmission time of a packet that

is L bits long is M.L/C. This isM times longer than if the packet was transmitted

using the entire channel bandwidth. The exclusive nature of the channel

assignment can also result in an underutilized bandwidth resources when a sourcenodes momentarily lack packets to transmit.


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2] CSMA: Carrier Sense Multipple Access

CSMA= listen before transmit

If channel sensed idle transmit entire packet.

If channel sensed busy, defer transmission

1.

Persistant CSMA: Retry immediately with probability p when channel

becomes idle(may cause instability)

2.Non-persistant CSMA: retry after random interval

Human Analogy: dont Interrupt Others

While the channel is busy, persistent CSMA continuously listens to determine

when the activity ceases. When the channel returns to an idle state, the protocol

immediately transmits a packet.


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Collisions will occur when multiple nodes are waiting for an idle channel. Non-

persistent CSMA reduces the likelihood of such collisions by introducingrandomization. Each time a busy channel is detected, a source node simply waits a

random amount of time before testing the channel again.

This process is repeated with an exponentially increasing random interval until the

channel is found idle.


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This procedure continues until either the packet is sent, or the channel becomes

busy. A busy channel forces a source node to wait a random amount of time beforestarting the procedure again.


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3] Aloha:

Explanation:

A station which has a frame ready will send it.

Then it waits for some time.

If it receives the acknowledgement then the transmission is successful.

Otherwise the station uses a backoff strategy, and sends the packet again.

After many times if there is no acknowledgement then the station aborts the idea of

transmission.

Types:


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(i) Pure ALOHA

In pure ALOHA, the stations transmitframes whenever they have data to send.

When two or more stations transmit simultaneously, there is collision and the frames are

destroyed.

In pure ALOHA, whenever any station transmits a frame, it expects the acknowledgement from

the receiver.

If acknowledgement is not received within specified time, the station assumes that the frame (or

acknowledgement) hasbeen destroyed.

If the frame is destroyed because ofcollision the station waits for a random amount of time and

sends it again. This waiting time must be random otherwise same frames will collide again and

again.

Therefore pure ALOHA dictates that when time-out period passes, each station must wait for a

random amount of time before resending its frame. This randomness will help avoid more

collisions.

Figure shows an example of framecollisions in pure ALOHA.

In fig there are four stations that .contended with one another for access to shared channel. All

these stations are transmitting frames. Some of these frames collide because multiple frames are

in contention for the shared channel. Only two frames, frame 1.1 and frame 2.2 survive. All other

frames are destroyed.


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Whenever two frames try to occupy the channel at the same time, there will be a collision and

both will be damaged. If firstbit of a new frame overlaps with just the last bit of a frame almost

finished, bothframes will be totally destroyed and bothwill have to be retransmitted.

(ii) Slotted ALOHA

Slotted ALOHA was invented to improve the efficiency of pure ALOHA as chances of

collision in pure ALOHA are very high.

In slotted ALOHA, the time of the sharedchannel is divided into discrete intervalscalled slots.

The stations can send a frame only at thebeginning of the slot and only one frame is sent in

each slot.

In slotted ALOHA, if any station is not able to place the frame onto the channel at the

beginning of the slot i.e. it misses the time slot then the station has to wait until the beginning of

the next time slot.

In slotted ALOHA, there is still apossibility of collision if two stations try to send at the

beginning of the same time slotas shown in fig.

Slotted ALOHA still has an edge overpure ALOHA as chances of collision are reduced to one-

half.


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CENTRALIZED MAC PROTOCOLS

In this section, we provide an overview of two of the most prevalent centralized

wireless networks. Cellular telephony is the most predominant form of wireless

system in current operation. Wireless ATM is generating a lot of interest for itsability to deliver broadband multimedia services across a wireless link

Cellular Telephony:

The Advanced Mobile Phone System (AMPS) is an FDMA based cellular

system . The system features 832 full-duplex channels that are grouped into

control and data channels.

Each cell has a full-duplex control channel dedicated to system management,

paging and call setup. There are also 45-50 data channels that can be usedfor voice, fax or data. The base station grants access to a data channel in

response to a call setup request sent on the control channel.

Protocol Flow Chart For Cellular Telephony


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A data channel remains assigned to a specific node until it is relinquished or

the node moves outside the current cell. Access to the control channel isdetermined using a CSMA based MAC protocol.

The base station periodically broadcasts the status of the control channel,

and a node transmits its setup request (possibly in contention with othernodes) when the control channel is idle.

Collisions among setup requests are resolved using randomized

retransmissions. The IS-136 cellular system is a digital version of the AMPSsystem . As such, it operates within the same spectrum using the same

frequency spacing of the original AMPS system.

Each data channel is then slotted and a time frame of 6 slots is used. This

allows the system to support multiple users within a single AMPS data

channel. An assignment of one slot per frame can support a total 6 users

transmitting at a rate of 8.1 kb/s. Higher data rates can be achieved bysuccessively doubling the number of assigned slots up to a maximum of 48.6

kb/s. Channel access remains relatively unchanged from the original AMPSsystem.

The IS-95 cellular system is a CDMA based wireless network in which all

the base stations share a common frequency band with individual

transmissions being distinguished by their PN sequences.

Strict power control ensures that all transmitted signals reach the base station

with the same power level.This type of system requires complex and costly

hardware both within the base stations and nodes.


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Wireless ATM

Asynchronous transfer mode (ATM) is a high performance connection-

oriented switching and multiplexing technology that uses fixed sized packets

to transport a wide range of integrated services over a single network. These include voice, video and multimedia services which have different

QoS requirements. The ability to provide specific QoS services is one of thehallmarks of ATM.

Wireless ATM is designed to extend these integrated services to the mobile

user. Similar to cellular systems, wireless ATM nodes send requests to the

base station for service.

The specific QoS requirements of an application are included in theserequest messages. The base station then collects these requirements, and

allocates the uplink and downlink channels accordingly.


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Thus wireless ATM MAC protocols typically follow a three phase model. In

the first phase, a request message is sent on a random access control channel,usually using a slotted ALOHA protocol.

The second phase involves the base station scheduling uplink and downlink

transmissions according to the QoS requirements of the current traffic mix.Preference is given to delay sensitive data, such as voice packets, whiledatagram services must make due with any remaining capacity.

The third phase involves the transmission of packets according to theschedule created in phase two.


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PRMA protocol flow structure:

Packet Reservation Multiple Access Protocol


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Hybrid Protocols:

Random Reservation Access - Independent Stations Algorithm (RRA-ISA)

RRA-ISA proposes a different access policy, using Independent Stations

Algorithm .

This algorithm tries to distribute access rights (i.e., the right to transmit in a

slot) among nodes so as to maximize the throughput from slot to slot.

The performance of these protocols is compared based on the number of

voice calls that can be supported on a 720 kbls upstream channel.

This has been a common metric to compare RRA protocols. The voicesource model is modeled as a two-state Markov model and includes featuressuch as silence suppression.

The delay shown in the table is the delay experienced by data traffic, whenthe number of calls is 90 percent of the maximum that can be supported by

that particular protocol. FRMA and C-PRMA are modifications to PRMAand they perform better.


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The RRA-ISA protocol performs better than PRMA when the number of

nodes is small. As the number of nodes increases, the performance is verysimilar to PRMA.

Polling Protocol:

In electronic communication, 'polling' is the continuous checking of other

programs or devices by one progam or device to see what state they are in, usually

to see whether they are still connected or want to communicate.

Specifically, in multipoint or multidrop communication (a controlling device with

multiple devices attached that share the same line), the controlling device sends a

message to each device, one at a time, asking each whether it has anything to

communicate (in other words, whether it wants to use the line).


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Conclusion:

Wireless Medium Access control protocols have been extensively studied since

1970s. The design of these protocols must take into consideration issues such as

time-varying channel, bursty errors, and localized carrier sensing, which makes it achallenging problem.

We have classified the protocols first on the basis of their network architecture into

distributed and centralized MAC protocols. These protocols can be further

classified into random, guaranteed, hybrid protocols.

Most MAC protocols have been analyzed assuming error free channels. It is

intresting to study the performance of different MAC protocols in fading-channel

and bursty-error environments. The design of high speed distributed wireless

protocols is another area that needs significant research. It is an additional

challenge to provide Quality of Service in these networks.

wireless mac protocols1

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