Lecture 7 8 ad hoc wireless media access protocols

Chandra Prakash

Assistant Professor

LPU

Ad Hoc Wireless Media Access Protocols

Lecture (7-8)

1 Chandra prakash, Lovely Professional University, Punjab

Introduction

Introduction

Problems in Ad Hoc Channel Access: Issues and need

Classifications of Multicast Routing

Protocols Synchronous

Asynchronous MAC Protocols

Receiver Sender initiated MAC Protocols

Various ADHOC MAC protocols


Introduction Ad hoc network does not rely on a pre-existing infrastructure,

such as routers in wired networks or access points in managed (infrastructure) wireless networks.

In Ad-Hoc network Wireless media can be shared and any nodes can transmit at any point in time. This could result in possible contention over the common channel.

Each node participates in routing by forwarding data for other nodes,and so the determination of which nodes forward data is madedynamically based on the network connectivity.

Ad-hoc mobile device can be highly mobile, powerful and heterogeneous.

Routing protocol in ad hoc networks need to deal with the mobility of nodes and constraints in power and bandwidth.


4

Wireless LAN configuration

LAN

Server

WirelessLAN

Laptops

Base station/access point

Palmtop

radio obstruction

A B C

DE

Chandra prakash, Lovely Professional University, Punjab

When do we need MAC?

2 types of links:

Point-to-point.

Shared.

If more than 1 node transmits at the same time:

Collision at receiver!

MAC protocol:

Arbitrate access to medium.

Determine who can transmit when.

Chandra prakash, Lovely Professional University, Punjab5

Expanded Data Link Layer

Sublayers of data link layer:

Logical Link Control (LLC): flow and error control.

Multiple Access Control (MAC): multiple access resolution.


Media Access Protocols

7

MAC protocol is a set of rules or procedures to allow the

efficient use of a shared medium, such as wireless.

Node: any host that is trying to access the medium.

Sender: is a node that is attempting to transmit over the

medium.

Receiver: is a node that is the recipient of the current

transmission.

The MAC protocol is concerned with per-link

communications, not end-to-end. While Routing

Protocol deal with end-to-end communication.


Types of MAC

Channel Access Policy

Random access (or contention-based) No scheduled time for transmissions.

No order for transmissions.

Controlled access Stations coordinate access to channel.

Station only transmits when it has right to send.

Channelization Bandwidth of channel is statically partitioned.


Problems in Ad Hoc Channel

Access- Issues and need

Distributed operation

fully distributed involving minimum control overhead

Synchronization

Mandatory for TDMA-based systems

Hidden terminals

Can significantly reduce the throughput of a MAC protocol

Exposed terminals

To improve the efficiency of the MAC protocol, the exposed nodes

should be allowed to transmit in a controlled fashion without

causing collision to the on-going data transfer

Access delay9 Chandra prakash, Lovely Professional University, Punjab

A and C cannot hear each other.

A sends to B, C cannot receive A.

C wants to send to B, C senses a “free” medium.

Collision occurs at B.

A cannot receive the collision.

A is “hidden” for C.

Hidden Terminal Problem

BA C


Exposed Terminal Problem

A starts sending to B.

C senses carrier, finds medium in

use and has to wait for A->B to

end.

D is outside the range of A,

therefore waiting is not necessary.

A and C are “exposed” terminals

A B

CD


MAC protocol categories

12

Based on operation :

Synchronous protocols: All nodes need to be synchronized.

Global time synchronization is difficult to achieve.

Asynchronous protocols: These protocols use relative time

information for effecting reservations.

Based on who initiates a communication request.

Receiver-initiated protocols

Sender-initiated protocols


Types of protocol1. Synchronous MAC Protocols

In synchronous MAC protocols, all nodes in the network are synchronized to the same time.

Achieved by a timer master broadcasting a regular beacon. All nodes listen for this beacon and synchronize their clocks to the master's

time.

Central coordination is, needed to synchronize time events.

2. Asynchronous MAC Protocols

Nodes do not necessarily follow the same time.

A more distributed control mechanism is used to coordinate channel access

Access to the channel tends to be contention-based.


Contention-Based Protocols

A nodes does not make any resource reservation a priori.

Whenever it receive a packet to be transmitted, it contends

with its neighbor nodes for access to the shared channel.

Nods are not guaranteed periodic access to the channel

Thus can’t provide QoS guarantees to sessions .

E.g. :

pure ALOHA, slotted ALOHA, CSMA, IEEE 802.11, etc

The "listen before talk" operating procedure in IEEE 802.11 is the

most well known contention-based protocol.


Problems in Ad Hoc Channel Access


Shortcomings of the RTS-CTS Solution

Exposed Node Problem



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• Found in contention-based protocols.

A contention-based protocol (CBP) is a communications

protocol for operating wireless telecommunication equipment that

allows many users to use the same radio channel without pre-

coordination.

• Two nodes are said to be hidden from one another (out of

signal range) when both attempt to send information to the

same receiving node, resulting in a collision of data at the

receiver node


Possible Solution


RTS-CTS handshake Protocol

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To avoid collision, all of the receiver's neighbouring nodes need to be informed about the status of the channel.

This can be achieved by using a handshake protocol

reserving the channel using control messages

Resolves hidden node problems

An RTS (Request To Send) message can be used by a node to indicate its wish to transmit data.

The receiving node can allow this transmission by sending a grant using the CTS (Clear To Send) message.

Because of the broadcast nature of these messages, all neighbors of the sender and receiver will be informed that the medium will be busy, thus preventing them from transmitting and avoiding collision.


CSMA/CA: RTS-CTS Solution

With collision avoidance, stations exchange small control

packets to determine which sender can transmit to a

receiver.


RTS-CTS handshake


Shortcomings of RTS-CTS Solution

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The RTS-CTS method is not a perfect solution to the hidden

terminal problem.

Problematic scenario occurs when

1. RTS and CTS control messages are sent by

different nodes.

2. Multiple CTS messages are granted to different

neighboring nodes, causing collisions.


a) Shortcomings of RTS-CTS Solution


a) Shortcomings of RTS-CTS Solution

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Cases when collisions occur and the RTS and CTS control messages are sent by different nodes.

1. Node B is granting a CTS to the RTS sent by node A.

2. This collides with the RTS sent by node D at node C. Node D is the hidden terminal from node B.

3. As node D does not receive the expected CTS from node C, it retransmits the RTS.

4. When node A receives the CTS, it is not aware of any collision at node C and hence it proceeds with a data transmission to node B.

5. Unfortunately,It collides with the CTS sent by node C in response to node D's RTS.


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b) Shortcomings of RTS-CTS SolutionMultiple CTS messages are granted to different neighbouring nodes,

causing collisions.


b) Shortcomings of RTS-CTS Solution

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Multiple CTS messages are granted to different neighboringnodes, causing collisions.

1. Two nodes are sending RTS messages to different nodes at different points in time.

2. Node A sends an RTS to node B. When node B is returning a CTS message back to node A, node C sends an RTS message to node B.

3. Because node C cannot hear the CTS sent by node B while it is transmitting an RTS to node D, node C is unaware of the communication between nodes A and B.

4. Node D proceeds to grant the CTS message to node C.

5. Since both nodes A and C are granted transmission, a collision will occur when both start sending data.


Exposed Node Problem

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Overhearing a data transmission from neighbouring nodes

can inhibit one node from transmitting to other nodes. This is

known as the exposed node problem.

An exposed node is a node in range of the

transmitter, but out of range of the receiver.


Solution to the exposed node problem

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Use of separate control and data channels

Power-Aware Multi-Access Protocol with Signaling

(PAMAS)

Dual Busy Tone Multiple Access (DBTMA) .

Use of Antennas

Directional antennas


Use of antennas.

29

Mobile node using an Omni-directional antenna can result in several surrounding nodes being "exposed“

Thus prohibiting them from communicating with other nodes.

Lowers network availability and system throughput.

Omni-directional antenna radiates radio wave power uniformly in all directions in one

plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to

zero on the antenna's axisChandra prakash, Lovely Professional University, Punjab

Use of antennas (cont…)

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If directional antennas are employed, the problem of network availability and system throughput can be mitigated.

Node C can continue communicating with the receiving palm pilot device without impacting the communication between nodes A and B.

The directivity provides spatial and connectivity isolation not found in omni-directional antenna systems.


MAC protocol categories

31

Based on operation :

Synchronous protocols

Asynchronous protocols

Based on who initiates a communication request.

Receiver-initiated protocols

Sender-initiated protocols

Single-channel sender-initiated protocols: A node that wins the

contention to the channel can make use of the entire bandwidth.

Multichannel sender-initiated protocols:The available bandwidth

is divided into multiple channels.



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Receiver-Initiated MAC Protocols

The receiver (node B) first has to contact the sender (node A),

informing the sender that it is ready to receive (RTR) data.

This is a form of polling, as the receiver has no way of knowing for

sure if the sender indeed has data to send.


Receiver-Initiated MAC Protocols

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Passive form of initiation since the sender does not have to

initiate a request.

There is only one control message used, compared to the

RTS-CTS approach.

Example: MACA-BI (By Invitation)



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Sender-Initiated MAC Protocols

This require the sender to initiate communications by informing

the receiver that it has data to send.


Sender-initiated MAC protocols

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Node A sends an explicit RTS(ready to send) message to

node B (the receiver) to express its desire to communicate.

Node B can then reply if it is willing to receive data from

node A. If positive, it returns a CTS (Clear to send ) message

to node A. Node A then subsequently proceeds to send data.

Examples:

MACA (Multiple Access with Collision Avoidance),

MACAW (MACA with Acknowledgment), and

FAMA (Floor Acquisition Multiple Access).


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Classifications of MAC protocols

Ad hoc network MAC protocols can be classified into three types: Contention-based protocols Contention-based protocols with reservation mechanisms Contention-based protocols with scheduling mechanisms Other MAC protocols

MAC Protocols for Ad Hoc

Wireless Networks

Contention-Based

Protocols

Contention-based

protocols with

reservation mechanisms

Other MAC

Protocols

Contention-based

protocols with

scheduling mechanisms

Sender-Initiated

Protocols

Receiver-Initiated

Protocols

Synchronous

Protocols

Asynchronous

Protocols

Single-Channel

Protocols

Multichannel

Protocols

MACAW

FAMA

BTMA

DBTMA

ICSMA

RI-BTMA

MACA-BI

MARCH

D-PRMA

CATA

HRMA

RI-BTMA

MACA-BI

MARCH

SRMA/PA

FPRP

MACA/PR

RTMAC

Directional

Antennas

MMAC

MCSMA

PCM

RBAR


Existing Ad Hoc MAC Protocols

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Multiple Access with Collision Avoidance (MACA)

MACA-BI (By Invitation)

Power-Aware Multi-Access Protocol with Signalling

(PAMAS)

Dual BusyTone Multiple Access (DBTMA)


1. Multiple Access with Collision Avoidance (MACA)

Aims to create usable, ad hoc, single-frequency networks.

Sender initiated MAC Protocol

Proposed to resolve the hidden terminal and exposed node problems.

Ability to perform per-packet transmitter power control, thus increase the

carrying capacity of a packet radio.

Uses a three-way handshake, RTS-CTS-Data.

Sender first sends an RTS to the receiver to reserve the channel. This blocks the

sender's neighboring nodes from transmitting.

The receiver then sends a CTS to the sender to grant transmission. This results

in blocking the receiver's neighboring nodes from transmitting, thus avoiding

collision.

Sender can now proceed with data transmission.38 Chandra prakash, Lovely Professional University, Punjab

Wireless LAN Protocols

• MACA protocol solved hidden, exposed terminal:– Send Ready-to-Send (RTS) and Clear-to-Send (CTS) first

– RTS, CTS helps determine who else is in range or busy (Collision

avoidance).

sender receiverother node in

sender’s rangeRTS

CTS

ACK

data

other node in

receiver’s range


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When a node wants to transmit a data packet, it first transmit a RTS

(Request To Send) frame.

The receiver node, on receiving the RTS packet, if it is ready to receive

the data packet, transmits a CTS (Clear to Send) packet.

Once the sender receives the CTS packet without any error, it starts

transmitting the data packet.

If a packet transmitted by a node is lost, the node uses the binary

exponential back-off (BEB) algorithm to back off a random

interval of time before retrying. In this each time a collision occurs the node

doubles its maximum back-off windows.

The binary exponential back-off mechanism used in MACA might

starves flows sometimes.

The problem is solved by MACAW.Chandra prakash, Lovely Professional University, Punjab

42

MACA Protocol

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

(b) B responding with a CTS to A.


MACA Has Power control features :

Key characteristic of MACA is that it inhibits a transmitter when

a CTS packet is overheard so as to temporarily limit power

output . This allows geographic reuse of channels.

For example,

If node A has been sending data packets to node B, after some time, A

would know how much power it needs to reach B.

If node A overhears node B's response to an RTS (i.e., a CTS) from a

downstream node C, A need not remain completely silent during this

time.

By lowering its transmission power from the level used to reach node

B, node A can communicate with other neighboring nodes (without

interfering with node B) during that time with a lower power.43 Chandra prakash, Lovely Professional University, Punjab

44

MACA avoids the problem of hidden terminals

A and C want to

send to B

A sends RTS first

C waits after receiving

CTS from B

MACA avoids the problem of exposed terminals

B wants to send to A, C

to another terminal

now C does not have

to wait for it cannot

receive CTS from A

MACA examples

A B C

RTS

CTSCTS

A B C

RTS

CTS

RTS


MACA There is no carrier sensing in MACA so Collision occurs during the RTS-CTS

phase

Each mobile host adds a random amount of time to the minimum interval

required to wait after overhearing an RTS or CTS control message.

In MACA, the slot time is the duration of an RTS packet.

If two or more stations transmit an RTS concurrently, resulting in a collision, these

stations will wait for a randomly chosen interval and try again, doubling the average

interval on every attempt.

The station that wins the competition will receive a CTS from its responder,

thereby blocking other stations to allow the data communication session to

proceed.

Compared to CSMA, MACA reduces the chances of data packet collisions.

Since control messages (RTS and CTS) are much smaller in size compared

to data packets, the chances of collision are also smaller45 Chandra prakash, Lovely Professional University, Punjab

MACAW (MACA wireless )

The binary exponential back-off mechanism used in MACA

might starves flows sometimes.

The problem is solved by MACAW.

The packet header has current back-off counter value of transmitting

node.

It implements per flow fairness as opposed to the per node fairness

in MACA.


MACAW

MACAW is proposed as a series of improvements to the

basic MACA algorithm.

1. Suggest a less aggressive backoff algorithm:

a multiplicative increase and linear decrease(MILD) backoff

mechanism is used.

Increasing BO by 1.5 after a timeout, and decreasing it by 1 after a successful

RTS-CTS pair.

2. proposes that receivers should send an ACK to the sender

after successfully receiving a data message.

3. Propose two related techniques for allowing transmitters to

avoid contention more effectively:

Data sending (DS)

Request-for-request-to-send (RRTS) Concept 47 Chandra prakash, Lovely Professional University, Punjab

NPDU’s DATA Sending (DS) Packets :

A DS packet should be sent after a successful RTS-CTS exchange, just before

the data message itself. The idea is to explicitly announce that the RTS-CTS

succeeded, so that if a node can hear an RTS but not the CTS response, it

does not attempt to transmit a message during the subsequent data transfer

period.

In MACA , an exposed node can received only the RTS and not the

CTS packet

READY for RTS (RRTS):

if a receiver hears an RTS while it is deferring any transmissions, at

the end of the deferral period it replies with an RRTS (“ready for

RTS”) packet, prompting the sender to resend the RTS. 48 Chandra prakash, Lovely Professional University, Punjab

MACA : RTS-CTS-DATA exchange mechanism

MACAW : RTS-CTS-DS-DATA-ACK exchange mechanism49 Chandra prakash, Lovely Professional University, Punjab

2. MACA-BI (By Invitation) MACA-BI uses only a two-way handshake.

No RTS. , the CTS message is renamed as RTR (Ready To

Receive).

Type Receiver initiated MAC Protocol

A node cannot transmit data unless it has received an

invitation from the receiver.

Receiver node does not necessarily know that the source has data

to transmit.

receiver needs to predict if node has data to transmit to it.

The timeliness of the invitation will affect communication

performance.



Packet queue length and arrival rate information is

piggyback into each data packet so that the receiver is aware

of the transmitter's backlog.

For constant bit rate (CBR) traffic, the efficiency of

MACA-BI will be high since the prediction scheme will work

fine. However, will not perform well in case of bursty traffic.

To enhance the communication performance of MACA-BI

under non-stationary traffic situations

a node may still transmit an RTS if the transmitter's queue

length or packet delay exceeds a certain acceptable threshold

before an RTR is issued.

MACA-BI now reverts back to MACA.52 Chandra prakash, Lovely Professional University, Punjab

MACA Vs MACA-BI As MACA-BI only uses a single control message, this turn around

limitation is reduced (i.e., up to 25 microseconds).

MACA-BI is less likely to suffer from control packet collision since it

uses half as many control packets as MACA


3. Power-Aware Multi-Access Protocol

with Signaling (PAMAS) PAMAS is based on the MACA protocol with the addition of a

separate signalling channel.

2 channel interface

Signalling channel :

RTS-CTS dialogue exchanges occur over this channel.

Data channel

PAMAS conserves battery power by selectively powering off

nodes that are not actively transmitting or receiving packets.

In PAMAS, nodes are required to shut themselves off if they are

overhearing other transmissions not directed to them. 54 Chandra prakash, Lovely Professional University, Punjab

Power-Aware Multi-Access Protocol with

Signaling (PAMAS)


Power-Aware Multi-Access Protocol

with Signaling (PAMAS) Each node makes an independent decision about whether to

power off its transceiver.

The conditions that force a node to power off include:

If a node has no packets to transmit, it should power off if one of its

neighboring nodes is transmitting.

If a node has packets to transmit, but at least one of the neighboring

nodes is transmitting and another is receiving, then it should power

off its transceiver.

Use of probing to detect when a node should appropriately

power up. The duration of power-off is critical since it affects delay and

throughput performance.

A node can selectively power down only its data

interface and leave the signalling interface power on.56 Chandra prakash, Lovely Professional University, Punjab

4. Dual Busy Tone Multiple Access

(DBTMA) The single shared channel is further split into 2 sub channels :

Data Channel : Data packets are sent over the data channel

Control channels: control packets (such as RTS and CTS) are sent + busy

tone (BTt (Transmit busy tone) and BTr (Receive busy tone))

One busy tone signifies transmit busy, while another signifies receive

busy.

These two busy tones are spatially separated in frequency to

avoid interference.


Dual Busy Tone Multiple Access

(DBTMA) Uses two busy tone on the control channel BTt (Transmit

busy tone) and BTr (Receive busy tone) are used to notify

neighboring nodes of any on-going transmission.

BT(t) used by the node that transmits data over the data channel

BT(r) used by the node receiving data


Dual Busy Tone Multiple Access

(DBTMA)

59

BTMA was used to solve the hidden terminal problem

BTMA relies on a wireless last-hop network architecture, where

centralized base station serves multiple mobile hosts.

When the base station receives packets from a specific mobile host, it

sends out a busy tone signal to all other nodes within its radio cell.

Hence, hidden terminals sense the busy tone and refrain from

transmitting

Zygmunt Haas from Cornell applied this concept further for use in ad hoc

wireless networks


DBTMA (Cont…) An ad hoc node wishing to transmit first sends out an RTS message.

When the receiver willing to accept the data, it sends out a receive busy tone message followed by a CTS message.

All neighboring nodes that hear the receive busy tone are prohibited from transmitting.

Upon receiving the CTS message, the source node sends out a transmit busy tone message to surrounding nodes prior to data transmission.

Neighboring nodes that hear the transmit busy tone, are prohibited from transmitting and will ignore any transmission received.


Dual Busy Tone Multiple Access (DBTMA)


5. Media Access with Reduced

Handshake: MARCH MARCH exploits the overhearing characteristic

associated with an ad hoc mobile network employing

omni-directional antenna.

MARCH is a sender and receiver initiated protocol

MARCH supports fast data transfer over a multi-hop route

MARCH provides data flow control by using a

sequence number contained inside the CTS packet

Improves communication throughput in wireless multi-

hop Ad-hoc networks by reducing the amount of control

overhead.


MARCH: Media Access with Reduced Handshake


MARCH (cont…) Exploits the broadcast characteristic of omni-directional

antennas to reduce the number of required handshakes.

In MARCH, a node has knowledge of data packet arrivals at its

neighboring nodes from the overheard CTS packets. It can then initiate an

invitation for data to be relayed.

Node C will receive the CTS1 message sent by node B.

This characteristic implies that the overheard CTS1 packet can also be used

to convey the information of a data packet arrival at node B to node C.

Subsequently, after the data packet has been received by node B, node C

can invite node B to forward that data via the CTS2 packet. Hence, the

RTS2 packet can be suppressed here.

RTS-CTS handshake is now reduced to a single CTS (CTS-only)

handshake after the first hop.


For an ad hoc route of n hops, the number of handshakes needed to

send a data packet from the source to the destination is 2n is in

MACA, n in MACA-BI, but only (n + 1) in MARCH.

If n is large, MARCH will have a very similar number of handshakes

as in MACA-BI.

There are n – 1 intermediary nodes between the source and

destination.

MARCH can be viewed as a request-first, pull-later protocol since

the subsequent nodes in the path just need to send invitations to

pull the data toward the destination node.

The RTS-CTS message in MARCH contains:

The MAC addresses of the sender and receiver

The route identification number (RTID)65 Chandra prakash, Lovely Professional University, Punjab

MARCH: Media Access with Reduced

Handshake


Lecture 7 8 ad hoc wireless media access protocols

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Transcript of Lecture 7 8 ad hoc wireless media access protocols