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 ChannelAccess: Issues and need
Classifications of Multicast Routing
Protocols Synchronous
Asynchronous MAC Protocols
Receiver Sender initiated MAC Protocols
VariousADHOC 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 made
dynamically 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
Wireless
LAN
Laptops
Base station/
access point
Palmtop
radio obstruction
A B C
D
E
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.
 MultipleAccess 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
ChannelAccess 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 forTDMA-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 delay

 A and C cannot hear each other.
 A sends to B, C cannot receiveA.
 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
C
D

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 protocol
1. 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
 HiddenTerminal Problem
 Shortcomings of the RTS-CTS Solution
 Exposed Node Problem

16
• 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
19
 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 (RequestTo 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 (ClearTo 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
22
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
24
 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 nodeA.
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 nodeA 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.

25
b) Shortcomings of RTS-CTS Solution
Multiple CTS messages are granted to different neighbouring nodes,
causing collisions.

b) Shortcomings of RTS-CTS Solution
26
Multiple CTS messages are granted to different neighboring
nodes, 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
27
 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
28
 Use of separate control and data channels
 Power-Aware Multi-Access Protocol with Signaling
(PAMAS)
 Dual BusyTone 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…)
30
 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 nodesA 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.

32
 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
33
 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)

34
 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
35
 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
nodeA. If positive, it returns a CTS (Clear to send ) message
to nodeA. NodeA then subsequently proceeds to send data.
 Examples:
 MACA (MultipleAccess with CollisionAvoidance),
 MACAW (MACA with Acknowledgment), and
 FAMA (FloorAcquisition MultipleAccess).

36
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
37
 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

Multiple Access with Collision Avoidance (MACA)

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 receiver
other node in
sender’s range
RTS
CTS
ACK
data
other node in
receiver’s range

41
Multiple Access with Collision Avoidance (MACA)
 When a node wants to transmit a data packet, it first transmit a RTS
(RequestTo 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.

42
MACA Protocol
The MACA protocol. (a)A sending an RTS to B.
(b) B responding with a CTS toA.

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 fromA
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

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.

 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 (ReadyTo
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.

MACA-BI (By Invitation)

MACA-BI (By Invitation)
 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.

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.

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.

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-
hopAd-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

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Andere mochten auch

Andere mochten auch (6)

Ähnlich wie Lecture 7 8 ad hoc wireless media access protocols

Ähnlich wie Lecture 7 8 ad hoc wireless media access protocols (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Lecture 7 8 ad hoc wireless media access protocols