Slides

1. Wireless and Mobile Computing
Delivered
By
Khadija Manzoor
Department of CS& IT
University of Gujrat

2. Lecture Agenda
 The aim of this lecture is to provide the reader
with a comprehensive view of the role and details
of the protocols that define and control access to
the wireless channel, i.e., wireless media access
protocols (MAC) protocols.
 Mac layer for Wireless and its issues
 MAC Classifications
 Design Goals

3. Introduction
 In the broadest terms, a wireless network consists of nodes
that communicate by exchanging “packets” via radio
waves.
 These packets can take two forms. A unicast packet
contains information that is addressed to a specific node,
whereas a multicast 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.

4. Introduction
 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 strict 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.

6. Issues
 The main issues need to be addressed while designing a
MAC protocol for ad hoc wireless networks:
• Bandwidth efficiency is defined at the ratio of the bandwidth
used for actual data transmission to the total available
bandwidth. The MAC protocol for ad-hoc networks should
maximize it.
• Quality of service support is essential for time-critical
applications. The MAC protocol for ad-hoc networks should
consider the constraint of ad-hoc networks.
• Synchronization can be achieved by exchange of control
packets.

7. Issues
 The main issues need to be addressed while designing a MAC
protocol for ad hoc wireless networks:
 Hidden and exposed terminal problems:
 Hidden nodes:
 Hidden stations: Carrier sensing may fail to detect
another station.
 Fading: The strength of radio signals diminished rapidly
with the distance from the transmitter.
 Exposed nodes:
 Exposed stations: B is sending to A. C can detect it. C
might want to send to E but conclude it cannot transmit
because C hears B.
 Collision masking: The local signal might drown out
the remote transmission.
 Error-Prone Shared Broadcast Channel
 Distributed Nature/Lack of Central Coordination
 Mobility of Nodes: Nodes are mobile most of the time.

8. Near and far terminals
 Consider the situation A and B are both sending with the same
transmission power. As the signal strength decreases
proportionally to the square of the distance, B’s signal drowns out
A’s signal.
 As a result, C cannot receive A’s transmission.
 Now think of C as being an intermediary for sending rights (e.g.,
C acts as a base station coordinating media access). In this case,
terminal B would already drown out terminal A on the physical
layer. C in return would have no chance of applying a fair scheme
as it would only hear B.

9. Near and far terminals
 The near/far effect is a severe problem of wireless networks. All
signals should arrive at the receiver with more or less the same
strength.
 Otherwise a person standing closer to somebody could always
speak louder than a person further away. Even if the senders were
separated by code, the closest one would simply drown out the
others.
 Precise power control is needed to receive all senders with the
same strength at a receiver.

10. issues
 The combination of network architecture, communication
model, and duplexing mechanism define the general
framework within which a MAC protocol is realized.
 Decisions made here will define how the entire system
operates and the level of interaction between individual
nodes. They will also limit what services can be offered and
define MAC protocol design.
 However, the unique characteristics of wireless
communication must also be taken into consideration.

11. issues
 Here, we explore these physical constraints and discuss
their impact on protocol design and performance.
 Radio waves propagate through an unguided medium
that has no absolute or observable boundaries and is
vulnerable to external interference. Thus, wireless links
typically experience high bit error rates and exhibit
asymmetric channel qualities.
 Techniques such as channel coding, bit interleaving,
frequency/space diversity, and equalization increase the
survivability of information transmitted across a
wireless link.
 However, the presence of asymmetry means that

12. The 802.11 MAC Sublayer
Protocol

13. Design goals of a MAC Protocol
 Design goals of a MAC protocol for ad hoc wireless networks
• The operation of the protocol should be distributed.
• The protocol should provide QoS support for real-time traffic.
• The access delay, which refers to the average delay experienced by
any packet to get transmitted, must be kept low.
• The available bandwidth must be utilized efficiently.
• The protocol should ensure fair allocation of bandwidth to nodes.

14. Design goals of a MAC Protocol
• Control overhead must be kept as low as possible.
• The protocol should minimize the effects of hidden and exposed
terminal problems.
• The protocol must be scalable to large networks.
• It should have power control mechanisms.
• The protocol should have mechanisms for adaptive data rate control.
• It should try to use directional antennas.
• The protocol should provide synchronization among nodes.

15. Classifications of MAC protocols
 Ad hoc network MAC protocols can be classified into
four types:
• Contention-based protocols
• Contention-based protocols with reservation mechanisms
• Contention-based protocols with scheduling mechanisms
• Other MAC protocols
15
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

16. Classifications of MAC Protocols
 Contention-based protocols
• Sender-initiated protocols: Packet transmissions are initiated by
the sender node.
• 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.
• Receiver-initiated protocols: The receiver node initiates the
contention resolution protocol.

17. Classifications of MAC Protocols
 Contention-based protocols with reservation
mechanisms
• 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.

18. Classifications of MAC Protocols
 Contention-based protocols with scheduling
mechanisms
• Node scheduling is done in a manner so that all nodes are
treated fairly and no node is starved of bandwidth.
• Scheduling-based schemes are also used for enforcing priorities
among flows whose packets are queued at nodes.
• Some scheduling schemes also consider battery characteristics.
 Other protocols are those MAC protocols that do not
strictly fall under the above categories.