Available in airports, hotels, McDonnells, Coffee Shops, train stations,
Eircom plan to provide service from Payphones on streets.
WLAN standardized by IEEE 802.11. Specifies only the physical
and MAC layers, just like
IEEE 802.3 Ethernet
IEEE 802.5 Token Ring
IEEE 802.4 Token Bus
There are many versions of WLAN
802.11 original standard, 1 and 2 Mbps physical layer,
CSMA/CA MAC adopted 1997
802.11a, enhanced to provide 54Mbps in 5GHz band
802.11b, provides 11Mbps in 2.4GHz, 1999
802.11g, provides 54Mbps in 2.4GHz band, 2004.
802.11? To provide 100Mbps
Advantages of Wireless Operation
No cabling or connector problems. No planning, (?) self configuring
plug & play. Unobtrusive. Robust, reconfiguring. Flexible. Provides
service in areas like airport lounges, coffee shops, train terminals etc
Disadvantages: A lower QoS, lower bandwidth, (1-10 Mbps), higher
error rates (10-4 as opposed to 10-10 for fibre), higher delays.
Cost: WLAN adapters cost approx 10 times more
Not fully standardized at present. Some proprietary solutions.
Restrictions:Wireless products have to conform to national regulation
to minimise interference i.e low power.
Not as secure as wired networks.
WLANs should operate uniformly throughout the world. Same
frequencies. WLAN devices can be carried from one country to another.
Should be low power with power-saving modes of operation.
Should be licence-free, i.e legal in all countries. Should be simple to use.
Wireless LAN Requirements
•High Throughput. Efficient use of available spectrum
•Capacity. May need to support hundreds of nodes in a small area.
•Connection to a fixed backbone LAN
•Service Area. A typical area of coverage is of diameter 100<x<300m.
•Low Power. Mobile devices operate on batteries. Must conserve power
•Robust & Secure. Wireless links are poor quality. Easily eavesdropped
•Must be capable of operating in the presence of other networks
•Handover/Roaming. The MAC protocol must allow roaming over cells
•Dynamic configuration to allow users to join and leave at random.
Ad Hoc and Infrastructure based WLANs
Many WLANs require an infrastructure. The infrastructure provides
access to other networks, forwarding functions, and medium access
control functions. Communication typically takes place only between
the wireless nodes and an Access Point (AP), not directly between
wireless nodes. The AP acts as a bridge to other networks, this way
several interconnected wireless networks appear as one large logical
wireless network, the interconnectivity provided by the bridge.
Cellular ‘phone networks (e.g GSM) are infrastructure wireless
networks. Also, satellite based ‘phones are infrastructure based.
Ad hoc requires no infrastructure. Any node can communicate with
any other within radio range. Ad hoc networks are more complex,
since each node has to implement MAC. Example= Bluetooth.
An ad-hoc network
Ad-Hoc networks interconnected via an infrastructure.
Communication is via the AP, not client-to-client. GSM is
an infrastructure based wireless network, no
mobile-to-mobile communication, except through MSC
IEEE WLAN Objectives
Specify a simple & Robust WLAN with time-bounded and
Specify physical and MAC layers only. Higher layers oblivious to
type of physical network.
MAC layer must be able to accommodate several different physical
Layers, e.g infrared, spread-spectrum etc.
Capable of world-wide operation, use unlicensed bands.
WLANs are normally owned by a private organization, e.g Hotel
Connection of 802.11 and 802.3
BSS: Basic Service Set DS: Distribution System AP:Access Point
Portal ESS:Extended Service Set
Note that IEEE does not specify form of DS. But it does specify its services.
Bridging between 802.11 and 802.3
Association:Establishes an association between a station and an AP within a BSS. The AP transfers the details of the
Station to other APs when the station roams. This is similar to the registration of a mobile user in the HLR in GSM.
Re-association: This is the transfer of a mobile station from one BSS to another.
Disassociation: A notification from a station or from an AP that an existing association is terminated. i.e on shutdown
or on exit from an ESS.
Integration:enables the transfer of data between an 802.11 station and an 802.x (i.e Ethernet) wired LAN.Refers to a
wired LAN that is physically connected to the DS and whose stations may be logically connected to the 802.11 LAN via
the integration service
There are 5 separate physical layer specifications in 802.11.
1. A frequency hopping spread spectrum in ISM 2.4 GHz band.
Provides either 1Mbps or 2 Mbps depending on modulation used.
2. A direct sequence spread spectrum (DSSS), in ISM 2.4GHz band.
Provides either 1Mbps or 2 Mbps. (2 Mbps is optional).
3. An Infrared specification. Either 1Mbps or 2Mbps. About 20m range
4. 802.11a. Uses a wider bandwidth, 5GHz, orthogonal frequency
division multiplexing (OFDM).Provides a range of speeds, 6,9,12
18,24,36, 48, 54 Mbps. Not Spread Spectrum.
5. 802.11b. Uses the 2.4GHz ISM band and delivers either 5.5Mbps
or 11Mbps. DSSS. Same bandwidth as 1. Different modulation (CCK)
6. 802.11.g with OFDM, data rate 54Mbps
The Physical Layer Options
With OFDM, the serial data stream
is split into several parallel streams via a
serial to parallel converter at the sending
site. The reverse happens at the receiver
The Physical Layer (Contd.)
The FHSS specification permits the overlapping of several networks
in the same area, each having a different hopping sequence. It defines
79 channels, each of width 1Mhz,(most countries except Japan, France
Spain). The overall bandwidth of the ISM band is from 2.4-2.4835Mhz
This band is unregulated, there is interference from garage doors,
cordless phones, radio controlled toys, microwave ovens, Bluetooth. It
uses Gaussian Frequency Shift Keying (GFSK) with either 2/4 states
to deliver either 1or 2 Mbps.
The DSSS, separates networks from each other with codes, not
frequency. It offers a robust network in face of interference, and
insensitivity to multipath effects.
It uses Differential Binary Phase Shift Keying (DPSK) to yield 1Mbps
and Quadrature Binary Phase Shift Keying (QPSK) to yield 2Mbps.
The LLC Layer
•Ethernet offers best effort transport. There is no error or flow control.
•The LLC (IEEE protocol) hides the differences between the various
kinds of 802.x networks by providing a common format and interface
to the network layer. LLC is based on HDLC. It provides error and flow control.
•LLC provides 1)unreliable datagram service 2)ack’ed datagram service 3)Reliable
connection oriented service, the user chooses most appropriate service.
•The frame header contains a source and destination access points addresses,
(SSAP & DSAP), seq numbers and acks similar to HDLC.
•When used over 802.3 Ethernet, the combination provides a reliable flow
controlled transport service
The MAC in Ethernet (CSMA/CD)
The MAC is intended to reduce the amount of collisions. Collisions
occur because of the non zero propagation delay of a stations
transmission until it is observable by all other stations on the network.
Before accessing the network, a station is required to ‘listen’ or sense the
network to see whether it is idle or busy. If it is busy, the station
defers, and tries again later. If it is idle, then the station transmits its
frame, while at the same time monitoring the network for possible
collisions which can occur during the vulnerable period within the
If the vulnerable period expires without a collision, then the station
has acquired the network, and will succeed in transmitting its frame.
If a collision occurs, each station backs off for a random period of
time after which they will retry.
There are some differences between a fixed LAN and a wireless LAN.
1.Except during the collision window, in a wired LAN, all stations can ‘hear’ the transmissions of
every other station, and therefore will know not to interfere. This is generally not the case in
wireless LANs. Radio waves propagate omnidirectionally from the source and attenuate ~1/d2.
Therefore it is possible that some stations may not hear the transmissions of other stations, i.e
they may be out of range.
Suppose A wishes to transmit to B, and suppose further that B is already receiving from another
station C which is out of range of A. A can sense the medium and find it idle, and begin to
transmit to B. It’s transmission will corrupt C’s transmission to B, i.e a collision. A cannot detect
this collision. This is a case of a collision occurring at the destination.
The conclusion is that CSMA with collision detection (CD) does not work in a wireless LAN.
2.Detecting a collision at the source is also difficult in a wireless LAN. It would require radio
equipment which could receive its own transmission, i.e to be able to send and receive
simultaneously. For cost reasons, in order to keep the equipment cheap, the
standard does not require the ability to send and receive simultaneously.
3 For the above reasons, CSMA with collision avoidance is used in wireless LANs. (CSMA/CA).
The Hidden Station Problem
The MAC for 802.11 is different from Ethernet. Hidden station problem. Low power
levels means that not all stations in a cell (BSS) are within radio range of each
In (a), C is transmitting to B. If A senses the medium it will find it idle, and start
transmitting to B. Collision. A’s transmission will corrupt C’s at B. C is hidden to A
and vice versa. The exposed station problem (b), B does not transmit to C since it
finds the medium busy (A is transmitting to D, not shown but out of range of C). It
could since A’s transmission is out of range of C. B is exposed to A. Delay not
THE MAC LAYER
There are 3 separate MAC Layer specifications. Each relies on a
‘Clear Channel Assessment’(CCA) from the physical layer to
determine if the medium is idle. Each Physical layer specification is
required to provide a CCA to the MAC.
1. A mandatory basic CSMA/CA. Supports broadcasting.
2. An optional method, an enhancement of the basic CSMA/CA to
cope with the hidden station problem.
3. A contention free polling MAC for a time bounded service.
3. The methods 1 and 2 above are referred to as DCF, i.e Distributed
Coordination Function. Method 3 is referred to as PCF, i.e Point
PCF requires a fixed infrastructure (polling performed by an AP). It is
not available in ad-hoc networks.
Inter Frame Spacing
1. The MAC Basic CSMA/CA Protocol (DCF)
If the medium is idle, and remains idle for interval DIFS, a node can access the medium at once.
If the medium is busy, a node has to wait for DIFS. If still idle after DIFS, a node calculates a
random backoff time interval within a contention window. (A slotted window, window size
determined by propagation RTT, transmitter delay, and other physical characteristics).
If, on expiry of the timer, the medium is idle, it can be seized at once.
If, on expiry of the timer, the medium is busy, then the node stops it’s timer, waits for the channel
to become idle again for DIFS, and starts the timer again. As soon as the timer expires the node
seizes the channel. (The timer has not been initialized, but counts down from the stop-value.
Deferred stations do not choose a randomized waiting time again, but count down). Longer
waiting stations have advantage over newly contending stations, they only have to wait for the
remainder of the backoff timer interval from the previous cycle.
When the source transmits a frame, a timer is started. If an acknowledgement is not received in
the lifetime of the timer, it is assumed there was a collision, and a retransmission is started.
DIFS: Distributed Coordination Interframe Spacing
PIFS: Point Coordination Interframe Spacing
If the medium is busy, all access attempts are deferred.
When the medium becomes idle, an interval of SIFS (Short
Inter Frame Spacing) must elapse before the next
transmission. This is the shortest delay, i.e the highest
priority. Used for returning acks.
MAC CSMA/CA Basic Protocol (Contd.)
The MAC Basic protocol tries to adapt the size of the contention
window to minimise the extent of collissions. Under heavy load, there
can be significant collisions. The initial size of the contention window
is 7 slots. Each time a collission occurs, its size is doubled to a max of
255. (Values are 7,15,31,63,127,255). Under light load conditions, a
Small CW results in less delay in accessing the medium.
For unicast transmission, an additional feature involving an ACK is
provided to cope with poor quality radio links. After receiving a frame,
the receiver returns an ACK, after waiting only SIFS. No other station
can access the medium within SIFS to cause a collission. If no ACK is
returned, sender retransmits. For a retransmission, sender has to
compete with other users, no special priveliges. Max limit on number of
retransmissions. Failure is reported to higher layers.
It is more efficient to correct failed transmissions at this level than wait
for recovery via upper layers e.g TCP. (See diagram 7.12 next slide.)
2. DCF with RTS/CTS
The standard defines an optional additional mechanism using two
Control packets, Request To Send (RTS) and Clear To Send (CTS).
After waiting for DIFS plus a random backoff time the sender issues
RTS. The RTS identifies the intended receiver, plus the duration
necessary to transmit the whole data frame and its related ack. Every
node that receives this RTS sets its Network Allocation Vector (NAV)
to reflect the duration for which it must not attempt to access the
medium. This is an internal timer set by those nodes which have sensed
the RTS. When the receiver answers after SIFS with a CTS, this packet
contains the duration required for data transmission also. All stations
within reach of the CTS-issueing node (possibly a different set) set their
NAV to allow the data transmission to take place. The sender sends the
data after SIFS. An ACK is returned after SIFS if successful. NAVs are
cancelled, cycle starts again. Collissions can only occur on sending RTS.
The use of a returned Ack after SIFS in Unicasting.
2. Enhanced DCF with RTS/CTS
A situation where a station can sense the presence of two other stations,
but the other two stations cannot sense each other, there can be
collisions. This is called the hidden station problem. Also there is the
exposed station problem. To cope with this problem an enhanced MAC
3. The PCF MAC
The PCF provides a contention-free medium access mechanism via
polling. It’s use requires an Access Point (AP). Not in ad hoc networks.
Works alongside DCF.
Time is split into ‘superframes’. A superframe slot contains a
contention-free period plus a contention period. During the contention-
free period, nodes (which have signed up for polling via a special
beacon frame) are polled. After the polling cycle is complete, a special
end marker CFend is transmitted to indicate the start of the contention
period. The length of the contention-free period depends on how many
nodes have to be polled. Polling has priority over DCF.
To decrease the probability of corruption to transmitted frames, the
MAC specifies a fragmentation mechanism. The frame size is adjusted
by the MAC to reflect the link error rate.
The sender sends RTS (after DIFS) to reserve the medium. It includes
the duration parameter for first fragment and related ack. Nodes
within reach set their NAVs. The CTS asserts the duration parameter
also, nodes within reach set their NAVs. The first fragment is sent
The first fragment also includes a duration parameter in respect of
the next fragment. This serves to reserve the medium for 2nd fragment
and related ACK. An ACK is returned with a duration parameter for 2nd
Fragment. Stations within reach set their NAVs, the medium is reserved
For 2nd fragment . The last fragment does not include a duration, so
medium reservation is terminated. (See diagram in next slide 7.14)
THE MAC FRAME
Frame Control:Protocol version, frame-type (data,control,mgt), frag?
2 DS bits relating to the 4 addresses.
Duration: Used with the RTS/CTS reservation scheme.
Addresses: standard MAC addresses, source, source AP, dest AP and
destination node. All 48 bits.
Sequence Control: A sequence number to deal with duplicates. A four
Bit field for fragmentation control, 12 bits for frame sequencing.
Data: the payload of the frame from a higher layer. Max 2312 bytes.
CRC: 4 octet checksum.
Each 802.11 node maintains an internal clock. Synchronization of
clocks is required for power management and for coordination of
PCF, hopping sequence etc.Performed by AP in infrastructure
Within a BSS, timing is provided by the quasi periodic transmission
of a beacon frame, which contains a timestamp, power mgt data, and
roaming data.Beacon frame transmission may be delayed due to
busy medium, hence quasi periodic.
For ad-hoc networks, synchronization is more difficult. Here each
node transmits a beacon signal based on its own clock. There are
fixed beacon transmission intervals. Standard Access rules apply to
beacon transmission, so generally only one beacon signal survives.
All stations base their clock on the received beacon signal.
Wireless devices are normally battery powered.Power saving modes
are required. Switch off transceiver when idle. A station has 2 states.
Sleep and awake. When a sender wishes to communicate with a
sleeping station it has to buffer the transmission until the station wakes
up. Sleeping stations have to wake up periodically, and stay awake for
a minimum time. During this time all senders announce the
destinations of their bufferred data. When a station discovers that it is
a target destination of some bufferred data, it must stay awake until it
receives the data.
Waking up a the right time requires the Timing Synchronization
Power management in infrastructure networks is easier than in ad-hoc
networks. The AP buffers all data for sleeping nodes. With every
beacon frame sent by the AP, a Traffic Indication Map (TIM) is
transmitted which contains a list of station for which there is buffered
data (unicast) in the AP.For multicast/broadcast, station must stay
Typical networks within buildings require more than one AP. The range
of an AP is (10-20)m. When a user is mobile the station moves from
one AP to another. For uninterrupted service a handover has to take
place from AP to AP.
When the current link quality is too poor, the station starts scanning for
another access point. In ad hoc networks, it can lead to the creation of
a new BSS. Passive scanning involves listening for other networks.
Active scanning involves sending a probe on each channel, and waiting
for a response. Based on signal strength, the station selects an AP. It
sends an association request to the selected AP. The AP responds with
an association response message.
The AP accepting an association request indicates the new station in its
BSS to the DS. The DS updates its DataBase to reflect the current
location of each wireless station. Also the DS innforms the old AP that
the station is no longer in its BSS.
A HLR for 802.11.
WLAN flexible, lower quality than wire based.
Ad hoc versus Infrastructure based
WLAN concerned with Phy and MAC only. LLC mediates with
Unlicenced in 2.4 GHz band, available worldwide
Large network via a DS.
Phy based on either optical (infraRed) or radio. (FHSS or DSSS or OFD
Roaming via AP
IEEE 802.11b allows either 5.5 or 11 Mbps
IEEE 802.11a allows up to 54 Mbps in 5GHz range
Hidden Station and Exposed Station Problems
3 separate MACs. CCA always available from any of 5 Physical layers
1. Basic. No consideration of hidden Sation problem
2. Enhanced. Takes care of Hidden station problem via RTS/CTS
3. Contention free, via polling by AP.
MAC does fragmentation, synchronization and power management
1. Mobile Communications. Jochen Schiller. Addison Wesley.
2. Wireless Communications and Networks.William Stallings.
Prentice Hall. ISBN=0-13-040864-6
3. Computer Networks. Andrew S. Tanenbaum. 4th edition.
Prentice Hall. ISBN=0-13-038488-7
4. IEEE 802.11 Wireless LAN Working Group Web Site.