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Wireless LAN

Introduction

Wireless LAN is a half-duplex (simplex) communication medium i.e. the same frequency is used to transmit as to receive. Spread Spectrum was invented in 1942 and started to be used in ISM bands in 1986. More information requires a wider frequency spectrum (bandwidth), e.g. CB Radio uses at 3KHz, FM Radio uses 175KHz and television uses 4.5MHz. In addition, the more complex the transmission and modulation/compression; the more vulnerable to noise is the signal.

In very broad terms, 'Wireless' covers a multitude of technologies. Relating to data networking these include:

Infrared - 1 to 10Mbps, local coverage
Narrowband - 9.6 to 19.6Kbps, local coverage
Spread Spectrum - 1 to 50Mbps, local coverage
Personal Communications Service (PCS) - 9.6 to 192Kbps, metropolitan coverage
2.5G, 3G, 3.5G GSM - 10 to 384Kbps, wide area coverage
Cellular Digital Packet Data (CDPD) - 19.6 to 56Kbps, wide area coverage
Free Space Optics - Laser running from 10Mbps to 1Gbps, metropolitan coverage
Microwave - 192Kbps to 54Mbps, wide area coverage
Satellite - 19Kbps to 1Mbps, wide area coverage

In the late 1980s Wireless LANs (WLAN) manufacturers used proprietary Direct Sequence Spread Spectrum (DSSS) technology over 900MHz radio with data throughput of 860Kbps. This could run at quite large distances, however the problem with this is that there were no open standards and many countries did not allow the 900MHz radio frequency to be used. 900MHz DSSS used one channel at 860Kbps, two channels at 344Kbps or three channels at 215Kbps each.

There are a number of different Wireless LAN technologies in addition to the 802.11 Wireless LAN. These include:

Wireless LAN Interoperability Forum (WLIF) - Proxim and RangeLAN2
Home RF - Frequency Hopping Technology
Home RF 2.0 - disbanded in 2003
HiperLAN - European 5GHz WLAN being superceded by 802.11a
Bluetooth - PAN on 2.4GHz using Fast Frequency Hopping giving up to 1Mbps over short distances.

With the move to 2.4GHz in 1990 the data throughput jumped to 1Mbps and 2Mbps albeit at shorter distances. 1992 saw the beginning of the drafting of the 802.11 standard for wireless LAN technologies. This culminated in 1997 with the 2.4GHz standard which used the following technologies at the physical layer:

DSSS - for 2Mbps and 11Mbps
Frequency Hopping Spread Spectrum (FHSS) - for 1Mbps and 2Mbps
Infra Red

This standard had 1Mbps as a standard data rate and 2Mbps as a Turbo mode. In September 1999 the 802.11a and 802.11b standards were developed and ratified, followed by 802.11g in June 2003.

Frequency Bands

The frequency bands used for WLAN belong to the unlicensed frequency bands. These are:

900MHz - 902MHz to 928MHz - Industrial, Scientific and Medical (ISM)
2.4GHz - 2.4GHz to 2.483GHz (or 2.495GHz in Japan), prone to interference fromcordless phones, microwave ovens, Bluetooth and wireless video
5GHz - 5.150GHz to 5.350GHz and 5.725GHz to 5.825GHz, prone to interference from HiperLAN, Maritime and Satellite frequencies. Unlicensed National Information Infrastructure (UNII)

Some countries use frequencies that are slightly different from those listed.

802.11 Committee

Wireless LAN technologies are based around the 802.11 committee standards which fall into the following categories:

802.11a - 5GHz i.e. the range 5.15GHz to 5.35GHz and 5.725GHz to 5.825GHz, for up to 11Mbps. This is also used by HiperLAN/1 and HiperLAN/2.
802.11b - 2.4GHz i.e. the range 2.4GHz to 2.4835GHz, for up to 11Mbps. This band is available globally and no licence is required.
802.11d - World Mode enables an Access Point (AP) to inform a client which radio setting it should use in order to be legally compliant locally.
802.11e - Quality of Service enhancements to the 802.11 MAC, called Wi-Fi Multimedia (WMM) or Wireless Multimedia Extensions (WME).
802.11f - Inter-Access Point Protocol (IAPP) is a recommended guideline for a protocol for APs to communicate, aid with roaming and with traffic load balancing.
802.11g - data rates greater than 20Mbps at 2.4GHz. Backwardly compatible with 802.11b
802.11h - appends to the MAC layer to provide Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) mechanisms. TPC limits the transmitted power to the minimum required to reach the furthest client and DFS selects the radio channel at the AP to minimise interference. This applies to the 5GHz band is required in Europe.
802.11i - Authentication and Security
802.11j - Channel selection for 4.9GHz and 5GHz in Japan.
802.11k - Defines radio and network information and provides management and maintenance.
802.11n - Currently at Draft 2, 802.11n provides for more bandwidth and greater reliability of transmission of data through a wireless LAN.

The range of 5GHz devices compared with 2.4GHz devices is about 30% less for the same data rates. This is mainly because there is more path loss at higher frequencies.

Wi-Fi Alliance

The Wi-Fi Alliance has been set up to provide interoperability certification for vendors products and the security mechanisms within the 802.11 umbrella of standards.

802.11b

Binary Phase Shift Keying (BPSK) is used to carry data. The carrier has two phase changes, one phase for binary '1' and one phase for binary '0' i.e. one bit per symbol. This leads to a data rate of 1Mbps. Quadrature Phase Shift Keying (QPSK) uses four phases on the carrier leading to two binary bits of data per symbol and a data rate of 2Mbps. To add further complexity Complementary Code Keying (CCK) uses complex functions known as complementary codes that result in data rates of 5.5Mbps and 11Mbps with less multipath distortion.

Direct Sequence Modulation applies a row of chips to a data bit e.g. data bit '1' could have a Chipping Code of '00110011011' applied and data bit '0' could have a chipping code of '11001100100' (the inverted bits). The FCC requires a chip rate of 10 chips for BPSK/QPSK and 8 chips for CCK. 802.11b uses 11 chips which is the Spreading Ratio. The greater the Spreading Ratio the better the chance of recovering the original data, but requires a faster chipping rate. Having a chipping code means that chips may be changed without losing the data. If more than 5 chips changed then the data would change its value. This means that you could lose half the signal before the data becomes corrupted. Chip streams are also called Pseudorandom Noise (PN) codes or Spreading Codes and do not carry data but are involved in the encoding and transmission of data by adding them to the data bit using modulo 2. They run at a much faster rates than the data bits themselves and take up a lot of power as the the high speed oscillators work to send them.

APs running DSSS generally have provision for shifting data rates during transmission of a client stream. As the client distance form the AP increases, the data rate is shifted down in steps from 11Mbps -> 5.5Mbps -> 2Mbps and finally 1Mbps. This can occur on a client by client basis.

802.11b is almost worldwide. In the US there are 11 22MHz channel sets, whereas the European Telecom Standards Institute (ETSI) allocates 13 channels, Israel has 8 and Japan has 14 channel sets available. The ETSI channels are outlined below:

Channel ID	Centre Frequency (MHz)	Spread (MHz)
1	2412	2402 - 2424
2	2417	2407 - 2429
3	2422	2412 - 2434
4	2427	2417 - 2439
5	2432	2422 - 2444
6	2437	2427 - 2449
7	2442	2432 - 2454
8	2447	2437 - 2459
9	2452	2442 - 2464
10	2457	2447 - 2469
11	2462	2452 - 2474
12	2467	2457 - 2479
13	2472	2462 - 2484

The problem with these channels is that the frequencies within the bands overlap because each channel is 22MHz wide. If two adjacent channels e.g. 1 and 2, are operating in close proximity e.g. one AP using channel '1' and the AP using channel '2', then there will be interference. In order to minimise interference you can pick channels that are far enough apart so as not to interfere. In the Americas and ETSI regions you can have up to three non-overlapping channels in one area, giving a theoretical data rate of 33Mbps. The above table illustrates using colours, how you can select channels 1, 6 & 11, or 2, 7 & 12 or 3, 7 & 13 to ensure that there is no interference if locating APs close to one another. Once an AP running channel 1 say, is far enough away so that the signal is very small, you can use the same channel again.

In the US the Federal Communications Commission (FCC) sets the maximum Effective Isotropic Radiated Power (EIRP) at 36dBm. The ETSI maximum EIRP is 20dBm.

802.11g

802.11g is backwardly compatible with 802.11b, it uses the same channels and uses OFDM for data rates 6, 9, 12, 18, 24, 36, 48 and 54Mbps; whereas DSSS is used for 1, 2, 5.5 and 11Mbps. Because OFDM is more efficient than DSSS you get better throughput with 'g' for the same distance as 'b'. Multipath Interference occurs because the same signal is reflected off different surfaces and takes multiple paths to the receiver. As a consequence the signals can occur out of phase at the receiver and have an attenuating impact on the final signal. OFDM spreads the data over different frequencies and so this reduces the chance of a signal being attenuated as two frequencies are unlikely to fade at the same point in the signal at the receiver end.

Mixing 802.11b clients with 802.11g clients reduces overall data throughput due to the differing modulation schemes. Generally the data throughput in either of a, b or g modes is about 50% of the data rate, however mixing modes reduces this substantially.

Due to sideband noise with OFDM, to handle peaks of modulation the power must be backed off. For Complementary Code Keying (CCK):

100mW - 20dBm
50mW - 17dBm
30mW - 15dBm
20mW - 13dBm
10mW - 10dBm
5mW - 7dBm
1mW - 0dBm

And for OFDM:

30mW - 15dBm
20mW - 13dBm
10mW - 10dBm
5mW - 7dBm
1mW - 0dBm

802.11a

802.11a uses Orthogonal Frequency Division Multiplexing (OFDM) and there are 12 channels in the 5GHz bands. The lowest 8 channels currently used lie in the 100MHz bands UNII-1 and UNII-2. In the USA UNII-1 can only be used indoors. Both UNII-2 and UNII-3 can be used indoors or outdoors. 802.11h adds a further 11 channels plus the use of UNII-3. Sidebands can cause interference if you use adjacent channels.

OFDM breaks up each 20MHz high speed carrier into 64 subchannels that are about 300KHz wide. 48 of these subchannels are used for data, while 4 are used for monitoring path shifts, InterCarrier Interference (ICI). The remaining 12 subchannels are called Zero Subcarriers and these are located on the sides as frequency guard bands thereby giving 16.5MHz occupied bandwidth. The central zero is used for DC offset/carrier leak rejection.

The modulation used with sub-channels varies depending on the data rates. Data rates up to 24Mbps have to be supported according to the standard. In the table below 16-QAM means 16-state Quadrature Amplitude Modulation and 64-QAM is 64-state QAM.

Modulation Technique	Data rate per subchannel (Kbps)	Total Data Rate (Mbps)
BPSK	125	6
BPSK	187.5	9
QPSK	250	12
QPSK	375	18
16-QAM	500	24
16-QAM	750	36
64-QAM	1000	48
64-QAM	1125	54

The 5GHz frequency bands are divided up as follows:

UNII-1 - 5.15GHz to 5.25GHz for indoor use only with 6dBi integrated antennae and a maximum power output of 40mW. This has the channels, 34 (5.170GHz), 36 (5.180GHz), 38 (5.190GHz), 40 (5.200GHz), 42 (5.210GHz), 44 (5.220GHz), 46 (5.230GHz), 48 (5.240GHz).
UNII-2 - 5.25GHz to 5.35GHz can use removeable antennae and can transmit at up to 200mW. This has the channels, 52 (5.260GHz), 56 (5.280GHz), 60 (5.300GHz), 64 (5.320GHz).
UNII-3 - 5.725GHz to 5.825GHz can transmit at up to 1W with 6dBi antennae for point-to-multipoint and 23dBi antennae for point-to-point. This has the channels, 5.745GHz, 5.765GHz, 5.785GHz, 5.805GHz.

As with 802.11b/g, data rate shifting can occur as the client moves away from the AP. This occurs on a transmission by transmission basis in the following steps 54Mbps -> 48Mbps -> 36Mbps -> 24Mbps -> 18Mbps -> 12Mbps.

802.11n

The 802.11n standard is designed to provide improvements on existing Wi-Fi technologies, for data, voice and video applications. 802.11n reached Draft 2.0 status in June 2007 and products are now being created to this Draft 2.0 standard. Estimates are that it will be ratified sometime around June 2009. 802.11 divides each of the 2.4 and 5GHz bands into channels. For example the 2.4000-2.4835 GHz band is divided into 13 channels each of width 22 MHz but spaced only 5 MHz apart, with channel 1 centered on 2412 MHz and 13 on 2472 MHz.

Improvements on existing Wi-Fi radio technologies include:

Throughput - as much as 5 times current levels, data throughput typically 74Mbps, may be up to 248Mbps
Reliability - requiring fewer packet retries and therefore taking up bandwidth
Predictability - consistent coverage (up to 70m indoors) and throughput
Compatibility - backwards support for 802.11a/b/g, 802.11n uses both 2.4 and 5GHz frequency bands

In order to achieve these improvements there are three main components that have been introduced in 802.11:

Multiple Input Multiple Output (MIMO) - multiple radios
Packet Aggregation - at the MAC layer multiple packets may be aggregated in a single stream
40MHz Channels - the normal 20MHz channels may be combined to make 40MHz if they are next to each other in the frequency spectrum

Multiple Input Multiple Output (MIMO)

There are three technologies that work within MIMO these are described below:

Maximal Ratio Combining

Most environments create multiple paths that the radio signal traverses as it bounces around reflective surfaces. As a result the actual true signal becomes compromised somewhat by the weaker signals that suffer from propagation delay. The radio receiver on the MIMO AP uses Maximal Ratio Combining on its 3 antennae to take advantage of the multiple signals that each carry an identical copy of the data, by combining the received signals and performing algorithms that increase the sensitivity to the received signal.

A non-802.11n client is able to benefit from this as well as an 802.11n client.

Transmit Beam Forming

The transmitter on a MIMO AP is able to adjust the transmitted signal by modifying the transmitted beam from each of its antenna according to the reflective environment. This ensures an improved in-phase signal at the client antennae. This improves receive sensitivity fro 802.11n and non-802.11n clients.

Spatial Multiplexing

Multiple antennae combine to transmit the same data across all of them, this therefore increases the bandwidth available, however it requires the client to have multiple antennae and be 802.11n compliant.

40MHz channels

802.11n supports both the traditional 20MHz and 40MHz channels. This applies to the AP and doubles the bandwidth for transmission, however it has no bearing on available bandwidth in the spectrum as a whole as this remains constant.

Packet Aggregation

Rather than have a header for each data unit, it is possible now to aggregate data units under one header which in turn releases a little more bandwidth.

For a 1500 Byte frame being transmitted at 300Mbps on 802.11n this would take 220µs which is a vast improvement on 360µs when running at 54Mbps. There is a problem however in that small packets e.g. 64 Byte frames take 181µs at 300Mbps rather than 145µs at 54Mbps. This is due to the increase header size present in 802.11n and will have a bearing on Voice over Wi-Fi deployments.

802.11 Frames

There are three main types of 802.11 frames, the Data Frame, the Management Frame and the Control Frame. The follwoign diagram illustrates the 802.11 frame that uses the Frequency Hopping PHY.

The fields have the following functions:

Synch - this is the preamble which for thr FH PHY is 80 bits. For the DSSS PHY it is 128 bits in length. These bits are alternating '0's and '1's.
SRD - the Start Frame Delimiter is 16 bits 0000 1100 1011 1101
PLW - the PLCP_PDU Length Word is a field of 12 bits that indicate the number of bytes in the packet. This is the first portion of the PLCP header. No matter what the speed of the 802.11 network, the PLCP header is transmitted only at 1Mbps!
PSF - the PLCP Signalling Field uses 4 bits to show the rate of the MAC paylod transmission. Bit 0 is reserved and is always '0'. Bits 1 to 3 are organised to indicate the data rates as follows:
- 000 - 1.0Mbps
- 001 - 1.5Mbps
- 010 - 2.0Mbps
- 011 - 2.5Mbps
- 100 - 3.0Mbps
- 101 - 3.5Mbps
- 110 - 4.0Mbps
- 111 - 4.5Mbps
HEC - the Header Error Check is a 16 bit error check for the PLCP header.

The MAC Data header begins with the Frame Control - this has a number of fields:

Ver - The Protocol Version number is always 0

Type - This is a two bit field that indicates whether the frame is a Management, Control or Data frame. The Subtype field uses four bits to describe the detail of the frame type. The following table lists both the fields' options:

Type (binary)	Main Type	Subtype (binary)	Description
00	Management	0000	Association Request
00	Management	0001	Association Response
00	Management	0010	Reassociation Request
00	Management	0011	Reassociation Response
00	Management	0100	Probe Request
00	Management	0101	Probe Response
00	Management	0110-0111	Reserved
00	Management	1000	Beacon
00	Management	1001	ATIM
00	Management	1010	Disassociation
00	Management	1011	Authentication
00	Management	1100	Deauthentication
00	Management	1101-1111	Reserved
01	Control	0000-1001	Reserved
01	Control	1010	PS-Poll
01	Control	1011	RTS
01	Control	1100	CTS
01	Control	1101	ACK
01	Control	1110	CF End
01	Control	1111	CF End and CF-ACK
10	Data	0000	Data
10	Data	0001	Data and CF-ACK
10	Data	0010	Data and CF-Poll
10	Data	0011	Data and CF-ACK and CF-Poll
10	Data	0100	Null
10	Data	0101	CF-ACK with no data
10	Data	0110	CF-Poll with no data
10	Data	0111	CF-ACK and CF-Poll with no data
10	Data	1000-1111	Reserved
11	Reserved	0000-1111	Reserved

To DS - set if the frame is to be sent by the AP to the Distribution System
From DS - set if the frame is from the Distribution System
More Frag - set if this frame is a fragment of a bigger frame and there are more fragments to follow.
Retry - set if this frame is a retransmission, maybe through the loss of an ACK
Power Mgmt - indicates what power mode ('save' or 'active') the station is to be in once the frame has been sent
More Data - set by the AP to indicate that more frames are destined to a particular station that may be in power save mode. These frames will be buffered at the AP ready for the station should it decide to become 'active'.
WEP - set if WEP is being used to encrypt the body of the frame
Order - set if the frame is being sent according to the 'Strictly Ordered Class' (rarely used)

Duration & ID - In Power save poll messages this is the station ID, whereas in all other frames this is the duration used when calculating the NAV
Address 1 - The recipient station address on the BSS. If To DS is set, this is the AP address; if From DS is set then this is the station address
Address 2 - The transmitter station address on the BSS. If From DS is set, this is the AP address; if To DS is set then this is the station address
Address 3 - If Address 1 contains the destination address then Address 3 will contain the source address. Similarly, if Address 2 contains the source address then Address 3 will contain the destination address.
Address 4 - If a Wireless Distribution System (WDS) is being used (with AP to AP communication), then Address 1 will contain the receiving AP address; Address 2 will contain the transmitting AP address; Address 3 will contain the destination station address and Address 4 the source station address.
Sequence Control - contains the Fragment Number and Sequence Number that define the main frame and the number of fragments in the frame
Frame Body - contains the actual data e.g. IP datagrams and can be up to 2312 octets in size
CRC - 32-bit Cyclic Redundancy Check on the whole 802.11 frame.

Control Frame Types

The Control frame types are illustrated below:

For the RTS Frame the Destination Address (DA) is that of the receipient of the next frame, and the the Source Address (SA) is that of the station transmitting the RTS frame. The Duration time is in microseconds and is how long the next frame will take to transmit plus the time for a CTS frame, an ACK frame and three SIFS One for each of the RTS Frame, the CTS frame and the final ACK frame).

In the CTS Frame the Destination Address is the address that has been copied from the Source Address field in the previous RTS Frame. The duration is reduced by the time taken for one CTS frame and its SIFS interval.

In the ACK Frame the address is the Destination Address that has been copied from the Source Address field in the previous frame (RTS or otherwise). The duration is reduced from the previous frame's duration by the time it takes to transmit the ACK frame and its SIFS interval. If there are more fragments to come (i.e. the More Frag bit is set within the Frame Control field, then the duration is set to '0'.

Management Frames

Beacon

The AP regularly sends a beacon frame to announce itself and send information, such as SSID, timestamp, and other parameters to wireless stations nearby. Wireless NICs are always scanning all 802.11 channels listening to beacons in order to choose an AP to associate with.

Probe Request

A probe request is sent by a station when it needs to obtain information from another station such as which APs are within range.

Probe Response

A station responds to a probe Request with a Probe Response, detailing capability information, supported data rates, etc.

Authentication

The station wireless NIC starts authentication by sending an Authentication frame to the AP containing its identity. With the default Open Authentication, the NIC sends only one authentication frame, and the AP responds with an authentication frame as a response indicating acceptance (or rejection). With Shared Key Authentication, the NIC sends an Authentication frame, and the AP responds with an Authentication frame containing challenge text. The NIC then sends a WEP encrypted version of the challenge text back to the AP. The AP checks the encrypted version by decrypting it and comparing the resultant text with the original challenge. The AP replies to the NIC with an Authentication frame signifying the result.

Deauthentication

A station sends a Deauthentication frame to another station if it wishes to stop encrypted communications.

Association Request

Association with an AP enables allocation of resources and synchronisation with a wireless NIC. The NIC begins the association process by sending an Association Request to an AP. This frame carries information about the NIC (e.g., supported data rates the version of Cisco Compatible Extensions (CCX)) and the SSID of the network. When the AP receives the Association Request, it considers associating with the NIC, reserves memory space and establishes an Association ID for the NIC.

Association Response

An AP sends an Association Response containing an acceptance or rejection notice to the NIC. If it is an acceptance, the frame includes information regarding the association, such as the Association ID and supported data rates. The NIC can use the AP to communicate with other NICs on the network and systems on the Distribution System.

Reassociation Request

If a NIC roams from the currently associated AP and finds another AP with a stronger beacon signal, the NIC will send a Reassociation to the new AP. The new AP then coordinates the forwarding of data frames that may still be in the buffer of the previous AP waiting for transmission to the NIC.

Reassociation Response

The AP sends a Reassociation Response containing an acceptance or rejection notice to the NIC requesting reassociation. The frame includes information regarding the association, such as Association ID and supported data rates.

Disassociation

A station sends a disassociation frame to another station if it wishes to terminate the association. For example, a NIC that is shut down gracefully can send a Disassociation frame to inform the AP that the NIC is powering off. The AP can then relinquish memory allocations and remove the NIC from the association table.

Structure and Operation

Basic Service Set (BSS)

802.11 describes the concept of the Independent Basic Service Set (BSS) which is in effect a Wireless LAN subset, or cell, consisting of the clients and perhaps a Base Station called an Access Point (AP). The most basic BSS is that between just two clients. This is called an Ad-Hoc Network where clients have wireless adapters and can talk to each other i.e. Peer-to-peer, but there is no connection point to a Distribution System (DS) via an Access Point. Basic AP functions such as beaconing and synchronisation can be carried out by the client in Ad-Hoc mode, however functions such as power save and relaying frames cannot be done. A Distribution System is the backbone network connection which can be a wired or a Wireless LAN. A number of BSSs along with the associated DS form the Extended Service Set (ESS). A Portal is a function that provides 'bridging' between a 802.11 LAN and another 802 LAN. This function is typically performed by the AP.

An individual station becomes associated with a particular BSS. This association is dynamic because a station is not fixed to one place as it would be in a wired environment. An Access Point is a particular type of station that provides access to the DS, which is normally a wired LAN.

Station Transmission and CSMA/CA

The MAC layer in 802.11 is similar to the MAC layer in other LAN technologies however in wireless it has additional tasks such as packet retransmission, fragmentation and acknowledgements. The Physical layer covers the technologies DSSS, FHSS, Infra Red etc. used to carry the data over the radio.

There are two provisions for accessing the wireless LAN; the Point Coordination Function (PCF) and the Distributed Coordination Function (DCF). The Point Coordination Function uses a smaller interval designated PIFS in order that the AP may give time-sensitive traffic such as VoIP or Video greater priority. The Distributed Coordination Function uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The difference of a wireless medium c.f. wired medium is that we cannot guarantee that every station will detect if another is sending data (unlike the wired environment where there is an obvious voltage change). For instance, a station 'A' that is detected by an AP may be too far from another station 'B' that is also detected by that AP. This is known as the Hidden Node Problem. For this reason the use of Clear to Send (CTS) has been introduced into the collision detection algorithm.

The steps taken in order for a station to transmit are as follows:

A station wishing to transmit a frame first sends a small control packet called Request to Send (RTS). This contains the source, the destination and the duration of the transaction. This duration time includes the time for the ACK to be sent and received.
The receiving station responds with a control packet called Clear to Send (CTS) with the same duration information
Any station that sees either the RTS or CTS sets its Network Allocation Vector (NAV)

Virtual Carrier Sense

NAV State

The station now physically senses the radio waves to see if another station is transmitting
It waits for a period of time called the Distributed Inter-Frame Space (DIFS) and then transmits
The receiving station (given by the destination MAC address) checks the CRC of the frame and sends back an ACK. This is where the 802.11 MAC differs from other MACs.
If the originating station receives the ACK then it can assume that no collision occurred. If no ACK is received then the transmitting station tries again
If there is a collision then the Exponential Backoff Algorithm is invoked.

When a collision occurs, each station chooses a number of slot times between '0' and a number 'n' from which the station picks a random number. Each station then waits the allotted time called the Contention Window before trying to communicate again. If a further collision occurs then the number 'n' is increased exponentially so that a much greater range of number of slot times are available for this obviously very busy network of talkative stations. This Exponential Backoff Algorithm is invoked on dtection of a collision, after a retransmission has occurred and after a successful transmission too.

Fragmentation and Reassembly

In this radio LAN environment there is more chance of larger frames being corrupted than smaller ones. This is mainly due to radio being less reliable than wired environments, but also due to there being more chance of collisions and interruptions due to frequency hopping that stops the radio for short periods e.g. 20ms. To counter this 802.11 allows for fragmentation and reassembly of frames. A data frame is called a MAC Service Data Unit (MSDU) which can be as large as 2346 octets, however it may be fragmented into smaller MAC Protocol Data Units (MPDU). Each MPDU has a copy of the MAC header and a CRC at the end. The sending station has to receive an ACK for each MPDU before it sends the next ACK. Losing a smaller MPDU through collision or some other interruption causes less disruption that losing the whole MPDU.

Inter Frame Spaces (IFS)

802.11 define four inter frame spaces:

Short Inter Frame Space (SIFS) - separates transmissions within a session between two stations. This is different depending on the Physical layer technology is being used e.g. the value is 28us for FHSS.
Point Coordination IFS (PIFS) - is the time used by the AP to gain access to the radio band before other stations. This is the IFS plus the slot time and amounts to 78us.
Distributed IFS (DIFS) - is the time used when a station wishes to begin a fresh transmission. It is caclulated as the PIFS plus one slot time and is 128us.
Extended IFS (EIFS) - this is used when a station cannot understand the duration time in a received packet. It therefore prevents a collision with a packet within the same data stream.

Joining a BSS

A station joins a BSS using one of two methods:

Passive Scanning - where the station just waits for a periodic Beacon Frame from the AP. This frame provides synchronisation information amongst other things.
Active Scanning - where the station sends a Probe Request Frame to discover an AP. The AP responds with a Probe Response Frame

Synchronisation is important because frequency hopping has to occur seamlessly with no errors in transmissions. The AP sends periodic Beacon frames that contain the value of the AP's clock at the time of transmission. The stations use this clock value to synchronise their own clocks.

Power Saving

By default wireless LANs operate in Constant Access Mode (CAM) which means that the Wireless transmitter is always on listening for traffic. Stations can however go into a sleep mode called Polled Access Mode (PAM) without losing frames, the APs buffer packets due for the station until the station comes out of sleep mode. The AP sends out the information on which stations have frames due to them within frames called Traffic Information Maps (TIM). These TIMs are received by the client and the client wakes just long enough to receive whatever frames have been buffered for it before it goes back to sleep. If broadcast traffic is available then the AP sends a Delivery Traffic Information Map (DTIM). DTIM and TIM timers can be adjusted at the AP.

802.11e - WMM

Wi-Fi Multimedia provides basic QoS for Wireless LANs. Although bandwidth is not guaranteed there are four Access Classes (AC) defined:

Background -
Best Effort
Video
Voice

Physical Architecture

The components of a Wireless network consist of the following:

Client Adapter - for client access to the wireless network in PCI or PCMCIA format, typical indoor range @11Mbps is 40m when running at Transmit power of 100mW or 30m at 30mW.
Access Point (AP) - Central point of shared access for wireless clients, analogous to the shared Ethernet hub in a wired environment. The AP normally has a 10/100BaseT uplink port which can often use inline power to power the AP. The Transmit power of the AP can be varied to reduce cell size. Often, up to 3 APs can be located together to provide 33Mbps aggregate bandwidth, or load balancing based on user numbers, error rates or signal strengths. You can also have a redundant hot standby AP to take over from the primary in case of failure. At this point, filtering based on broadcasts/multicasts and IP/IPX sockets numbers can be set up to maximise the available bandwidth.
Workgroup Bridge - this provides connectivity for a number of wired Ethernet devices to a wireless LAN. You need a hub to connect these devices, and then you connect the hub to the 10BaseT port on the Workgroup Bridge.
Wireless Bridge - This is designed to provide long range external wireless connectivity, typically between buildings and can provide Point-to-point or Point-to-multipoint connectivity. Operating power comes from the inline power within the Ethernet connection. The Wireless Bridge comes with two RP-TNC connectors for antennae.
Antennae - for transmitting and receiving specific frequencies e.g. 2.4GHz. The following antennae are typical:
- Client Antennae
- Access Point Antennae which can also be used to bridge between buildings up to 1km
- Bridge Antennae specifically for bridging between buildings within line of site, mainly with the use of masts. Distances exceeding 18km can be achieved.
Low Loss Antennae Cable - In order to extend the distance between the antenna and the bridge use this cable which typically has a loss of 6.7dB/30m.
Access Control Server - an Access control server that can run RADIUS or TACACS+ for AAA can be used to enhance security within the Wireless network

A typical Wireless Topology could look like the following:

Cisco's Aironet 350 system uses 2.4GHz and Direct Sequence Spread Spectrum (DSSS) technology (where the radio frequency is spread continuously over the specified frequency band) at 100mW transmit power. The modulation used at 1Mbps is Differential Binary Phase Shift Keying (DBPSK), at 2Mbps - Differential Quadrature Phase Shift Keying (DQPSK) and at 5.5 and 11Mbps - Complementary Code Keying (CCK). There can be up to 3 non-overlapping channels allowing 11 x = 33Mbps aggregate throughput if desired. Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) is used at the data link layer, note the difference of 'Collision Avoidance' compared with wired Ethernet LAN technologies which use 'Collision Detection'. This difference exists because there is no way of detecting a collision in the wireless environment.

Cisco also has a 340 series. The differences are that the Power Output is 30mW (c.f. 100mW with 350 series) and it uses a single dipolar antenna by default in the client adapters. The resultant maximum range indoors @11Mbps is 30m (c.f. 39.6m with 350 series).

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