Study On Wi Fi And Communications Theory Computer Science Essay

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Wi-Fi, which stands for Wireless Fidelity, is a means of wireless communication technology which uses radio waves to provide Internet and network connections between different nodes. The term Wi-Fi is a trademark of the Wi-Fi Alliance, which is an association set up in 1999 that promotes Wireless LAN technology, establishes and enforces standards for interoperability between Wi-Fi devices and backward compatibility. On the other hand the IEEE committee provides a set of standards known as IEEE 802.11 to standardize wireless LANs. In fact, Wi-Fi is also referred to as 802.11, which comes up in several different variants of protocols, to be discussed later.

Today the use of Wi-Fi has expanded considerably, being used by millions of people using various devices such as personal computers, laptops, mobile phones, tablet PCs, cameras among many others. It is also used in various environments including homes, office buildings, airports, universities and coffee shops. It is worth noting that not all IEEE 802.11-compliant devices are submitted to be certified by the Wi-Fi alliance, mostly due to the costs involved with the ceritification process. However, this does not mean that these devices are incompatible with other Wi-Fi devices. Often manufacturers use the Wi-Fi trademark to officially certify their Wi-Fi compatible device to enhance competitivity of the product and for marketing purposes.http://a.fsdn.com/gc/wp-content/uploads/2010/10/wifi.jpg

The Wi-Fi Alliance also owns and controls the Wi-Fi CERTIFIED logo, a registered trademark, which is permitted only on equipment which has

Figure : The Wi-Fi CERTIFIED logopassed testing. The testing is rather rigorous and tha main focus of criteria is interoperability. This ensures that different devices from different manufacturers are able to interoperate in a wide variety of configurations. Other types of testing include backward compatability to ensure that new products are able to work with existing ones; and security measures, which will be discussed later.

Since interoperability is the primary target of certification, it is ensured by three types of tests:

Compatability: This ensures that certified equipment is able to connect to other certified equipment. The testing is done using multiple devices from different vendors.

Conformance: This type of test ensures that certified devices adhere to specific and critical elements defined by the IEEE 802.11 standard, such as the performance of a device in security-related issues.

Performance: The product is verified to ensure that it meets the minimum performance requirements for a good user experience as established by the Wi-Fi Alliance.

Basic Operation

Since Wi-Fi is radio wave dependent, all devices must be equipped with short-range radio transmitters and receivers to be able to communicate. Wireless networking can operate in two modes; in the presence of a base station and without. In the former case all communication is done via the base station, known as an access point. This type infrastructure is used in an environment to provide what is known as a hotspot, such as office areas. The WLAN equipment can be installed instead of a wired system, and can provide considerable cost savings and congestion due to physical connections. A backbone wired network is still required and is connected to a portal, which connects the 802.11 system to the outside world. The wireless network is then split up into a number of cells, each serviced by a base station (access point).

In the latter case, computers communicate with each other directly, without the need of an access point. This is called ad-hoc networking and is considered very popular especially in multiplayer handheld devices. Therefore in this case there is no need for access points and special algorithms within the protocols are used to enable one of the peripherals to take over the role of master to control the network with the others acting as slaves.

When working on the IEEE 802.11 standard, the committee had various challenges it needed to tackle: finding a suitable frequency range in which to operate, acknowledging the fact that radio waves have a finite range, security and privacy issues, social and economical factors. One of the problems which had to be solved is the fact that a radio wave can be reflected off solid objects and may therefore be received multiple times. Such interference is called multipath fading.

After some work, the committee came up with a standard in 1997 that addressed these and other concerns. The wireless ran LAN it came up with ran at either 1 Mbps or 2 Mbps. However it was criticised for being too slow and work began on faster standards. Various types of protocols have been described since then, each with its own benefits and setbacks. These will be discussed later in another section.

Carrier Sense Multiple Access - Collision Avoidance (CSMA/CA)

802.11 utilizes a multiple access protocol called CSMA/CA. Other protocols such as CSMA/CD (CSMA with Collision Detection) have been adopted successfully to other standards such as the Ethernet (IEEE 802.3). However when using Wi-Fi, much of the sent energy is lost in transmisson, therefore a collision may add only 5 to 10% additional energy, which is not suitable for collision detection. Hence it was therefore decided that due to the difficulty of detecting collisions in wireless networks, a different approach should be taken in this case; that of avoiding collisions in the first place as much as possible. In order to achieve this, CSMA/CA uses three particular strategies: Interframe Space (IFS), Contention Window and Acknowledgments.

When the sending station has some data to send, it first senses the channel to check if it is idle. If in the positive, the transmission is still deferred and waits a period of time called IFS. This is because even though the channel may appear idle when it is sensed, a distant station may have already started transmitting. If after the IFS the channel is still idle, the station proceeds to the contention window, which is an amount of time divided into slots. A station which is ready to start sending, chooses a random number of slots as its wait time. The number of slots in the window changes according to the binary exponential back-off strategy. The station will then need to sense the channel after each time slot. However, if the station finds the channel busy, it does not restart the process but it rather just stops the timer and restarts it when the channel is sensed as idle. Finally, with all these precautions, there still may be the risk of a collision which results in destroyed data. Furthermore, the data may also be corrupted during transmission. A further positive acknowledgment from the receiver and the timer ensure that the sent frame has arrived successfully to the receiver.

Figure : CSMA/CA flowchart describing its basic operation

Protocols

As remarked in the introduction, the 802.11 has different variant protocols; the major ones are summarised in the table below:

Table

802.11

Protocol

-

A

B

G

N

Date of standard approval

Jun 1997

July 1999

July 1999

June 2003

Not yet ratified

Maximum data rate (Mbps)

2

54

11

54

72.2

Modulation

DSSS, FHSS

OFDM

CCK or DSSS

CCK, DSSS, or OFDM

CCK, DSSS, or OFDM

RF Band (GHz)

2.4

5

2.4

2.4

2.4 or 5

Number of spatial streams

1

1

1

1

1 - 4

Channel width (MHz) nominal

20

20

20

20

20

Approximate Indoor range (m)

20

35

38

38

70

Approximate Outdoor range (m)

100

120

140

140

250

IEEE 802.11 (Legacy Mode)

The original version of the standard IEEE 802.11 was released in 1997 and clarified in 1999, but is today obsolete. Back then it ran at two net bit rates of either 1 or 2 megabits per second  (Mbit/s) and an additional forward error correction code. It specified three alternative physical layer technologies : diffuse infrared operating at 1 Mbit/s, frequency-hopping spread spectrum operating at 1 Mbit/s or 2 Mbit/s and direct-sequence spread spectrum operating at 1 Mbit/s or 2 Mbit/s. The latter two radio technologies used microwave transmission over the Industrial Scientific Medical frequency band at 2.4 GHz, which is part of the frequency spectrum that does not require licensing. This protocol used two types of modulation techniques; FHSS and DSSS.

Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. FHSS uses 79 channels, each 1 MHz wide, starting at the low end of the 2.4 GHz Industrial, Scientific and Medical (ISM) band. A pseudorandom number generator is used to produce the sequence of frequencies hopped to.The stations will hop to the same frequencies simultaneously as long as they use the same seed to the pseudorandom number generator and stay synchronized in time. The amount of time spent at each frequency, called the dwell time, is an adjustable parameter but must be less than 400 msec. The advantages that FHSS offers are that the randomization process provides a fair way to allocate spectrum in the unregulated (license free) ISM band, and it also provides a means of security since an intruder cannot pick on transmissions without knowing the hopping sequence or dwell time. Another advantage is that it offers good resistance to multipath fading and is also relatively insensitive to radio interference. A disadvantage is its low bandwidth.

On the other hand, DSSS phase modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. DSSS uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

Legacy 802.11 with direct-sequence spread spectrum was rapidly phased out due to complaints of being too slow and was popularized by 802.11b.

IEEE 802.11a

802.11a is considered to be the first of the high-speed wireless LANs and uses OFDM (Orthogonal Frequency Divison Multiplexing) to deliver up to 54 Mbps in the wider 5 GHz ISM band. Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed. Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented.

In OFDM, 52 frequencies are used, 48 for data and 4 for synchronization. Splitting the signal into many narrow bands has some key advantages over using a single wide band, including better immunity to narrowband interference and the possibility of using noncontigous bands. A complex encoding system is used, based on phase-shift modulation for speeds up to 18 Mbps and on QAM (Quadrature amplitude modulation) above that. OFDM has a good spectrum efficiency in terms of bits/Hz and good immunity to multipath fading.

This standard is able to transfer data with raw data rates up to 54 Mbps, and has a good range, although not when operating at its full data rate. The 802.11a standard uses basic 802.11 concepts as its base, and it operates within the 5GHz ISM band.

IEEE 802.11b

802.11b is the slowest and least expensive standard and is considered to have a better range than 802.11a. 802.11b has a maximum data rate of 11Mbit/s and operates in the 2.4 GHz ISM band. It is a direct extension of the modulation technique defined in the original standard and uses the same CSMA/CA media access method as well. Compared to the original standard, it has a much higher throughput (5.9 Mbit/s using TCP and 7.1 Mbit/s using UDP). Due to this reason and the relatively low cost, 802.11b was regarded as the definitive wireless LAN technology and was implemented in many home and small business networks. In fact, this standard was approved before 802.11a and got to the market first.

802.11b is a direct extension of DSSS modulation technique defined in the original standard but it actually incorporates Complementary Code Keying (CCK) as its modulation technique. This is sometimes refereed to as HR-DSSS (High Rate Direct-Sequence Spread Spectrum). CCK was adopted to supplement the Barker code in wireless digital networks to achieve data rate higher than 2 Mbit/s at the expense of shorter distance. The Barker code was implemented originally in the DSSS and is defined as a sequence of N values of +1 and −1, aj for j = 1,2,.....,N such that

for all 1 ≤ v < N.

Barker, R. H. (1953). "Group Synchronizing of Binary Digital Sequences". Communication Theory. London: Butterworth. pp. 273-287.

In CCK there is a shorter chipping sequence (8 bits versus 11 bits in Barker code) that means less spreading to obtain higher data rate but is more susceptible to narrowband interference resulting in shorter radio transmission range. Beside shorter chipping sequence, CCK also has more chipping sequences to encode more bits (4 chipping sequences at 5.5 Mbit/s and 64 chipping sequences at 11 Mbps) increasing the data rate even further. The Barker code, however, only has a single chipping sequence.

The main disadvantage of 802.11b is that it suffers from interference from other products operating in the 2.4 Ghz band which include microwave ovens, Bluetooth devices, baby monitors and cordless telephones.

IEEE 802.11g

802.11g was the third modulation standard for Wireless LAN. It uses the OFDM modulation method of 802.11a for speeds of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps but for 5.5 and 11 Mbps it uses Complementary Code Keying (CCK), while for 1 and 2 Mbps it uses DSSS. It operates in the narrow 2.4 GHz ISM band along with 802.11b and in theory it can operate at up to 54 Mbps or about 19 Mbit/s net throughput. 802.11g hardware is also fully compatible with 802.11b hardware. 802.11g was fully ratified in June 2003, however by January of the same year, this standard was already adopted by a number of consumers due to the high speeds offered. Like the 802.11b, it suffers from interference due to the crowded 2.4GHz range. Due to its popularity, it has also caused usage-density problems because crowding in urban areas.

IEEE 802.11n

802.11n is another standard which is able to provide much better performance and be able to keep pace with the rapidly growing speeds provided by technologies such as Ethernet. A number of new features that have been incorporated into the IEEE 802.11n standard to enable the higher performance. The major innovations are summarized below:

Changes to implementation of OFDM

Introduction of MIMO

MIMO power saving

Wider channel bandwidth

Antenna technology

Reduced support for backward compatibility under special circumstances to improve data throughput

It is worth noting that all these innovations have made the system much more complex to produce, however much of these can be incorporated in chipsets which allows the added cost increase to be absorbed by the mass production efficiency of these chipsets. This has made 802.11n an instant success despite not being officially ratified. We will now take a brief individual look at each innovation.

OFDM implementation: A modification to OFDM was done to improve the data throughput of the single signal path. This lead to the data rate being increased from 54 Mbps achieved for 802.11a and g to 65 Mbps.

Use of MIMO in IEEE 802.11 n: MIMO (Multiple Input Multiple Output) is a technique that exploits multipath propagation. It involves the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput by using a technique known as spatial division multiplexing. The data is split into a number of what are termed spatial streams and these are transmitted through separate antennas to corresponding antennas at the receiver. Doubling the number of spatial streams will double the throughput, enabling a far greater utilization of the available bandwidth. The 802.11n standard allows for up to four spatial streams.

IEEE 802.11 n power saving: One disadvantage with using MIMO is that since more transmitters and receivers need to be supported, more current is drawn and therefore power is increased. However this is counteracted by increasing the efficiency of operation. In wireless networks, data is normally transmitted in "bursts" which means that there are long periods of time when the system remains idle or running at a very slow speed. Therefore during this time, MIMO is not required and the circuitry can be held inactive so that it does not consume power.

Increased bandwidth: With this new standard, there is an option for the system to be used at 40MHz bandwidth rather than 20MHz.  However this will result in having less channels that can be used for other devices and therefore the choice of bandwith is done according to the number of devices in use.

Antenna technology for 802.11n: For 802.11n, the antenna associated technologies have been significantly improved by the introduction of beam forming and diversity.

Beam forming focuses the radio signals directly along the path for the receiving antenna to improve the range and overall performance. A higher signal level and better signal to noise ratio will mean that full use can be made of the channel.

Diversity uses the multiple antennas available and combines or selects the best subset from a larger number of antennas to obtain the optimum signal conditions. This can be achieved because there are often surplus antennas in a MIMO system. As 802.11n supports any number of antennas between one and four, it is possible that one device may have three antennas while another with which it is communicating will only have two. The supposedly surplus antenna can be used to provide diversity reception or transmission as appropriate.

http://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Prinzip_MIMO.png/280px-Prinzip_MIMO.png

Figure : Different variants of antennae technologies (MIMO at the bottom)

Backward compatibility switching: Although 802.11n provides backward compatibility for devices in a network using earlier versions of 802.11, this adds a significant overhead to any exchanges, thereby reducing the data transfer capacity. To provide the maximum data transfer speeds when all devices in the network at to the 802.11n standard, the backwards compatibility feature can be removed. When earlier devices enter the net, the backward compatibility overhead and features are re-introduced. As with 802.11g, when earlier devices enter a net, the operation of the whole net is considerably slowed. Therefore operating a network in 802.11n only mode offers considerable advantages.

Wi-Fi Security

The 802.11 standard prescribes a data link-level security protocol called WEP (Wired Equivalent Privacy), which is designed to make the security of a wireless LAN as good as that of a wired LAN. Since the default for wired LANs is no security at all, this goal is rather easy to achieve.

When 802.11 security is enabled, each station has a secret key shared with the base station. How the keys are distributed is not specified by the standard. They could be preloaded by the manufacturer. They could be exchanged in advance over the wired network. Finally, either the base station or user machine could pick a random key and send it to the other one over the air encrypted with the other one's public key. Once established, keys generally remain stable for months or years.

WEP encryption uses a stream cipher based on the RC4 argorithm. RC4 was designed by Ronald Rivest and kept secret until it leaked out and was posted to the Internet in 1994. In WEP, RC4 generates a keystream that is XORed with the plaintext to form the ciphertext.

http://www1.cse.wustl.edu/~jain/cse574-06/ftp/wireless_security/fig13.gif

Figure

Each packet payload is encrypted using the method of Fig. 4. First the payload is checksummed using the CRC-32 polynomial and the checksum appended to the payload to form the plaintext for the encryption algorithm. Then this plarntext is XORed with a chunk of keystream its own size. The result is the ciphertext. The IV used to start RC4 is sent along with the ciphertext. When the receiver gets the packet, it extracts the encrypted payload from it, generates the keystream from the shared secret key and the IV it just got, and XORs the keystream with the payload to recover the plaintext. It can then verify the checksum to see if the packet has been tampered with.

Unfortunately, WEP suffers from several flaws and weaknesses. One of the major reasons behind WEP weaknesses is its key length. WEP has a 40-bit key, which can be broken in less than five hours using parallel attacks with the help of normal computer machines. This issue urged vendors to update WEP from using 40-bit to 104-bit key; the new release is called WEP2.

This update helped to resolve some security issues with WEP. The main disadvantage of WEP however, is the lack of key management. Some SOHO users (Small Office/ Home Office) never change their WEP key, which once known the whole system is in jeopardy. In addition to that, WEP does not support mutual authentication. It only authenticates the client, making it open to rouge AP attacks.

Another issue is the use of CRC to ensure integrity. While CRC is a good integrity provision standard, it lacks the cryptography feature. CRC is known to be linear. By using a form of induction, knowing enough data (encrypted packets) and acquiring specific plaintext, the WEP key can be resolved.

RC4 suffers from a serious flaw. It tends to repeat IV values (even if it is auto generated), making the exposing of the traffic easier. Mathematically, if the same IV is used to encrypt two packets (WEP key did not change also) and you have a pair of encrypted/plaintext message, then by applying the following simple rule:

C1 XOR C2 = P1 XOR P2

making it very easy to know the content of the new encrypted packet P2.

These weaknesses forced the designers of WLAN security modules to be more cautious. It demonstrates the result of not designing the security module from the ground up taking into consideration all applicable risks. In the next section we will go through the new standards that came after WEP to overcome its vulnerabilities.

802.11i Standard

The 802.11i (released June 2004) security standard  is supposed to be a definite solution to wireless security issue. It improves authentication, integrity and data transfer. Due to the market need of a better substitute to WEP, vendors (Wi-Fi Alliance) took a subset of it and market the new product  before the final release under the name WPA (WiFi Protected Access), which was released in April 2003. After the final release of 802.11i the vendors implemented the full specifications under the name WPA2.

802.11i supports two methods of authentication. The first method uses 802.1x and EAP to authenticate users. For users who can not or do not want to implement the first method, another method was proposed to use per-session key per-device. This method is implemented by having a shared key called GMK (Group Master Key), which represent the base key to derive the other. GMK is used to derive PTK (Pair Transient Key) and PSK (Pair Session Key) to do the authentication and data encryption.

To solve the integrity problem with WEP, a new algorithm named Michael is used to calculate an 8-byte integrity check called MIC (Message Integrity Code). Michael differs from the old CRC method by protecting both data and the header. Michael implements a frame counter which helps to protect against replay attacks.

To improve data transfer, 802.11i specifies three protocols: TKIP, CCMP and WRAP. TKIP (Temporal Key Integrity Management) was introduced as a "band-aid" solution to WEP problems. One of the major advantages of implementing TKIP is that you do not need to update the hardware of the devices to run it. Simple firmware/software upgrade is enough. Unlike WEP, TKIP provides per-packet key mixing, a message integrity check and a re-keying mechanism. TKIP ensures that every data packet is sent with its own unique encryption key. TKIP is included in 802.11i mainly for backward compatibility.

WRAP (Wireless Robust Authenticated Protocol) is the LAN implementation of the AES encryption standard (Advanced Encryption Standard) which is a block cipher. It was ported to wireless to get the benefits of AES encryption. WRAP has intellectual property issues, where three parties have filed for its patent. This problem caused IEEE to replace it with CCMP.

CCMP (Counter with Cipher Block Chaining Message Authentication Code Protocol) is considered the optimal solution for secure data transfer under 802.11i. CCMP uses AES for encryption. The use of AES will require a hardware upgrade to support the new encryption algorithm.

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