Wireless LANs and Mobile Networks : A Survey

Wireless LAN
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1. History of Wireless LAN[1]

    Norman Abramson, a professor at the University of Hawaii, developed the world’s first wireless computer communication network, ALOHAnet (operational in 1971), using low-cost ham-like radios. The system included seven computers deployed over four islands to communicate with the central computer on the Oahu Island without using phone lines. 

In 1979, F.R. Gfeller and U. Bapst published a paper in the IEEE Proceedings reporting an experimental wireless local area network using diffused infrared communications.

In 1980, P. Ferrert reported on an experimental application of a single code spread spectrum radio for wireless terminal communications in the IEEE National Telecommunications Conference.

Later on, M. Kavehrad reported on an experimental wireless PBX system using code division multiple access. These efforts prompted significant industrial activities in the development of a new generation of wireless local area networks and it updated several old discussions in the portable and mobile radio industry.

European Telecommunications Standards Institute, ETSI, ratified in1996 with High Performance Radio LAN (HiperLAN 1) standard. Later, ETSI, rolled out in June 2000, an flexible Radio LAN standard called HiperLAN 2, designed to provide high speed access 54Mbps than HiperLAN 1(20 Mbps)

The IEEE finalized the initial standard for wireless LANs, IEEE 802.11 in June 1997. This initial standard specifies a 2.4 GHz operating frequency with data rates of 1 and 2 Mbps. With this standard, one could choose to use either FHSS or DSSS (two non compatible forms of spread spectrum modulation).

Because of relatively low data rates (as compared to Ethernet), products based on the initial standard did not flourish as many had hoped. In late 1999, the IEEE published two supplements to the initial 802.11 standard: 802.11a and 802.11b (Wi-Fi).

The unlicensed PCS Unlicensed Personal Communications Services and the proposed SUPERNet, later on renamed as U-NII, bands also presented new opportunities.

A HomeRF group formed in 1997 to promote a technology aimed for residential use, but it disbanded at the end of 2002

Bluetooth is an industry specification for short-range RF-based connectivity for portable personal devices with its functional specification released out in 1999 by Bluetooth Special Interest Group. Bluetooth communicates on a frequency of 2.45 gigahertz.

In 2009 802.11n was added to 802.11. It operates in both the 2.4 GHz and 5 GHz bands at a maximum data transfer rate of 300 Mbit/s. Most newer routers are able to utilise both wireless bands, known as dualband. This allows data communications to avoid the crowded 2.4 GHz band, which is also shared with Bluetooth devices and microwave ovens. The 5 GHz band is also wider than the 2.4 GHz band, with more channels.

2. Types of Wireless LANs:

Local area networks can be constructed in wireless fashion mainly so that wireless users moving within a certain organization, such as a university campus, can access a backbone network.

Wireless LANs are generally categorized according to the transmission technique that is used. All current wireless LAN products fall into one of the following categories:

1. Infrared (IR) LANs
2. Spread spectrum LANs
3. Narrowband microwave LANs

a) Infrared LANs :

Infrared communication technology is used in several home devices, such as television remote controls.

Advantages of Infrared LANs

• The bandwidth for infrared communication is large and can therefore can achieve high data rates.
• IR light is diffusely reflected by light-coloured objects; thus it is possible to use ceiling reflections to achieve coverage of an entire room.
• IR light does not penetrate walls or other opaque objects. This has two advantages:
  - IR communications can be more easily secured against eavesdropping than microwave
  - A separate IR installation can be operated in every room in buildings without interference, enabling the construction of very large IR LANs
• Equipment for infrared communication is relatively inexpensive and simple.

The one major disadvantage of infrared technology is that background radiation from sunlight and indoor lighting can cause interference at the infrared receivers.

There are three alternative transmission techniques commonly used for IR data transmissions:

• Direct beam Configuration
• Omni directional Configuration
• Diffused Configuration

Direct beam Configuration:

This configuration involves point-to-point connections. The range of communications is limited by the transmitted power and the direction of focus. With proper focusing, ranges up to a kilometre can be achieved. This technology can be used in token-ring LANs and interconnections between buildings as shown below

Omni directional configuration

This configuration consists of a single base station that is normally used on ceilings. The base station sends an omnidirectional signal, which is picked up by all transceivers. The transceivers in turn use a directional beam focused directly at the base-station unit.

Diffused-Configuration

In this the infrared transmitters direct the transmitted signal to a diffused reflected ceiling. The signal is reflected in all directions from this ceiling. The receivers can then pick up the transmitted signal.

b) Spread-Spectrum LANs

The spread-spectrum techniques use three different frequency bands: 902-928 MHz, 2.4 GHz-2.4835 GHz and 5.725 GHz-5.825GHz. Higher-frequency ranges offer greater bandwidth capability. However, the higher-frequency equipment is more expensive. The spread-spectrum LANs makes use of multiple adjacent cells arrangement. Adjacent cells make use of different centre frequencies within the same band to avoid interference. Within each of these cells, a star or peer-to-peer topology can be deployed.

If a star topology is used, a hub as a network center is mounted on the ceiling. This hub acts as an interface between wired and wireless LANs, can be connected to other wired LANs. All users in the wireless LAN transmit and receive signals from the hub.

A peer to peer topology is one in which there is no hub. A MAC algorithm such as CSMA is used to control access. This topology is appropriate for ad hoc LANs.

c)  Narrowband RF LANs

Narrowband radio frequency LANs use a very narrow bandwidth. These LANs can be either licensed or unlicensed. In licensed narrowband RF, a licensed authority assigns the radio frequency band. Most geographic areas are limited to a few licenses. Adjacent cells use different frequency bands. The transmissions are encrypted to prevent attacks. The licensed narrowband LANs guarantee communication without interference. The unlicensed narrowband RF LANs use the unlicensed spectrum and peer-to-peer LAN topology.

3. Wireless LAN Standards

a) IEEE 802.11 Wireless Standard:

802.11 networks use free frequency bands (ISM: Industrial, Science, Medical). Thus everybody can run 802.11 devices without licensing a frequency band.

802.11 Pros and Cons:

• Mobility
• Flexible configuration
• Relatively cheap
• Weak security (WEP Wired Equivalent Protection, but fixed with WPA Wired Protection Access)
• Relatively low bandwidth for data (compared to wired networks)
• Electromagnetic interference with other devices (Bluetooth)
• Simple installation, but high skills needed for exploitation of full potential of technology

Different 802.11standards:

• 802.11a: 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band) uses OFDM
• 802.11b: Up to 11Mbps, simple (cheap) technology uses DSSS.
• 802.11g: Up to 54Mbps and uses OFDM,DSSS
• 802.11n: <600Mbps (MIMO=Multiple In Multiple Out antenna technology, uses multi-path transmission for better signal recovery at the receiver) uses OFDM
• 802.11ac: Forthcoming standard for higher throughput (802.11n enhancements).
• 802.11ad: Standard in progress, even higher throughput (<7Gpbs).

Operation modes of 802.11: Below picture taken from copyrighted content of Peter. R. Egli[4]

Each wireless LAN user has a wireless LAN adapter for communication over the wireless medium. This adapter is responsible for authentication, confidentiality and data delivery. To send data to a user in the wired LAN, a user in the wireless LAN first sends the data packet to the access point. The access point recognizes the wireless user through a unique ID called the SSID. SSID is like a password protection system that enables any wireless client to join the wireless LAN. Once the wireless user is authenticated, the access point forwards data packets to the desired wired user through the switch or hub.

Access points build a table of association that contains the MAC addresses of all users in the wireless network.

b) HiperLAN 1/2: HiperLAN 1 standard provides highspeed communications (20Mbps) between portable devices in the 5GHz range. Similar to IEEE802.11, HiperLAN/1 adopts CSMA to connect end user devices together. On top of that, HiperLAN/1 supports isochronous traffic for different type of data such as video, voice, text, etc.

HIPERLAN/2 has a very high transmission rate (up to 54 Mbps at PHY layer) to a variety of networks including 3G mobile core networks, ATM networks and IP based networks, and also for private use as a wireless LAN system. This is achieved by making use of a modularization method called Orthogonal Frequency Digital Multiplexing (OFDM).

c) Bluetooth: One of the ways Bluetooth devices avoid interfering with other systems is by sending out very weak signals of 1 milliwatt. The low power limits the range of a Bluetooth device to about 10 meters, cutting the chances of interference between a computer system and a portable telephone or television. Bluetooth operates at 2.4 GHz frequency band and supports data rates of 1 Mbps, with next generation products allowing anywhere from 2 to 12 Mbps, to be determined at a later date.

   

d) HomeRF: HomeRF is an open industry specification developed by Home Radio Frequency Working Group that defines how electronic devices such as PCs, cordless phones and other peripherals share and communicate voice, data and streaming media in and around the home. HomeRF-compliant products operate in the license-free 2.4GHz frequency band and utilize frequency-hopping spread spectrum RF technology for secure and robust wireless communications with data rates of up to 1 Mbps (HomeRF1). Unlike Wi-Fi, HomeRF already has quality-of-service support for streaming media and is the only wireless LAN to integrate voice.

3. Challenges[3] :

The key challenges in wireless networks are:

a) Data Rate Enhancements.

b) Low power networking.

c) Security.

d) Radio Signal Interference.

e) System Interoperability.

a) Enhancing Data Rate: Improving the current data rates to support future high speed applications is essential, especially, if multimedia service are to be provided. Data rate is a function of various factors such as the data compression algorithm, interference mitigation through error-resilient coding, power control, and the data transfer protocol. Therefore, it is imperative that manufacturers implement a well thought out design that considers these factors in order to achieve higher data rates.

   

b) Low Power Design: The size and battery power limitation of wireless mobile devices place a limit on the range and throughput that can be supported by a wireless LAN. The complexity and hence the power consumption of wireless devices vary significantly depending on the kind of spread spectrum technology being used to implement the wireless LAN. Normally, direct sequence spread spectrum (DSSS) based implementations require large and power-hungry hardware compared to frequency hopped spread spectrum (FHSS). They tend to consume about two to three times the power of an equivalent FHSS system. But, the complex circuitry provides better error recovery capability to DSSS systems compared to FHSS. The right time has come for researchers and developers to approach these issues in wireless LAN technologies together and from a global perspective.

c) Security: Security is a big concern in wireless networking, especially in e-commerce applications. Mobility of users increases the security concerns in a wireless network. Current wireless networks employ authentication and data encryption techniques on the air interface to provide security to its users. The IEEE 802.11 standard describes wired equivalent privacy (WEP) that defines a method to authenticate users and encrypt data between the PC card and the wireless LAN access point. In large enterprises, an IP network level security solution could ensure that the corporate network and proprietary data are safe. Virtual private network (VPN) is an option to make access to fixed access networks reliable. Since hackers are getting smarter, it is imperative that wireless security features must be updated constantly.

d) Radio Signal Interference: Interference can take on an inward or outward direction. A radio-based LAN, for example, can experience inward interference either from the harmonics of transmitting systems or from other products using similar radio frequencies in the local area. Microwave ovens operate in the S band (2.4GHz) that many wireless LANs use to transmit and receive. These signals result in delays to the user by either blocking transmissions from stations on the LAN or causing bit errors to occur in data being sent. Newer products that utilize Bluetooth radio technology also operate in the 2.4GHz band and can cause interference with wireless LANs, especially in fringe areas not well covered by a particular wireless LAN access point. The other issue, outward interference, occurs when a wireless network’s signal disrupts other systems, such as adjacent wireless LANs and navigation equipment on aircraft.

e) System Interoperability: With wireless LANs, interoperability is taken as a serious issue. There are still pre-802.11 (proprietary) wireless LANs, both frequency-hopping and direct sequence 802.11 versions, and vendor-specific enhancements to 802.11-compliant products that make interoperability questionable. To ensure interoperability with wireless LANs, it is best to implement radio cards and access points from the same vendor, if possible.

Handoff: Handoff is the mechanism by which an ongoing connection between a Mobi lehost (MH) and a corresponding Access point (AP) is transferred from one access point to the other. Handoff occurs during cell boundary crossing, weak signal reception and while a QoS deterioration occurs in the current cell. Present handoff mechanisms are based only on signal strength and do not take into account the load of the new cell. There is no negotiation of QoS characteristics with the new AP to ensure smooth carryover from the old AP to new AP. Now, several methods are proposed by researchers to have a seamless handoff between access points.   

4. Future Research areas:

FutureWLANs must have to mature in the following areas

• Higher Speeds      
• Improved Security
• Seamless end-to-end protocols
• Better Error control
• Long distance Coverage
• Better interoperability
• Global networking

Anywhere, anytime, any-form connectivity

Mobile Networks
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1. History of Mobile Networks

You can get very nice information at http://www.indigoo.com/dox/itdp/12_MobileWireless/

2. MANET[2]:

The concept of mobile ad hoc networking is not a new one and its origins can be traced back to the DARPA Packet Radio Network project in 1972. Infrastructured wireless networks require a fixed network infrastructure with centralised administration is required for their operation, potentially consuming a lot of time and money for set-up and maintenance. Furthermore, an increasing number of devices such as laptops, personal digital assistants (PDAs), pocket PCs, tablet PCs, smart phones, MP3 players, digital cameras, etc. are provided with short-range wireless interfaces. In addition, these devices are getting smaller, cheaper, more user friendly and more powerful. This evolution is driving a new alternative way for mobile communication, in which mobile devices form a self creating, self-organising and self-administering wireless network, called a mobile ad hoc network.

Nodes that lie within each other’s send range can communicate directly and are responsible for dynamically discovering each other. In order to enable communication between nodes that are not directly within each other’s send range, intermediate nodes act as routers that relay packets generated by other nodes to their destination. These nodes are often energy constrained— that is, battery-powered— devices with a great diversity in their capabilities. Furthermore, devices are free to join or leave the network and they may move randomly, possibly resulting in rapid and unpredictable topology changes.

3. Challenges:

MANETs impose challenges in all layers of protocol stack. The physical layer must deal with rapid changes in link characteristics. The media access control (MAC) layer needs to allow fair channel access, minimise packet collisions and deal with hidden and exposed terminals. At the network layer, nodes need to cooperate to calculate paths. The transport layer must be capable of handling packet loss and delay characteristics that are very different from wired networks. Applications should be able to handle possible disconnections and reconnections.

The Key Challenges are:
a) Routing
b) Service or resource discovery
c) Addressing or internet connectivity
d) Security and node co-operation

a) Routing: As mobile ad hoc networks are characterised by a multi-hop network topology that can change frequently due to mobility, efficient routing protocols are needed to establish communication paths between nodes, without causing excessive control traffic overhead or computational burden on the power constrained devices.

A number of proposed solutions attempt to have an up-to-date route to all other nodes at all times. To this end, these protocols exchange routing control information periodically and on topological changes. These protocols, which are called proactive routing protocols, are typically modified versions of traditional link state or distance vector routing protocols encountered in wired networks, adapted to the specific requirements of the dynamic mobile ad hoc network environment.

Most of the time, it is not necessary to have an up-to-date route to all other nodes. Therefore, reactive routing protocols only set up routes to nodes they communicate with and these routes are kept alive as long as they are needed.

Combinations of proactive and reactive protocols, where nearby routes (for example, maximum two hops) are kept up-to-date proactively, while far-away routes are set up reactively, are also possible and fall in the category of hybrid routing protocols.

A completely different approach is taken by the location-based routing protocols, where packet forwarding is based on the location of a node’s communication partner. Location information services provide nodes with the location of the others, so that packets can be forwarded.

b) Service and resource discovery:  MANET nodes may have little or no knowledge at all about the capabilities or services offered by each other. Therefore, service and resource discovery mechanisms, which allow devices to automatically locate network services and to advertise their own capabilities to the rest of the network are an important aspect of self-configurable networks. Possible services or resources include storage, access to databases or files, printer, computing power, Internet access, etc.

Directory-less service and resource discovery mechanisms, in which nodes reactively request services when needed and/or nodes proactively announce their services to others, seem an attractive approach for infrastructureless networks.

The alternative scheme is directory-based and involves directory agents where services are registered and service requests are handled. This implies that this functionality should be statically or dynamically assigned to a subset of the nodes and kept up-to-date. Existing directory-based service and resource discovery mechanisms such as UPnP or Salutation are unable to deal with the dynamics in ad hoc networks.

Currently, no mature solution exists, but it is clear that the design of these protocols should be done in close cooperation with the routing protocols and should include context-awareness (location, neighbourhood, user profile, etc.) to improve performance.

Also, when ad hoc networks are connected to fixed infrastructure (for example, Internet, cellular network, etc.), protocols and methods are needed to inject the available external services offered by service and content providers into the ad hoc network.

c) Addressing and Internet connectivity: In order to enable communication between nodes within the ad hoc network, each node needs an address. In stand-alone ad hoc networks, a unique MAC addresses could be used to address nodes. However, all current applications are based on TCP/IP or UDP/IP. In addition, as future mobile ad hoc networks will interact with IP-based networks and will run applications that use existing Internet protocols such as transmission control protocol (TCP) and user datagram protocol (UDP), the use of IP addresses is inevitable.

Unfortunately, an internal address organisation with prefixes and ranges like in the fixed Internet is hard to maintain in mobile ad hoc networks due to node mobility and overhead reasons and other solutions for address assignment are thus needed.

One solution is based on the assumption (and restriction) that all MANET nodes already have a static, globally unique and pre-assigned IPv4 or IPv6 address. This solves the whole issue of assigning addresses, but introduces new problems when interworking with fixed networks. Connections coming from and going to the fixed network can be handled using mobile IP, where the pre-assigned IP address serves as the mobile node’s home address (HoA). All traffic sent to this IP address will arrive at the node’s home agent (HA) . When the node in the ad hoc network advertises to its home agent the IP address of the Internet gateway as its careof- address (CoA), the home agent can tunnel all traffic to the ad hoc network , on which it is delivered to the mobile node using an ad hoc routing protocol . For outgoing connections, the mobile node has to route traffic to an Internet gateway, and for internal traffic an  ad hoc routing protocol can be used.

The main problem with this approach is that a MANET node needs an efficient way to figure out if a certain address is present in the MANET or if it is necessary to use an Internet gateway, without flooding the entire network.

Another solution is the assignment of random, internally unique addresses. This can be realised by having each node picking a more or less random address from a very address detection (DAD) techniques in order to impose address uniqueness within the MANET. Strong DAD techniques will always detect duplicates, but are difficult to scale in large networks. Weak DAD approaches can tolerate duplicates as long as they do not interfere with each other; that is, if packets always arrive at the intended destination. If interconnection to the Internet is desirable, outgoing connections could be realised using network address translation (NAT), but incoming connections still remain a problem if random, not globally routable, addresses are used. Also, the use of NAT remains problematic when multiple Internet gateways are present. If a MANET node switches to another gateway, a new IP address is used and ongoing TCP connections will break.

Another possible approach is the assignment of unique addresses that all lie within one subnet (comparable to the addresses assigned by a dynamic host configuration protocol (DHCP) server). When attached to the Internet, the ad hoc network can be seen as a separate routable subnet. This simplifies the decision if a node is inside or outside the ad hoc network. However, no efficient solutions exist for choosing dynamically an appropriate, externally routable and unique network prefix (for example, special MANET prefixes assigned to Internet gateways), handling the merging or splitting of ad hoc networks, handling multiple points of attachment to the Internet, etc.

The above discussion makes clear that, although many solutions are being investigated, no common adopted solution for addressing and Internet connectivity is available yet. New approaches using host identities, where the role of IP is limited to routing and not addressing, combined with dynamic name spaces, could offer a potential solution.

d) Security and node cooperation: The wireless mobile ad hoc nature of MANETs brings new security challenges to the network design. As the wireless medium is vulnerable to eavesdropping and ad hoc network functionality is established through node cooperation, mobile ad hoc networks are intrinsically exposed to control packets or data packets. Securing ad hoc networks against malicious attacks is difficult to achieve.

Preventive mechanisms include among others authentication of message sources, data integrity and protection of message sequencing, and are typically based on key-based cryptography. Incorporating cryptographic mechanisms is challenging, as there is no centralised key distribution centre or trusted certification authority. These preventative mechanisms need to be sustained by detection techniques that can discover attempts to penetrate or attack the network.

The previous problems were all related to malicious nodes that intentionally damage or compromise network functionality. However, selfish nodes, which use the network but do not cooperate to routing or packet forwarding for others in order not to spill battery life or network bandwidth, constitute an important problem as network functioning entirely relies on the cooperation between nodes and their contribution to basic network functions.

To deal with these problems, the self-organising network concept must be based on an incentive for users to collaborate, thereby avoiding selfish behaviour. Existing solutions aim at detecting and isolating selfish nodes based on watchdog mechanisms, which identify misbehaving nodes, and reputation systems, which allow nodes to isolate selfish nodes.

Another promising approach is the introduction of a billing system into the network based on economical models to enforce cooperation. Using virtual currencies or micro-payments, nodes pay for using other nodes forwarding capabilities or services and are remunerated for making theirs available. This approach certainly has potential in scenarios in which part of the ad hoc network and services is deployed by companies or service providers.

Also, when ad hoc networks are interconnected to fixed infrastructures by gateway nodes, which are billed by a telecom operator (for example, UMTS, hot-spot access, etc.), billing mechanisms are needed to remunerate these nodes for making these services available. Questions such as who is billing, to whom and for what, need to be answered and will lead to complex business models.

We may conclude that in some ad hoc network scenarios, the network organisation can completely or partially rely on a trust relationship between participating nodes (for example, PANs). In many others security mechanisms, mechanisms to enforce cooperation between nodes or billing methods are needed and will certainly be an important subject of future research.

[1]en.wikipedia.org/wiki/Wireless_LAN

[2]Hoebeke, Jeroen, et al. "An overview of mobile ad hoc networks: Applications and challenges." Journal-Communications Network 3.3 (2004): 60-66.

[3]Chandramouli, Vijay. "A detailed study on wireless LAN technologies." URL: h ttp://crystal. uta. edu/~ kumar/cse6392/termpapers/Vijay_paper. pdf# search='A% 20Detailed% 20St udy% 20on% 20Wireless% 20LAN% 20Technologies (2002).

[4]http://www.indigoo.com/dox/itdp/12_MobileWireless/

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