Advanced Networking Trends WIRELESSLAN
CHAPTER 6
wireless LAN
6.1 Introduction
A wireless LAN (WLAN) is a data transmission system designed to provide location-independent network access between computing devices by using radio waves rather than a cable infrastructure.
In the corporate enterprise, wireless LANs are usually implemented as the final link between the existing wired network and a group of client computers, giving these users wireless access to the full resources and services of the corporate network across a building or campus setting.
The widespread acceptance of WLANs depends on industry standardization to ensure product compatibility and reliability among the various manufacturers.
The 802.11 specification as a standard for wireless LANS was ratified by the Institute of Electrical and Electronics Engineers (IEEE) in the year 1997. This version of 802.11 provides for 1 Mbps and 2 Mbps data rates and a set of fundamental signaling methods and other services. Like all IEEE 802 standards, the 802.11 standards focus on the bottom two levels the ISO model, the physical layer and link layer . Any LAN application, network operating system, protocol, including TCP/IP and Novell NetWare, will run on an 802.11-compliant WLAN as easily as they run over Ethernet.
There are several issues associated with Wireless LAN:
· Error Rate: For wired LAN, errors are relatively rare. But in the world of radio, error rate is much higher. Noise, multipath, attenuation, spread-spectrum interference, etc. are all common causes for errors in wireless environments.
· Security: Radio waves are not confined at the boundary of building or campus, there exists the possibility for eavesdropping and intentional interference.
· Interference: In wired LAN, the only machines you hear are the ones connected to the network. In a wireless LAN you may hear other networks, as well as cordless phones, microwave ovens, etc. Any of these can interfere with your transmission of data.
· Power conservation: Wireless LANs are typically related to mobile applications, and in this type of applications battery power is a scare resource
6.2 Characteristics of wireless LANs
Advantages
q Flexibility: very flexible within the reception area
q Planning: Ad-hoc networks without previous planning possible
q Design: (almost) no wiring difficulties (e.g. historic buildings, firewalls)
q Robustness: more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug
q Cost: Adding additional users to a wireless network will not increase the cost.
The major motivation and benefit from Wireless LANs is increased mobility. Untethered from conventional network connections, network users can move about almost without restriction and access LANs from nearly anywhere.
The other advantages for WLAN include cost-effective network setup for hard-to-wire locations such as older buildings and solid-wall structures and reduced cost of ownership-particularly in dynamic environments requiring frequent modifications, thanks to minimal wiring and installation costs per device and user. WLANs liberate users from dependence on hard-wired access to the network backbone, giving them anytime, anywhere network access. This freedom to roam offers numerous user benefits for a variety of work environments, such as:
· Immediate bedside access to patient information for doctors and hospital staff
· Easy, real-time network access for on-site consultants or auditors
· Improved database access for roving supervisors such as production line managers, warehouse auditors, or construction engineers
· Simplified network configuration with minimal MIS involvement for temporary setups such as trade shows or conference rooms
· Faster access to customer information for service vendors and retailers, resulting in better service and improved customer satisfaction
· Location-independent access for network administrators, for easier on-site troubleshooting and support
· Real-time access to study group meetings and research links for students
Disadvantages
q Quality of service: typically very low bandwidth compared to wired networks (1-10 Mbit/s)
q Proprietary solutions: many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11). Now, 802.11g is a popular solution.
q Restrictions: products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000
q Safety and security: Precautions have to be taken to prevent safety hazards. Secrecy and integrity must be assured.
Design goals for wireless LANs
q Global operation
q Low power for battery use
q License-free operation: no special permissions or licenses needed to use the LAN
q Robust transmission technology
q Simplified spontaneous cooperation at meetings
q Easy to use for everyone, simple management
q Protection of investment in wired networks
q Safety and security: security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation)
q Transparency for applications: transparency concerning applications and higher layer protocols, but also location awareness if necessary
6.3 Comparison: infrared vs. radio transmission
Infrared
q uses IR (Infra-Red) diodes, diffuse light, multiple reflections (walls, furniture etc.)
Advantages
q simple, cheap, available in many mobile devices
q no licenses needed
q simple shielding possible
Disadvantages
q interference by sunlight, heat sources etc.
q many things shield or absorb IR light
q low bandwidth
Example
q IrDA (Infrared Data Association) interface available everywhere
Radio
q typically using the license free ISM (Industrial, Scientific, Medical) band at 2.4 GHz
Advantages
q experience from wireless WAN and mobile phones can be used
q coverage of larger areas possible (radio can penetrate walls, furniture etc.)
Disadvantages
q very limited license free frequency bands
q shielding more difficult, interference with other electrical devices
Example
q WaveLAN (Lucent), HIPERLAN, Bluetooth
6.4 Comparison: infrastructure vs. ad-hoc networks
Infrastructure networks
q Provide access to other networks
q Include forwarding functions
q Medium access control
Ad-hoc networks is a group of computers each with wireless adapters, connected as an independent wireless LAN.
q Each node can communicate with other nodes
6.5 802.11 - Architecture of an infrastructure network
When two or more stations come together to communicate with each other, they form a Basic Service Set (BSS). The minimum BSS consists of two stations. 802.11 LANs use the BSS as the standard building block.
A BSS that stands alone and is not connected to a base is called an Independent Basic Service Set (IBSS) or is referred to as an Ad-Hoc Network. An ad-hoc network is a network where stations communicate only peer to peer. There is no base and no one gives permission to talk. Mostly these networks are spontaneous and can be set up rapidly. Ad-Hoc or IBSS networks are characteristically limited both temporally and spatially.
When BSS's are interconnected the network becomes one with infrastructure. 802.11 infrastructure has several elements. Two or more BSS's are interconnected using a Distribution System or DS. This concept of DS increases network coverage. Each BSS becomes a component of an extended, larger network. Entry to the DS is accomplished with the use of Access Points (AP). An access point is a station, thus addressable. So, data moves between the BSS and the DS with the help of these access points.
Creating large and complex networks using BSS's and DS's leads us to the next level of hierarchy, the Extended Service Set or ESS. The beauty of the ESS is the entire network looks like an independent basic service set to the Logical Link Control layer (LLC). This means that stations within the ESS can communicate or even move between BSS′s transparently to the LLC.
One of the requirements of IEEE 802.11 is that it can be used with existing wired networks. 802.11 solved this challenge with the use of a Portal. A portal is the logical integration between wired LANs and 802.11. It also can serve as the access point to the DS. All data going to an 802.11 LAN from an 802.X LAN must pass through a portal. It thus functions as bridge between wired and wireless.
The implementation of the DS is not specified by 802.11. Therefore, a distribution system may be created from existing or new technologies. A point-to-point bridge connecting LANs in two separate buildings could become a DS.
While the implementation for the DS is not specified, 802.11 does specify the services, which the DS must support. Services are divided into two sections
1. Station Services (SS)
2. Distribution System Services (DSS).
There are five services provided by the DSS
1. Association
2. Reassociation
3. Disassociation
4. Distribution
5. Integration
The first three services deal with station mobility. If a station is moving within its own BSS or is not moving, the stations mobility is termed No-transition. If a station moves between BSS's within the same ESS, its mobility is termed BSS-transition. If the station moves between BSS's of differing ESS's it is ESS transition. A station must affiliate itself with the BSS infrastructure if it wants to use the LAN. This is done by Associating itself with an access point. Associations are dynamic in nature because stations move, turn on or turn off. A station can only be associated with one AP. This ensures that the DS always knows where the station is.
Association supports no-transition mobility but is not enough to support BSS-transition. Enter Reassociation. This service allows the station to switch its association from one AP to another. Both association and reassociation are initiated by the station. Disassociation is when the association between the station and the AP is terminated. This can be initiated by either party. A disassociated station cannot send or receive data. ESS-transition are not supported. A station can move to a new ESS but will have to reinitiate connections.
Distribution and Integration are the remaining DSS's. Distribution is simply getting the data from the sender to the intended receiver. The message is sent to the local AP (input AP), then distributed through the DS to the AP (output AP) that the recipient is associated with. If the sender and receiver are in the same BSS, the input and out AP's are the same. So the distribution service is logically invoked whether the data is going through the DS or not. Integration is when the output AP is a portal. Thus, 802.x LANs are integrated into the 802.11 DS.
Station services are:
1. Authentication
2. Deauthentication
3. Privacy
4. MAC Service Data Unit (MSDU) Delivery.
With a wireless system, the medium is not exactly bounded as with a wired system. In order to control access to the network, stations must first establish their identity. This is much like trying to enter a radio net in the military.
Before you are acknowledged and allowed to converse, you must first pass a series of tests to ensure that you are who you say you are. That is really all authentication is. Once a station has been authenticated, it may then associate itself. The authentication relationship may be between two stations inside an IBSS or to the AP of the BSS. Authentication outside of the BSS does not take place.
There are two types of authentication services offered by 802.11. The first is Open System Authentication. This means that anyone who attempts to authenticate will receive authentication. The second type is Shared Key Authentication. In order to become authenticated the users must be in possession of a shared secret. The shared secret is implemented with the use of the Wired Equivalent Privacy (WEP) privacy algorithm. The shared secret is delivered to all stations ahead of time in some secure method (such as someone walking around and loading the secret onto each station).
Deauthentication is when either the station or AP wishes to terminate a stations authentication. When this happens the station is automatically disassociated. Privacy is an encryption algorithm, which is used so that other 802.11 users cannot eavesdrop on your LAN traffic. IEEE 802.11 specifies Wired Equivalent Privacy (WEP) as an optional algorithm to satisfy privacy. If WEP is not used then stations are "in the clear" or "in the red", meaning that their traffic is not encrypted. Data transmitted in the clear are called plaintext. Data transmissions, which are encrypted, are called ciphertext. All stations start "in the red" until they are authenticated. MSDU delivery ensures that the information in the MAC service data unit is delivered between the medium access control service access points.
6.6 802.11 - Architecture of an ad-hoc network
Direct communication within a limited range
The most basic wireless LAN topology is a set of stations, which have recognized each other and are connected via the wireless media in a peer-to-peer fashion. This form of network topology is referred to as an Independent Basic Service Set (IBSS) or an Ad-hoc network.
In an IBSS, the mobile stations communicate directly with each other. Every mobile station may not be able to communicate with every other station due to the range limitations. There are no relay functions in an IBSS therefore all stations need to be within range of each other and communicate directly.
q Station (STA):
terminal with access mechanisms to the wireless medium
q Independent Basic Service Set (IBSS):
group of stations using the same radio frequency
6.7 IEEE 802.11 Protocol
802.11 - Layers and functions
MAC
q access mechanisms, fragmentation, encryption
MAC Management
q synchronization, roaming, MIB (Management Information Base), power management
PLCP Physical Layer Convergence Protocol
q clear channel assessment signal (carrier sense)
PMD Physical Medium Dependent
q modulation, coding
PHY Management
q channel selection, MIB
Station Management
Coordination of all management functions
As any 802.x protocols, the 802.11 protocol covers the MAC and Physical Layer.
The Physical Layer is further divided into two sublayers: Physical Layer Convergence Procedure (PLCP) Sublayer, Physical Media Dependent (PMD) Sublayer.
PLCP adapts the capabilities of the physical medium dependent system to the Physical Layer service. PLCP defines a method of mapping the 802.11 PHY sublayer Service Data Units (PSDU) into a framing format suitable for sending and receiving user data and management information between two or more stations using the associated physical medium dependent system. This allows 802.11 MAC to operate with minimum dependence on the PMD sublayer.