The First Wireless Network Sta Essay, Research Paper

The First Wireless Network Protocol: 802.11

Blessing of the IEEE 802.11 criterion for wireless local country networking ( WLAN ) and rapid advancement made toward higher information rates have put the promise of genuinely nomadic calculating within range. While wired LANs have been a mainstream engineering for at least 15 old ages, WLANs are chartless district for most networking professionals. Some obvious inquiries come to mind when sing wireless networking:

+ How can WLANs be integrated with wired web substructure?

+ What is the implicit in wireless engineering?

+ How is multiple entree handled?

+ What about web security?

IEEE 802.11 is limited in range to the Physical ( PHY ) bed and Medium Access Control

( MAC ) bed ( Lough, 3 ) , but it portions MAC features with the IEEE 802.3 Ethernet criterion ( 3Com, 2 ) . The undermentioned overview explains major differences between wired and wireless LANs and should reply some of the inquiries that arise when measuring WLAN engineering.

Network Topology

WLANs can be used either to replace wired LANs, or as an extension of the wired LAN substructure. The basic topology of an 802.11 web is shown in Figure 1. A Basic Service Set ( BSS ) consists of two or more wireless nodes, or Stationss, which have recognized each other and have established communications. In the most basic signifier, Stationss communicate straight with each other on a peer-to-peer degree sharing a given cell coverage country. This type of web is frequently formed on a impermanent footing, and is normally referred to as an ad hoc web, or Independent Basic Service Set ( IBSS ) ( Geier, 3 ) .

Figure 1 ( Intel, 1 )

In more structured environments, the BSS contains an Access Point ( AP ) . The chief map of an AP is to organize a span between radio and wired LANs. The AP is similar to a basestation used in cellular phone webs. When an AP is present, Stationss do non pass on on a peer-to-peer footing. All communications between Stationss or between a station and a wired web client go through the AP. AP s are non nomadic, and organize portion of the wired web substructure. A BSS in this constellation is said to be runing in the substructure manner.

The Extended Service Set ( ESS ) consists of a series of overlapping BSSs ( each incorporating an AP ) connected together by agencies of a Distribution System ( Geier, 3 ) . Although the Distribution System could be any type of web, it is about constantly an Ethernet LAN. Mobile nodes can roll between APs and seamless campus-wide coverage is possible.

Radio Technology

IEEE 802.11 provides for two fluctuations of the physical bed. These include two RF engineerings, viz. , Direct Sequence Spread Spectrum ( DSSS ) , and Frequency Hopped Spread Spectrum ( FHSS ) . The DSSS and FHSS physical bed options were designed specifically to conform to FCC ordinances for operation in the 2.4 GHz ISM sets, which has worldwide allotment for unaccredited operation ( Geier, 3 ) . Both FHSS and DSSS physical beds presently support 1 and 2 Mbps. However, all 11 Mbps wirelesss are DSSS.

Multiple Access

The basic entree method for 802.11 is the Distributed Coordination Function ( DCF ) which uses Carrier Sense Multiple Access / Collision Avoidance ( CSMA / CA ) ( Lough, 4 ) similar to AppleTalk. This requires each station to listen for other users. If the channel is idle, the station may convey. However if it is busy, each station waits until transmittal Michigans, and so enters into a random back off process. This prevents multiple Stationss from prehending the medium instantly after completion of the predating transmittal.

Packet response in DCF requires acknowledgement. The period between completion of package transmittal and start of the recognition ( ACK ) frame is one Short Inter Frame Space ( SIFS ) or 28 microseconds ( Brenner, 8 ) . ACK frames have a higher precedence than other traffic. Fast recognition is one of the important characteristics of the 802.11 criterion, because it requires ACKs to be handled at the MAC sublayer.

Transmissions other than ACKs must wait at least one DCF inter frame infinite ( DIFS ) or 128 microseconds before conveying informations ( Brenner, 8 ) . If a sender senses a busy medium, it determines a random back-off period by puting an internal timer to an integer figure of slot times. Upon termination of a DIFS, the timer beg

Immigration and Naturalization Services to decrement. If the timer reaches zero, the station may get down transmittal. However, if the channel is seized by another station before the timer reaches zero, the timer scene is retained at the decremented value for later transmittal.

The method described above relies on the premise that every station can hear all other Stationss. This is non ever the instance. Mentioning to Figure 2, the entree point is within scope of the Station A, but Station B is out of scope. Station B would non be able to observe transmittals from Station A, and the chance of hit is greatly increased. This is known as the Hidden Node ( Lough, 4 ) .

Figure 2 ( Intel, 3 )

To battle this job, a 2nd bearer sense mechanism is available. Virtual Carrier Sense enables a station to reserve the medium for a specified period of clip through the usage of RTS/CTS frames. In the instance described above, Station A sends an RTS ( ready to direct ) frame to the AP. Station B will non hear the RTS. The RTS frame contains a duration/ID field, which specifies the period of clip for which the medium is reserved for a subsequent transmittal. The reserve information is stored in the all Stationss observing the RTS frame ( Brenner, 6 ) .

Upon reception of the RTS, the AP responds with a CTS ( clear to direct ) frame, which besides contains a duration/ID field stipulating the period of clip for which the medium is reserved. While Station B did non observe the RTS, it will observe the CTS and update itself consequently. Therefore, hit is avoided even though some nodes are hidden from other Stationss. The RTS/CTS process is invoked harmonizing to a user specified parametric quantity. It can be used ever, ne’er, or for packages that exceed an randomly defined length ( Brenner, 6 ) .

As mentioned above, DCF is the basic media entree control method for 802.11 and it is compulsory for all Stationss. The Point Coordination Function ( PCF ) is an optional extension to DCF. PCF provides a clip division duplexing capableness to suit clip bounded, connection-oriented services such as cordless telephone.

Logical Addressing

The writers of the 802.11 criterion allowed for the possibility that the wireless media, distribution system, and wired LAN substructure would all utilize different reference infinites. IEEE 802.11 merely specifies turn toing for over the wireless medium, though it was intended specifically to ease integrating with IEEE 802.3 wired Ethernet LANs. The IEEE802 48-bit turn toing strategy was hence adopted for 802.11, thereby keeping address compatibility with the full household of IEEE 802 criterions. In the huge bulk of installings, the distribution system is an IEEE 802 wired LAN and all three logical turn toing infinites are indistinguishable.

Security

IEEE 802.11 provides for security via two methods: hallmark and encoding ( Brenner, 13 ) . Authentication is the agencies by which one station is verified to hold mandate to pass on with a 2nd station in a given coverage country. In the substructure manner, hallmark is established between an AP and each station.

Authentication can be either Open System or Shared Key. In an Open System, any station may bespeak hallmark. The station having the petition may allow hallmark to any petition, or merely those from Stationss on a user-defined list. In a Shared Key system, merely Stationss which possess a secret encrypted key can be authenticated. Shared Key hallmark is available merely to systems holding the optional encoding capableness.

Encoding is intended to supply a degree of security comparable to that of a wired LAN. The Wired Equivalent Privacy ( WEP ) characteristic uses the RC4 PRNG algorithm from RSA Data Security, Inc. ( Brenner, 13 ) . The WEP algorithm was selected to run into the undermentioned standards:

+ Reasonably strong

+ Self-synchronizing

+ Computationally efficient

+ Exportable

+ Optional

Plants Cited

Brenner, Pablo. A Technical Tutorial on the IEEE 802.11 Protocol. SSS-Online. February 25, 2001. .

Geier, Jim. Overview of the IEEE.11 Standard. Wireless-Nets. February 24, 2001. .

Lough, Blankenship, and Krizman. A Short Tutorial on Wireless LANs and IEEE 802.11. Computer.org. February 24, 2001. .

What is Wireless Networking? Intel. February 24, 2001. .

What s New in Wireless LANs: The IEEE 802.11b Standard. 3Com. February 24, 2001. .