ive layers of the OSI model remains unchanged; hence, TCP and IP can be implemented in their respective layers. В
IEEE 802.11 protocol
The wireless network interface manages the use of air through the operation of a communications protocol. For synchronization, wireless networks employ a carrier sense protocol similar to the common Ethernet standard. This protocol enables a group of wireless computers to share the same frequency and space.
The lack of standards has been a significant issue with wireless networking. In response to this problem, the Institute for Electrical and Electronic Engineers (IEEE) has been involved in the development of wireless LAN standards for the last seven years. This effort is nearly complete, and the final standard (IEEE 802.11) will be ready by May of 1997. p> As with other 802 standards such as Ethernet and token ring, the primary service of the 802.11 standard is to deliver MSDUs (MAC Service Data Units) between LLC (Logical Link Control) connections to the network. In other words, the 802.11 standard will define a method of transferring data frames between network adapters without wires. In addition, the 802.11 standard will include:
ТЗ Support of asynchronous and time-bounded delivery service
ТЗ Continuity of service within extended areas
ТЗ Accommodation of transmission rates between 1 and 20 Mbps
ТЗ Support of most market applications
ТЗ Multicast service
ТЗ Network management services, Registration and authentication services
The IEEE 802.11 standard supports operation in two separate modes, a distributed coordination (DCF) and a centralized point-coordination mode (PCF). The IEEE 802.11 MAC is called DFWMAC (Distributed Foundation Wireless MAC), and the access mechanism is based upon the principal of CSMA/CA (Collision Sense Medium Access with Collision Avoidance), which is another adaptation of CSMA/CD used by Ethernet networks.
Under CSMA/CD, when a station has data to send, it first listens to determine whether any other station on the network is occupying the medium. If the channel is busy, the station will wait until it becomes idle before transmitting data. Since it is possible for two stations to listen at the same time and discover an idle channel, it is also possible that two stations could then transmit at the same time. When this occurs a collision will take place, and then a jamming signal is sent throughout the network in order to notify all stations of the collision. The stations will then wait for a random period of time before re-transmitting their respective frames.
CSMA/CA is a modified version of the CSMA/CD access system. Under the CSMA/CA technique, as before stations are listening to the medium at all times. A station that is ready to transmit a frame will sense the medium, if the medium is busy, it will wait for an additional predetermined time period of DIFS (DCF Interframe Space) length and then, based upon a random calculation, picks a time slot within a contention window to transmit its frame. If there were no other transmissions before this time slot has arrived, it will start transmitting its frame. On the other hand if there were transmissions by other stations during this back-off time period, the station will freeze its counter and will pick-up the count where it left off after the other station has completed its frame transmission. The collisions can now occur only when two or more stations select the same time slot to transmit. These stations will have to reenter the contention procedure to select new time slots to transmit the collided frames. The figure below illustrates DFWMAC access scheme. <В
Interworking Units for wireless connectivity
Just as in wired networks, the interworking unit (IWU) provides the protocol manipulation to connect networks with different protocols together. The IWUs act as access points between wireless stations and the Web. They address issues such as:
ТЗ Correct delivery of data to its destination.
ТЗ Congestion control.
ТЗ Differences in maximum PDU sizes.
To connect a wireless network that is using the 802.11 protocol to the Internet, IWUs are needed at access points. Access points are nodes that allow traffic flow in and out of the Wireless network. Alternatively, IWUs (IP Routers) control the traffic in and out of the Internet; thus routing wireless packets into and out of the Internet as shown below:
The 802.11 protocol can support data rates of 20 Mbps, thus making it an attractive wireless protocol for Internet connectivity. Companies such as Proxim that have been involved with the development of 802.11, are migrating rapidly to the new standard.
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Internet Mobile Host Protocol
An important part o...
