Radio-based wireless connectivity




 

The most widely sold wireless LAN products use radio waves as a medium between computers and the WEB or each other. An advantage of radio waves over other forms of wireless connectivity such as infrared and microwaves is that they propagate through walls and other obstructions with little attenuation. Even though several walls might separate the user from the server or an access point to the Web, users can maintain connections to the network, thus supporting true mobility. The disadvantage for radio frequencies is that governments manage the region and not all the spectrum can be used everywhere; hence, techniques such as FHSS and DSSS (as described) must be used.

There are three regions of the E-M spectrum utilized by these waves:

· 902-928 MHz

· 2.4-2.484 GHz

· 5.725-5.850 GHz

Presently Metricom is operating a two way radio based multi-user data communications system is San Francisco called Ricochet. The architecture is shown below:

The concept is to use wireless access points and network radio relays approximately one half mile apart to facilitate connectivity between users. The radios operate in the license-free 902-928 portion of the radio spectrum using FHSS. The underlying network protocol is TCP/IP, allowing it to interact with the Internet seamlessly.

An important goal for wireless communications has been to make the application layer transparent to the underlying protocol (TCP/IP) in order to have more acceptability by the Web users. To understand the kind of standards developed for wireless networks, it helps to see the affected layers in an OSI (Open System Interconnect) model. The bottom two layers are the ones of interest to us. At the very bottom is the Physical layer. This layer defines the electrical characteristics of the actual connection between network nodes. For wired networks, it covers topics like voltage levels and type of cabling. But for wireless networks, it addresses areas such as frequencies used and modulation techniques, including spread-spectrum technologies.

The next layer up is the Data Link Layer. It deals with how the network is shared between nodes. The Data Link Layer defines rules such as who can talk on the network, how long they can occupy network resources. This layer can be further divided into two separate layers (shown below).

· The Medium Access Control (MAC) layer.

· The Logical Link Control (LLC) layer.

The first five layers of the OSI model remains unchanged; hence, TCP and IP can be implemented in their respective layers.

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.

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:

· Support of asynchronous and time-bounded delivery service

· Continuity of service within extended areas

· Accommodation of transmission rates between 1 and 20 Mbps

· Support of most market applications

· Multicast service

· 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.



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