Wireless Lan(802.11)
Wireless networking is a rapidly evolving technology for connecting computers.the possibilities for building wireless networks are almost endless, ranging from using infrared signals within a single building to constructing a global network from a grid of low-orbit satellites.
802.11 supports additional features (e.g., time-bounded services, power management, and security mechanisms), but we focus our discussion on its base functionality.
Physical Properties
802.11 was designed to run over three different physical media—two based on spread spectrum radio and one based on diffused infrared.
The idea behind spread spectrum is to spread the signal over a wider frequency band than normal, so as to minimize the impact of interference from other devices. (Spread spectrum was originally designed for military use, so these “other devices” were often attempting to jam the signal.)
A first technique frequency hopping is a spread spectrum technique that involves transmitting the signal over a random sequence of frequencies; that is, first transmitting at one frequency, then a second, then a third, and so on.
A second spread spectrum technique, called direct sequence, achieves the same effect by representing each bit in the frame by multiple bits in the transmitted signal. For each bit the sender wants to transmit, it actually sends the exclusive-OR of that bit and n random bits. As with frequency hopping, the sequence of random bits is generated by a pseudorandom number generator known to both the sender and the receiver. The transmitted values, known as an n-bit chipping code.
The third physical standard for 802.11 is based on infrared signals. The transmission is diffused, meaning that the sender and receiver do not have to be aimed at each other and do not need a clear line of sight. This technology has a range of up to about 10 m and is limited to the inside of buildings only.
Collision Avoidance
The problem is more complicated in a wireless network, Collision detection is not feasible since all nodes are not within the reach of each other.
Hidden Station
- Suppose station B is sending data to A. At the same time, station C also has data to send to station A.
- Since B is not within the range of C, it thinks the medium is free and sends its data to A. Frames from B and C collide at A. Stations B and C are hidden from each other.
- Suppose station A is transmitting to station B and station C has some data to send to station D, which can be sent without interfering the transmission from A to B.
- Station C is exposed to transmission from A and it hears what A is sending and thus refrains from sending, even if the channel is available
Multiple Access with Collision Avoidance (MACA)
- The idea is for the sender and receiver to exchange short control frames with each other, so that stations nearby can avoid transmitting for the duration of the data frame.
- The control frames used for collision avoidance is Request to Send (RTS) and Clear to Send (CTS).
- Any station hearing RTS is close to sender and remains silent long enough for CTS to be transmitted back.
- Any station hearing CTS remains silent during the upcoming data transmission.
- The receiver sends an ACK frame to the sender after successfully receiving a frame.
- If RTS frames from two or more stations collide, then they do not receive CTS. Each node waits for a random amount of time and then tries to send RTS again
- B sends an RTS containing name of sender, receiver & duration of transmission.
- It reaches A, but not C.
- The receiver A acknowledges with a CTS message back to the sender B echoing the duration of transmission and other information.
- The CTS from A is received by both B and C. B starts to transmit data to A.
- C knows that some hidden station is using the channel and refrains from transmitting.
- The handshaking messages RTS and CTS does not help in exposed stations because C does not receive CTS from D as it collides with data sent by A.
Distribution System
- In wireless network, nodes are mobile and the set of reachable nodes change with time.
- Mobile nodes are allowed to connect with a wired network infrastructure called access points (AP).
- Access points are connected to each other by a distribution system (DS) such as ethernet, token ring, etc.
- Two nodes communicate directly with each other if they are reachable (eg, A and C)
- Communication between two stations in different APs occurs via two APs (eg, A and E)
The technique for selecting an AP is called scanning and involves the following four steps:
- The node sends a Probe frame.
- All APs within reach reply with a Probe Response frame.
- The node selects one of the access points and sends that AP an Association Request frame.
- The AP replies with an Association Response frame.
point.
Frame Format
802.11 frame format.
- Control--contains subfields that includes type (management, control or data), subtype (RTS, CTS or ACK) and pair of 1-bit fields ToDS and FromDS.
- Duration--specifies duration of frame transmission.
- Addresses--The four address fields depend on value of ToDS and FromDS subfields.
- When one node is sending directly to another, both DS bits are 0, Addr1 identifies the target node, and Addr2 identifies the source node
- When both DS bits are set to 1, the message went from a node onto the distribution system, and then from the distribution system to another node. Addr1 contains ultimate destination, Addr2 contains immediate sender, Addr3 contains intermediate destination and Addr4 contains original source.
- Sequence Control--defines sequence number of the frame to be used in flow control.
- Payload--can contain a maximum of 2312 bytes and is based on the type and the subtype defined in the Control field
- CRC--contains CRC-32 error detection sequence.
No comments:
Post a Comment