Wireless LANs Wireless proliferating rapidly. IEEE 802.11 --> link access standard designed for use in a limited geographic setting. Various versions 802.11a, 802.11e, 802.11g, 802.11n. Physical layer evolution -- increased rates . As an example, 802.11n uses multiple antennas -- can provide very high data rates.

Physical Properties Typically use 3 kinds of physical media -- two based on spread-spectrum and one based on IR. IR : transmission is diffused; limited range. (not much in use) Spread spectrum -- spread signal over a higher frequency -- provides reduced impact from external interference. more robustness to signal loss.

Fading Signal travels and reflects off objects. Multiple copies converge at receiver (Red copy and Green copy). Copies interfere -- may self destruct -- called multipath fading. Signal combination depends on frequency of transmission.

Spread Spectrum The use of larger bandwidth provides robustness to fading/interference. Wiped out frequencies

Frequency hopped Spread Spectrum Transmit signal over a random sequence of frequencies (not really random but pseudo-random). Computed using a pseudo-random sequence generator. Receiver uses the same generator -- they can synchronize (same seed).

Direct Sequence Spread Spectrum Each bit translated into ‘N’ random symbols called chips. Random chips generated using the pseudo-random number generator. Transmitted sequence called a n-bit chipping code. If receiver knows the chips, it can decode. Others cannot, they see a higher frequency signal -- can be filtered out as noise.

802.11 PHY layers One PHY layer uses frequency hopping over a 79.1 MHz range. A second version uses a 11 bit chipping sequence. Both run in the 2.4 GHz band. Note: For other than the intended receiver signal looks like noise.

Medium Access Control Can we use the same protocol as in the Ethernet ? Carrier Sensing -- Sense channel, transmit when channel is idle, back-off when collision occurs ? Not really -- why ?

Hidden Terminals B can talk to A and C but not D. C can talk to B and D but not A. A sends to B -- C cannot make out (cannot sense), and it sends to D. Collision at B :(. A and C are hidden from each other -- hidden terminal problem.

Exposed Terminals On the other hand, if B is sending A, C will sense channel to be busy. Will not send to D. Not good either! C is “exposed” to B’s transmission.

The MACA scheme 802.11 addresses these problems by using an algorithm called MACA -- multiple access with collision avoidance. Also referred to as “virtual carrier sensing”. Sender sends a “Request to Send” or RTS to Receiver. Tells sender’s neighbors of intent to send. Receiver sends a “Clear to send” or CTS to sender. Tells receivers neighbors of intent to receive.

Example A sends to B. A’s RTS tells everyone in its neighborhood that it is sending. B’s CTS tells everyone in its neighborhood that it is receiving. Now C knows that B is receiving and does not initiate communications with D.

Details RTS indicates the time for which the sender wishes to hold the channel. Receiver echoes this “duration” field to the sender. Every node knows -- how long the transmission is for.

Data transfer Upon a successful RTS/CTS exchange, nodes initiate data transfer. Receiver sends ACK after successfully receiving frame. Exposed terminal issue left alone Random wait when CTS is not received Back-off similar to what happens with Ethernet.

Access Points While 802.11 facilitates operations in an “ad hoc” mode, typically, some of the wireless nodes connected to a wireline infrastructure. These are called access points (APs) -- some people also call them base-stations (more appropriate for cellular networks) Other mobile hosts connect to the Internet via these APs.

Distribution System APs connected via the distribution system -- could be Ethernet or FDDI based (or anything else). Distribution system runs at Layer 2 -- not Layer 3 (Network Layer) entity.

Selection of APs Via a process called scanning. When a node wants to select an AP, it sends a probe message. APs that get this, respond with a Probe-Response. Node selects one of the APs (strongest signal ?),and sends an Association Request. Selected AP responds with an Association Response. Active scanning -- Probes sent actively when mobile joins the network or moves around and out of coverage. Passive scanning -- APs send beacons -- mobiles hear and if they find a more attractive AP, they can switch.