Wireless Networking So we talked about wired networks. What about wireless?

Standard is the name of the standard that defines how wireless networks should operate. –The basic service set defines how a simple standalone wireless LAN should operate. –The extended service set defines how several BSS’s can be connected to each other, and to a WAN, through a distribution system.

Two MAC sublayer protocols: –Distributed Coordination Function is the lower layer. CSMA/CA –Sender detects that the medium is idle using persistence with back-off –Waits the DIFS and then sends a Request To Send (RTS) frame. –Receiver gets the RTS, waits the SIFS, then sends a Clear To Send (CTS) frame to sender. –Sender gets CTS, waits SIFS, then sends data. –Receiver gets data, waits SIFS, sends ACK. –RTS frame contains a request for how long the sender needs to send. When an RTS is sent, other stations start a NAV timer and do not poll the medium until this timer has expired. –Collisions are not detected, but assumed if CTS frame is not received. –Point Coordination Function - Controlled access using polling.

FC - Frame control info D - duration of transmission Addresses are used differently depending on who’s doing the sending and receiving: One device to another - Add1 is Receiver, Add2 is Sender, Add3 is BSS ID; largely used for communication between device and access point. From outside the network to a device - Add1 is Receiver, Add2 is nearest AP; Add3 is original Sender From device to outside network - Add1 is nearest AP, Add2 is Sender, Add3 is outside Receiver From device to device on another wireless network - Add1 is nearest AP, Add2 is AP nearest Receiver; Add3 is Receiver, and Add4 is Sender

Which addressing is used is controlled by two bits in FC. Sequence Control: The frame number for flow control Frame body: the data FCS: CRC-32 error detection. Management and Control frames (RTS, CTS, and ACK) are much smaller. RTS only has FC, D, address of sender and receiver, and FCS. CTS and ACK only have FC, D, address of sender, and FCS.

Problems with wireless Hidden station problem Two devices can be out of range of each other, but still in range of an AP. If one is sending data to the AP, the other will think the medium is free and also start sending, causing a collision. CTS frames eliminate this possibility by informing all devices when the AP is being sent to. Exposed station problem If a device is in range of an AP, but also in range of another device using a different AP, it will refrain from sending because it will sense the medium is busy. Thus, it (and the AP) sit idle when they could be communicating.

Physical layer Four specifications: – –1 to 2 Mbps –Uses either FHSS and FSK, DSSS and PSK, or Infrared and PPM at 2.4 GHz. –802.11a –18 or 54 Mbps –OFDM with PSK or QAM at 5.725GHz –802.11b (most common) –5.5 and 11 Mbps –DSSS with PSK at 2.4GHz –802.11g (becoming more popular) –22 and 54 Mbps –OFDM at 2.4GHz