802.11 – Wireless PHY and MAC

802.11 – Wireless PHY and MAC Stallings 502-507

Types of 802.11 • 802.11 Infrared • 802.11 FHSS (frequency hopping spread spectrum) • 802.11 DSSS (direct sequence spread spectrum) • 802.11b HR-DSSS (HR = high rate) • 802.11a OFDM (orthogonal frequency division multiplexing)

Spread Spectrum • Put the signal onto many frequencies, not just one. • Two approach • Frequency hopping (Hedy Lamarr) • Pseudorandom number generator dictates which frequency to use. • Stay at a frequency for a short “dwell time.” • Direct sequence spread spectrum • Each symbol is spread over many frequencies. • 802.11b uses Barker code +1 –1 +1 +1 –1 +1 +1 +1 –1 –1 –1. If 1 is to be sent, then +1 –1 +1 +1 –1 +1 +1 +1 –1 –1 –1 is sent to the modulator. If 0 is to be sent, then -1 +1 -1 -1 +1 -1 -1 +1 +1 +1 +1 is sent to the modulator. (Like CDMA?)

Collision Avoidance in 802.11 • Two approaches • DCF (Distributed Coordination Function) • No RTS/CTS CSMA/CA (Carrier sense multiple access with collision avoidance (not /CD = carrier detection which is very difficult in wireless). If a node wants to broadcast, it checks if the channel is idle for a little while (DIFS). If not, use binary exponential backoff in slot times (like Ethernet). If the channel is idle, it broadcasts. When the receiver gets the frame, it check the CRC and if all is ok, it transmits an ACK. If the source fails to get an ACK, it will resend the (after getting control of the channel again).

Collision Avoidance in 802.11 • DCF – problems (is DCF better than Aloha?) • It is possible that two nodes are listening at the same time. But this is not very likely if there is not too much traffic. Remember, propagation delay is very small. • The hidden node problem. Suppose that node A want to transmit to node B and while node C is transmitting to node B….. A B C

Collision Avoidance in 802.11 • DCF with RTS/CTS • Suppose that node C wants to transmit to node B. • First, C broadcast a RTS (request to send). • If B is not currently hearing another broadcast (i.e., B gets the RTS), then B broadcasts a CTS (clear to send) to C. • When the packet has arrived, an ACK is sent. • Now, if A then wants to broadcast, it would have heard the CTS. So, instead, node A waits….

Collision Avoidance in 802.11 • DCF with RTS/CTS • Suppose that node C wants to transmit to node B. • First C broadcast a RTS (request to send) which includes information on how long the broadcast will last. • If B is not currently hearing another broadcast (i.e., B gets the RTS), then B broadcasts a CTS (clear to send) to C along with C’s estimate of the time the broadcast will last. • When the packet has arrived, an ACK is sent to • Now, if A then wants to broadcast, it would have heard the CTS. So it waits for the amount of time specified in the CTS. • Suppose you hear a RTS but no CTS, should you wait the length of time in the RTS?

Collision Avoidance in 802.11 • DCF with RTS/CTS • If a node wants to transmit, it checks that the channel is idle. And continues to check it for DIFS seconds. If it remains idle for this whole period it transmits. • Otherwise, it waits until it is idle and then wait a random amount of time Back off = Random(0,CW) * SlotTime Random(0,CW) is a random number between 0 and CW. • After waiting, goto 1. backoff Slot time DIFS channel busy transmit if idle channel still idle channel checked

DCF: CW calculation • CW may be increased when the channel fails to be clear when checked. • CW may be decreased when the channel is idle when checked. CW CWmax CWmin Channel found to be busy Channel found to be idle could be zero

DCF: When do you sense the channel? When you want to send a frame and • No CTS and RTS indicates that there might be an ongoing transmission. • There is no ongoing backoff. The network allocation vector (NAV) is an internal structure that records when the channel might be free. If the NAV indicates that the channel is busy, then we say that the virtual channel is busy. A transmission is only attempted when both the physical sense and the NAV agree that the channel might be idle.

Flowchart of CSMA/CA start NAV=0 ? yes sense channel random backoff Channel Idle ? no yes transmit frame collision ? no ? No - success

Fragmenting in DCF • The BER for wireless channels may be large. So frames are fragmented to increase throughput… • But if a many fragments are used, then the overhead of gaining access to the channel is large. • To alleviate this problem, two different time intervals are specified. • For a node broadcasts, it first checks if the channel is idle for DIFS. • Thus, after an ACK, the channel is guaranteed to be idle for at least DIFS. However, if another fragment needs to be sent or resent, or if an frame is to be resent (because the ACK did not arrive), it will be broadcast after waiting SIFS (short interfame spacing). With SIFS < DIFS.

Timing SIFS = 16s, PIFS = 25s, DIFS = 34s, EIFS = 43s, Slottime = 9s Sending a single data packet DIFS DIFS SIFS SIFS SIFS RTS CTS DATA ACK channel sense begin to sense channel decide that the channel to be idle Sending a fragmented data packet DIFS SIFS SIFS SIFS SIFS SIFS SIFS RTS CTS Frag 1 ACK Frag 2 ACK How long does it take to send an RTA, CTA, Data or ACK? Later

Sending back to back packets DIFS DIFS SIFS SIFS SIFS SIFS RTS CTS Data ACK RTS CTS Wait an random backoff, i.e., random(0,CW)*slottime) But don’t increment/decrement CW.

Problems with RTS/CTS

Frame Layout for 802.11b PLCP MPDU physical layer convergence protocol MAC protocol data unit Physical layer convergence protocol always transmitted at 1Mbps Preamble Enable synchronization Sync 128 bits SFD 16 bits Signal 8 bits Service 8 bits Length 16 bits CRC 16 bits contains the bit rate of the MPDU 1Mbps, 2Mbps, 5.5Mbps, 11Mbps time in microseconds to transmit the MPDU total time to transmit PLCP = ?

data frame Frame Layout for 802.11b PLCP MPDU physical layer convergence protocol MAC protocol data unit MPDU

Frame Layout for 802.11b RTS frame control 2 bytes 00011011X…X duration 2 bytes receiver address 6 bytes transmitter address 6 bytes error detection 4 bytes CTS frame control 2 bytes 00011100X…X duration 2 bytes receiver address 6 bytes error detection 4 bytes ACK frame control 2 bytes 00011101X…X duration 2 bytes receiver address 6 bytes error detection 4 bytes

What is the overhead for a data packet and for a fragment?

PCF • Definition AP access point – the central controller of the nodes. Often these are connected to the wired network. • The access point polls each node to ask if it wants to send something. Hence, no collisions. • When a node moves closer from one AP domain to another, it waits for a beacon. The beacon invites nodes to sign up for polling service.

802.11 – Wireless PHY and MAC