Wi-Fi

Basic structure: – Stations plus an access point – Stations talk to the access point, then to outside – Access point talks to stations – Stations talk to stations Design goal: – A MAC protocol to determine who talks next

Wireless communications Signal decays according to a power law with the distance, at least to the power of -2 with distance Comparing to Ethernet, what is the difference (as far as MAC is concerned)? When a station is sending, not all stations can hear. No real 100% carrier sense. – In Ethernet, everybody can hear everybody

Wireless communications When a station is sending, it cannot hear other stations – cannot decide if there is a collision. No CD in wireless LAN. – In Ethernet, the sender can determine if there is collision and abort immediately.

Wireless communications Being able to sense the carrier does not mean that you can decode the data If received signal having power P means that you can decode the data, it may be true that at power P/2 you can realize that there is something going on

Wireless communication The received signal can be decoded if the signal to noise ratio is larger than a certain threshold. Whether there is a collision depends on the signal to noise ratio at the receiver. You may allow two transmissions at the same time without collision. – In Ethernet, two simultaneous transmission means collision ADCB ADCB A->B, C->D A->B, D->C

Wireless communications Hidden terminal, A->B, C->D. C did not hear A. ADCB Exposed terminal. A->B, C->D. C hears A. A D C B

Medium Access Control (MAC) Layer Asynchronous Data Service – DCF (Distributed Coordination Function) Contention-Based Medium Access Control CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance For elastic applications like , file transfer Time-Bounded Service – PCF (Point Coordination Function) Contention Free Medium Access Control Optional access method works like polling For time-sensitive voice/video applications

Problems What problems will occur if apply Ethernet MAC? – No CD, does not know whether there is a collision – No CD, channel waste could be large using 1- persistent – Cannot hear all other people means the sender cannot be sure that he can reserve the whole channel.

Fixes No CD, use ACK. If there is no ACK, assume there is collision No CD, has to use non-persistent to reduce collision by AVOIDING COLLISION, CA Cannot hear other people, so devise some channel reservation technique

DCF’s Main Idea When get a packet to send, sense the channel. If channel is busy, wait until the channel is free for DIFS. Start to backoff for a random time. If busy before reaching zero, freeze bo counter, and reactivate when idle for DIFS again. If counted to 0 and channel is still idle, send. After receives a packet, send ACK. If no ACK received, double the window and retry.

Difference with Ethernet In Ethernet, a station will pick a backoff timer, sleep for that amount of time, then wake up. – When waking up, two cases: Medium is free: send Medium is busy: wait until free and send In Wi-Fi DCF, a station will pick a backoff timer backoff, while still monitor the medium – If the medium is busy during the backoff, will freeze the timer. Someone who waited longer will have a larger chance to send

Simplified DCF operation for unicast in implementation (Automating Cross-Layer Diagnosis of Enterprise Wireless Networks, Sigcomm 2007 ) The first packet does not have to experience the backoff before it is sent; backoff after a successful packet transmission. So if there is a packet following the first packet, it will go through the backoff process before transmission.

DCF Do you want the ACK to have the same priority as data packets? How do you make sure that ACK has higher priority? Use time. You have to wait for a certain amount time before you can send. High priority packets wait shorter.

DCF The SIFS, DIFS. SIFS is for control packets. DIFS is for data packets. When a station wants to send, if it is a control packet, sense the channel for SIFS, then send. If it is a data packet, sense the channel for DIFS, then send.

Rate supports multiple rates g has 6,9,12,18,24,36,48,54 Mbps. Lower rates have lower loss probabilities.

Research Challenge Any problem do you see in the design of MAC? Hint: wireless packets are subject to random loss, e.g., if you just walk by and blocked the line-of-sight path, the packet may be lost. In this case, what will MAC do? What should be done?

Further improvement Further improvement by improving carrier sense The problem is other people cannot hear me sending, so they will send. So, how to make sure that they will know I am sending?

RTS/CTS RTS/CTS in the place for carrier sense – RTS – reserves channel for a bit of time, if sender hasn’t heard other CTSes – CTS – sender replies if it hasn’t heard any other RTSes – Both messages include time. Network Allocation Vector (NAV) – If no CTS, exponential backoff – “RTS-CTS-DATA”

RTS/CTS standardized both CSMA/CA and RTS/CTS In practice, most operators disable RTS/CTS – Very high overhead! RTS/CTS packets sent at “base rate” (6Mbps for g) – Avoid collisions regardless of transmission rate – Most deployments are celluar (base stations), not ad hoc. Neighboring cells are often configured to use non-overlapping channels, so hidden terminals on downlink are rare Hidden terminal on uplink possible, but if clients mostly d/l, then uplink packets are small. THIS MAY CHANGE. And is likely not true in your neighborhood! – When CS range >> reception range, hidden terminal less important

PCF The AP acts as the master and sends out beacon signals for polling stations and stations can sign up for certain amount of bandwidth use Co-exists with DCF. How to make sure that beacon signals have higher priority? – PIFS