CS6956: Wireless and Mobile Networks Lecture Notes: //05 IEEE 80. Wireless Local Area Networks (WLANs) CSMA/CD Carrier Sense Multi Access/Collision Detection detects collision and retransmits, no acknowledgement, chance of channel errors is very low, can detect errors corrected through retransmissions at the upper layers, no link layer retransmissions (other than those due to collision detection) CSMA/CA Carrier Sense Multi Access/Collision Avoidance cannot detect collisions (not enough energy), uses acknowledgement- based mechanisms to retransmit unacknowledged frames, uses certain collision avoidance mechanisms (explained later in the lecture) Random Access vs Reservation schemes: Reservation schemes waste resources when they are not used. Random access schemes more useful when fewer number of nodes contend for the channel can possibly let one sender use the entire channel when no other senders are transmitting. Infrastructure and Ad Hoc Modes: Instrastructure mode communication through an Access point (AP), ad hoc mode direct communication possible, WiFi Direct infrastructureless but one node assumes the role of an access point. Basic service set (BSS): comprising user devices (laptops, smartphones, etc.) and an access point. Extended service set multiple BSS, possibly overlapping, with APs connected by a separate local area network. Association Protocol: A user device must associate with an Access Point. Access Points transmit or receive only on specified channels. A device could potentially passively scan all channels to hear active AP transmissions (or beacons sent by APs). Alternatively, a device could actively scan channels by sending probe requests on each channel. An AP on receiving a probe request, responds by sending a probe reply. A device can then choose to associate with that access point (if it is authenticated by the access point in case of secure WiFi networks). When a large number of users try to associate with only a few access points, the association protocol traffic can become very high due to user devices trying to find the best access points (because the access points they associate with are unable to provide satisfactory performance). The next few pages are slides from Kurose and Ross on the basics of WiFi LANs. These slides also include the IEEE 80. MAC protocol as well as material on the hidden terminal problem and the use of RTS/CTS.
Elements of a wireless network network infrastructure infrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network Wireless, Mobile Networks 6- Elements of a wireless network ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves Wireless, Mobile Networks 6-
Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B C A B A s signal C s signal Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other means A, C unaware of their interference at B space Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other interfering at B Wireless, Mobile Networks 6-3 80. LAN architecture BSS Internet hub, switch or router wireless host communicates with base station base station = access point (AP) Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only BSS Wireless, Mobile Networks 6-4
80.: Channels, association 80.b:.4GHz-.485GHz spectrum divided into channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing AP s name (SSID) and MAC address selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in AP s subnet Wireless, Mobile Networks 6-5 80.: passive/active scanning BBS BBS BBS BBS AP 3 AP AP 3 4 AP H H passive scanning: () beacon frames sent from APs () association Request frame sent: H to selected AP (3) association Response frame sent from selected AP to H active scanning: () Probe Request frame broadcast from H () Probe Response frames sent from APs (3) Association Request frame sent: H to selected AP (4) Association Response frame sent from selected AP to H Wireless, Mobile Networks 6-6 3
IEEE 80.: multiple access avoid collisions: + nodes transmitting at same time 80.: CSMA - sense before transmitting don t collide with ongoing transmission by other node 80.: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) C A B C A B A s signal C s signal space Wireless, Mobile Networks 6-7 IEEE 80. MAC Protocol: CSMA/CA 80. sender if sense channel idle for DIFS then transmit entire frame (no CD) if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 80. receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS sender data ACK receiver SIFS Wireless, Mobile Networks 6-8 4
Avoiding collisions (more) idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets! Wireless, Mobile Networks 6-9 Collision Avoidance: RTS-CTS exchange A AP B RTS(A) RTS(B) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) Wireless, Mobile Networks 6-0 5
Exposed Terminal Problem Currently transmitting Wants to transmit R S S R A node is prevented by CSMA from sending packets to other nodes due to a neighboring transmitter. Both S and S can transmit to R and R simultaneously but cannot do so because S finds the channel to be busy. If it is possible to disable CSMA when both S and S have data to transmit, then we can get around the exposed terminal problem this could be done in cases where S and S are access points and are controlled by a common access point controller than can schedule transmissions through these access points in a fine granular manner. We will look at an example of this when discussing the CENTAUR paper (from ACM MobiCom Conference 009). It is possible for the ACK from R to collide with the transmission from S (to R) at S. Again, this problem can be handled through tight scheduling of transmissions and the corresponding ACKs by centralized controllers. Reliable Broadcast/Multicast Consider the scenario where an access point (AP) wishes to reliably broadcast or multicast frames to the user devices in its BSS. The current IEEE 80. protocols support reliable unicast but do not support reliable broadcast or multicast. ACK vs NAK- based solution: A positive ack based solution will result in collisions. One could possibly stagger the ACKs through random delays but that might take a long time to send all the ACKs from the user devices to the AP (depending on the number of user devices that the AP is trying to reach). A NAK- based solution depends on the user devices getting their NAKs reliably through to the AP. However, NAKs can collide too. NAKs can possibly be staggered in time and given that the number of NAKs is likely to be much less than the ACKs from all the user devices, the time taken for sending NAKs might not be as large. Also, one NAK for a frame should be enough so, one could possibly build a NAK overhearing mechanism where all the other devices suppress their NAKs when they hear a device transmitting a NAK.