‘Neighborhood Capture’ in Wireless LANs

1. ‘Neighborhood Capture’ in Wireless LANs Mathilde Benveniste AT&T Labs, Research M. Benveniste -- AT&T Labs, Research

2. HC 3 HC 1 HC 2 ‘Neighbourhood Capture’ Effect Does backoff work when different co-channel HCs hear different Carrier Sense reservations? With variable packet lengths cause the countdown rates to be different:; higher for HC1 and HC3, lower for HC2 With fixed packet lengths or fixed CFPs/CFB lengths, the backoff timers of HCA and HCB will be set and counted down in parallel. HC1 hears only HC2; HC2 hears both HC1 and HC3; and HC3 hears only HC2. HC1 and HC3 hog the channel, while HC2 gets little through An example HC1 backoff 1 7 6 3 1 1 5 HC2 backoff 8 2 1 HC3 backoff 1 1 3 4 2 5 6 7 Time M. Benveniste -- AT&T Labs, Research

3. The Problem The capture problem described above occurs also in ordinary wireless LANs using CSMA/CA Because all nodes do not hear the same channel busy intervals due to the different attenuation experienced as a result of their location, channel capture is possible If packet lengths, or durations of contention-free sessions, are different, the countdown rates would be different, higher for periphery stations, and lower for centrally located stations As a result, the periphery stations would hog the channel Even when all stations have the same number of co-channel neighbors, serious instabilities may arise. M. Benveniste -- AT&T Labs, Research

4. BP, CFS and PCFS: Definitions Contention free session (CFS) Any frame exchange sequence that may occur without contention (frames are separated by a SIFS) following a successful channel access attempt. A CFS may involve one or more stations. A CFS may be initiated by any station. • A Contention-Free Burst and an RTS/CTS exchange are both examples of a CFS Periodic contention free period (PCFS) A CFS that must occur at regular time intervals. • A Contention-Free Period is an example of a PCFS Busy Period (BP) The generic term Busy Period will be used to indicate either frames or contention-free sessions (CFSs). Different BPs access the channel according to different protocols Stations use random backoff CFSs use CPMA PCFSs use fixed cycle time BPs are all assigned an AIFS Stations are assigned AIFS according to their priority CFSs and PCFSs use the same AIFS M. Benveniste -- AT&T Labs, Research

5. The solution: Slotted CSMA/CA Each BP accesses the channel according to its assigned AIFS, and access protocol (i.e. random backoff, deterministic backoff, or fixed cycle timer) Time is slotted at equal spacing; BPs must complete at slotted time; they must have the same length after adjustments for channel access When a BP is over, nodes contend for the channel [for instance by using RTS/CTS] Collisions will occur, but there will be some winners that will transmit a BP of length adjusted for the AIFS and backoff delay Nodes that attempted access unsuccessfully using a short reservation packet, RTS/CTS, which do not interfere with successful BP(s) will be able to use the channel concurrently. • Their length is adjusted to account for the.time devoted to contention By synchronizing nodes to end their BPs at the same time, equal access is offered to all nodes M. Benveniste -- AT&T Labs, Research

6. ‘Neighborhood Capture’ problem resolved through slotting The presence of neighboring mutually non-interfering nodes does not place a centrally located node at a disadvantage The channel becomes free at regular time intervals, when every node has a chance to seize the channel regardless of the number of interfering nodes surrounding it HC1 and HC3 no longer hog the channel, HC2 gets equal access HC1 backoff 7 3 7 5 5 3 1 1 7 1 HC2 backoff 1 2 6 2 1 HC3 backoff 5 7 1 1 7 3 1 3 5 7 Time M. Benveniste -- AT&T Labs, Research

7. Slotted CSMA/CA Slotted BP end times: All BPs are slotted to end at pre-specified times that are equally spaced Exceptions: Exceptions to the above rule are possible. BPs may end before the designated time boundary. Examples of exempt BPs are ‘pegs’ (short BPs generated for the purpose of maintaining contiguity of CFB sequences). Other packets may be exempted as well. Foreshortened BPs: Because of short BPs and a variety of other reasons, such as CSMA backoff idle slots, RTS collisions, etc., a BP start may be delayed; the BP will therefore not have the maximum possible length, BPLength in order to complete by the slot boundary In such a case, the node generates a shorter BP to ensure termination of the BP at the slotted time Delay (due to backoff idle gaps/ RTS collisions ...) X=Shield/Beacon BPLength Foreshortened BP X X BP BP Slotted BP end time Slotted BP end time Slotted BP end time M. Benveniste -- AT&T Labs, Research

8. Synchronization Nodes must send beacons ending at the time boundaries for synchronization When a node powers on, it synchronizes to the end times of active nodes Time offsets may arise when distant nodes [nodes that cannot hear one another] power on at different times This could happen early in the morning when a few stations are on. As more nodes power on and synchronize with their neighbors in the course of the day, asynchrony may arise Clock adjustment is necessary in order to eliminate time offsets Clock synchronization may be achieved as in an IBSS. • A beacon or probe response frame carrying a timestamp field will be transmitted by the nodes. • Each node will update it’s timer, TSF timer, if the received time stamp is later. • The initial setting of the TSF timer when a node powers on is 0. M. Benveniste -- AT&T Labs, Research

9. Fixed lengthCFSs and PCFSs Fixed length CFS This requirement implies that a station cannot simply transmit short packets as they come. Depending on the scenario under which a LAN operates, the packet must be either filled with null data to reach the target length, or a CFS of the target duration must be put together. Because a CFS must be filled, a station might transmit lower priority frames in the same CFS before another station, with higher priority packets, seizes the channel. Fixed length PCFS This requirement facilitates collision avoidance of PCFSs. According to the access mechanism for PCFSs [01/569], collisions with (E)DCF transmissions can be readily avoided if the length of a PCFS is fixed M. Benveniste -- AT&T Labs, Research