Collision Avoidance and Resolution Multiple Access with Transmission Groups

1. Collision Avoidance and Resolution Multiple Access with Transmission Groups R. Garces and J. J. Garcia-Luna-Aceves INFOCOM’97

2. Abstract • proposing a CARMA-NTG protocol for accessing wireless media • CARMA-NTG = Collision Avoidance and Resolution Multiple Access Protocol with Non-persisitent Trees and transmission Group • Based on transmission group • Once obtaining the medium, a station will have its right to keep on sending. • based on RTS/CTS messages

3. Introduction • Dynamically divide the channel into cycles of variable length. • Each cycle contains a contention period and a group-transmission period. • The group-transmission period is a train of packets sent by users already in the group. • New users contend to join transmission group by contending during the contention period. A, B, C Y, A, B, C Z, Y, A, B, C X, Z, Y, A, B, C media : contention period : group trans. period X Y Z

4. CAMA-NTG • Each cycle is composed of • contention period: allowing one user to be joined in one contention period. • group transmission period: allowing one packet to be transmitted per cycle per station. • To send in the transmission period, each station must know the following environment parameters: • the number of members in the transmission group • its position within the group • the beginning of the each group-transmission period • the successful RTS/CTS exchange of new users in the previous contention period

5. Group-Transmission Period • A station transmits once the previous station’s packet is received. • The spacing is twice the propagation delay. • If this is not heard during this period, • assume that the previous station fails • its membership is removed from the group • the failed station has to contend to join the group later. Exceed propagation delay A B C A C A C B contend later

6. Contention Period • Contending based on RTS/CTS exchange. • The contention period terminates once the first station successfully join the group. • Each station keeps the following variables: • a unique ID • LowID and HiID: to denote the current contention window in the current contention period • contention window: the allowable ID’s that can contend • an ID not within this range can not contend • a stack: the future potential contention windows

7. State Transition • A REMOTE station needs to sense the carrier before trying to contend. • It backoff repeatedly until no carrier is heard. • Then an RTS is sent. • NTG (non-persistent tree and transmission group) is used to resolve multi-RTS conflict. • Once CTS is received, the permitted station can send one packet immediately. BACKOFF carrier timeout send out first packet immediately RTS XMIT REMOTE no carrier send RTS CTS

8. NTG Scheme • Initially, LowID=1 and HiID=(max. ID in the system) • On RTS conflict, all stations divide (LowID, HiID) into • (LowID, (LowID+HiID)/2) • ((LowID+HiID)/2 + 1, HiID) // i.e., binary split • PUSH the first part into STACK • Contend if its ID is within the latter part. • If no RTS is heard after channel delay, POP the stack and repeat recursively. • ONLY stations in the RTS state can persist in trying. • new stations: backoff and wait until the next period • already-in-group stations: not until they leave the group

9. Contention Example • A system with 4 stations: n00, n01, n10, n11. • n00 and n01 are contending. (a) (c) (b) (d) n01 RTS n00 RTS n11 idle n10 idle after idle after 2nd collision after n01 success before 1st collision after 1st collision (a) (b) (c) (d) (00, 01) (00, 00) allowed interval (00, 11) (10, 11) (00, 01) (01, 01) (00, 11) n01 RTS n00 RTS n01 RTS packets

10. Status in the Contention Period • Idle: no RTS is heard • duration: one channel delay • Success: one RTS heard • duration: • one RTS delay + one channel delay • one CTS delay + one channel dealy • Collision: multiple RTSs heard • duration: • one RTS delay + one channel delay • RTS and CTS also have the effect of announcing the number of packets to be send during the stay in group. • So the group can be maintained more accurately.

11. Cost Analysis • Metric: average collision cost for a system with n stations and m RTS stations • Further divided into 3 costs: • Z(n, m): idle cost • S(n, m): success cost • C(n, m) collision cost • In our earlier example: • Z(4, 2) = 1 • S(4, 2) = 1 • C(4, 2) = 2 • Goal: evaluate the average cost for all m and n combinations.

12. 2 0 1 0 2 1 Average Collision Cost C(n, m) • In the earlier example (n=4, m=2), there are C(4, 2) = 6 combinations of contenders. This collision

13. Upper Bound • Success cost S(n, m) is always 1. • Theorem 1: For all m > 1 and n > 1, the average idle cost Z(n, m)  1/2. • Theorem 2: For all m > 1, the average collision cost C(n, m)  log(m) + 1.

14. Simulation on Throughput • CARMA-NTG vs. FAMA-NTG

15. Conclusions • propose the concept of group transmission • ONE RTS/CTS exchange is used for transmitting a train of packets • better fairness than IEEE 802.11 • NTG (non-persistent tree group) keeps the contention cost low. • Performance: • on high load, similar to TDMA • on low load, better than TDMA by getting rid of empty slots