1. Ch 12. Multiple Access

2. Multiple Access for Shared Link Dedicated link – Point-to-point connection is sufficient Shared link – Link is not dedicated – Wireless medium, Ethernet – Controlling the access to the medium Who will use the medium?

3. Taxonomy of Multiple Access

4. 12.1 Random Access Random access (or contention) – No station is superior to another station (no centralized authority) – No scheduled time for transmission – transmission is random – No rules specify which station would send next – stations compete with each other (contention)

5. Pure (Original) ALOHA Earliest random access, developed at the University of Hawaii in early 1970s Basic procedure – If a station has a frame to send, then it sends a frame immediately (hoping that none transmits simultaneously) – Then, check whether the transmission is successful (by receiving an ACK) Collision: two or more users’ transmissions overlap resulting in packet loss t Collision occurs Station 1 decides to Xmit Station 2 Xmits

6. Example of Four Stations Only two frames (1.1 and 3.2) are successful

7. Reliable Transmission Retransmission – The sender checks whether a transmission is successful by receiving an ACK from the receiver – If unsuccessful, the sender retransmits the frame Binary Exponential Back-off – Before retransmission, wait for random amount of time – If it is the first retransmission, random within [0,T], where T is a time unit (often, max round-trip propagation delay) – If it is the second retransmission, random within [0, 2T] – If it is the third retransmission, random within [0, 4T] –…–… – If it is the k-th retransmission, random within [0, 2 k T]

8. Detailed Algorithm of Pure ALOHA

9. Example 12.1 The stations on a wireless ALOHA network are a maximum 600km apart. If we assume that signals propagate at 3x10 8 m/s, we find T = (600x10 3 )/(3x10 8 ) = 2ms. Now we can find the value of backoff time T B for different Ks. – For K = 1, T B = 0 or 2ms – For K = 2, T B = 0, 2, 4, or 6ms – For K = 3, T B = 0, 2, 4, 6, 8, 10, 12, or 14ms – In many cases, there are maximum K. For example, if K > 10, the range is set to [0, …, 2 10 -1] T B

10. Vulnerable Time Vulnerable time is a time interval, in which there is a possibility of collision Assume that all stations send fixed-length frames

11. Throughput Analysis : Pure Aloha  N stations  Each station transmits λ (new) packets/ sec  Arrival from each station is Poisson  Fixed Packet lengths  Time to transmit a packet m=T fr

12. Throughput Analysis : Pure Aloha Due to collisions : Total rate of packets attempting transmission = λ′ λ′ > λ G = Actual traffic intensity or utilization G = N λ′m

13. Throughput Analysis : Pure Aloha Any transmissions by any station between times t-m and t+m will cause a collision Pr {Successful transmission} = Pr {No arrivals occur in time [t-m, t+m] | An arrival occurs at time t} = Pr {No arrivals occurring in time (t-m,t+m)} = Pr {No arrivals occur in time 2m} = e -N λ′ (2m) Hence, P s = Pr {Successful transmission} = e -2G (Because G = N λ′m) vulnerable time

14. Throughput Analysis : Pure Aloha

15. S = G e -2G : Pure Aloha throughput equation dS/dG = e -2G - 2G e -2G At the peak of S, e -2G (1 - 2G) = 0 G* = 1 / 2 = 0.5 S max = 0.5e -1 = 1 / (2e) S max ≈ 0.18 For Pure ALOHA the max. throughput is 18% of channel capacity.  Suitable for highly bursty traffic (computer traffic)

16. Throughput Analysis : Pure Aloha Ge -G Ge -2G G S 0.184 G* S max

17. Slotted ALOHA Time-slotted system Slotted ALOHA: stations must send a frame only at the beginning of the time slot

18. Vulnerable Time of Slotted ALOHA Improves the efficiency – Vulnerable time is T fr (Recall that pure ALOHA has vulnerable time of 2T fr ) – When a data arrives, it waits until the next slot

19. Throughput Analysis: Slotted Aloha  Pr {Success} = e -G  Similar to pure Aloha: S = G e -G, dS/dG = 0  G* = 1  S max = 1/e ≈ 0.36  Slotted Aloha improves throughput by a factor of 2!!!  However, maintaining slots consistently in a distributed system is difficult

20. Throughput Analysis: Slotted Aloha Ge -G Ge -2G G S 0.184 0.368 G* S max

21. Simulating Slotted-ALOHA Network nonAttempt = 0; collision = 0 for i=0 to 9 do – throughput(i) = 0 – variable p(i) is given within [0, 1] for t=1 to 1000 do /* simulate for 1000 time slots */ – for i=0 to 9 do /* simulate with 10 stations */ X(i) = 0 – for i=0 to 9 do X(i) = 1 with probability p(i) /* station i transmits at this time slot */ – Y = X(0) + X(1) + … + X(9) – case Y == 0: nonAttempt = nonAttempt + 1 == 1: throughput(i) = throughput(i) + 1 >= 2: collision = collision + 1 endfor print nonAttempt, collision, throughput(0), throughput(1),…, throughput(9)

22. Carrier Sense Multiple Access (CSMA) Intended to reduce collision – A station senses the medium before trying to use it (i.e., listen to the medium before sending) – If a station finds that there is an ongoing transmission in the medium, it should not send

23. Collision under CSMA CSMA can reduce collision, but cannot eliminate B C

24. Vulnerable Time of CSMA Vulnerable time = propagation time Tp

25. Persistence: CSMA 1-persistent – If the medium is busy, sense continuously – If the medium is idle, send a frame non-persistent – If busy, wait for a random amount of time – If idle, send a frame p-persistent – If busy, sense continuously – If idle, send a frame with prob. p, and wait for a time slot with prob. 1-p High chance of collision Low efficiency

26. Procedure of Persistence Methods

27. CSMA/CD CSMA with Collision Detection Station monitors the medium after it sends a frame – If it detects a collision, it stops transmitting immediately – Before sending the last bit of the frame, the sending station must detect the collision  restriction on the frame size

28. CSMA/CA CSMA with Collision Avoidance – Signal attenuation makes it hard to detect a collision (usual in wireless environments) – Inter-Frame Space (IFS): a station defers transmission for an IFS time, even if the channel is found idle – Contention windows: a station randomly picks a time slot of contention window as its wait time – Acknowledgement: data still can be corrupted

29. Cautions in CSMA/CA – IFS can be used to prioritize stations: stations with a higher priority will have a shorter IFS – Fairness: when a station finds that the channel is busy while it waits in the contention window, it freezes the timer, which will restart when the channel becomes idle

30. 12.2 Controlled Access Reservation access – A station needs to make a reservation before sending data time

31. Controlled Access (2) Polling – Primary station (central entity) manages other secondary stations – Select: check whether the primary station can send a frame to a secondary station – Poll: check whether a secondary station has a frame to send to the primary station

32. Controlled Access (3) Token passing – Stations in a network are organized in a logical ring – A special packet called token (there is only one token in the network) is circulated in the network – A station should wait for the token before sending data – The station who holds the token can send data – Once the station finishes its transmission, it passes the token to the next station – Token management: the token has to be monitored to ensure that it has not been lost or destroyed

33. 12.3 Channelization Frequency Division Multiple Access (FDMA)

34. Channelization (2) Time Division Multiple Access (TDMA)

35. Channelization (3) Code Division Multiple Access (CDMA)

36. Chip sequence of CDMA – Orthogonal sequences: c i * c j = 0 if i ≠ j – Each station j has its sequence c j, and sends c j for to send 1 or –c j for to send 0

37. CDMA (Cont) Decoding for Station 2’s receiver in CDMA

38. CDMA (Cont) At sender – d i = 0 or 1 At receiver of Station 3: – Result = c 3 * (d 1 * c 1 + d 2 * c 2 + d 3 * c 3 + d 4 * c 4 ) = 0 + 0 + d 3 * (c 3 * c 3 ) + 0

39. Homework Exercise in Chap. 12 – 11, 12, 13, 14, 15, 23 Additional problem – Simulate Slotted-ALOHA networks (see Slide 22) – 1) Draw nonAttempt when p(i) = p for all i, changing p = 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2 – 2) Repeat 1) with collision – 3) Repeat 1) with  i throughput(i) – 4) Obtain throughput(i)’s when p(i) = 0.02 x i, AND compare with the results when p(i) = 0.1 for all i