Chapter 3: Medium Access Control

1. Chapter 3:Medium Access Control

2. Motivation • The main question in connection with MAC in the wireless is whether it is possible to use complicated MAC schemes from wired networks. • For example: CSMA/CD • Let us consider carrier sense multiple access with collision detection (CDMA/CD) which works as follow: • A sender senses the medium (a wire) to see if it is free. If the medium is busy, the sender waits until it is free.

3. Motivation • If the medium is free, the sender starts transmitting data and continues to listen into the medium. • If the sender now detects a collision while sending it stops at once and sends a jamming signal. • why does this scheme fall in wireless networks? • CDMA/CD is not really interested in collisions at the sender, but rather in those at the receiver. • The signal should reach the receiver without collisions.

4. Motivation • But the sender is the one detecting collisions. • This is not a problem using a wire, and if a collision occurs somewhere in the wire, everybody will notice it. • The situation is different in wireless networks. • The strength of a signal decreases proportionally to the square of the distance to a sender. • The sender may now apply carrier senses and detect an idle medium. • Thus the sender start sending-but a collision happens at the receiver due to a second sender.

5. Motivation • This is hidden terminal problem. That is happen to the collision detection. • The sender detects no collision assumes that the data has been transmitted without errors, but actually a collision. • The destroyed the data at the receiver. • Thus this very common MAC scheme from wire network fails in a wireless scenario.

6. Motivation - hidden and exposed terminals • Hidden terminals • Consider the situation as show in figure. • A sends to B, C cannot receive A • C wants to send to B, C senses a “free” medium (CS fails) • collision at B, A cannot receive the collision (CD fails) • A is “hidden” for C

7. Motivation - hidden and exposed terminals A B C A B B C A

8. Motivation - hidden and exposed terminals • Exposed terminals • Consider the situation as show in figure. • B sends to A, C wants to send to another terminal (not A or B) • C has to wait, CS signals a medium in use • but A is outside the radio range of C, therefore waiting is not necessary • C is “exposed” to B

9. Motivation - near and far terminals • Consider the situation as show in figure. • A and B are both sending with the same transmission power. • As the signal strength decreases proportionally to the square of the distance, B’s signal drowns out A’s signal. • As a result, C cannot receive A’s transmission. • The near/far effect is a several problem of wireless networks using CDM. • All signals should arrive at the receiver with more or less the same strength.

10. Motivation - near and far terminals A B C

11. Motivation - near and far terminals • E.g. A person standing closer to somebody could always speak louder than a person further away. • Even if the sender were separated by code, the closest one would simply drown out the others. • Thus, precise power control is needed to receive all senders with the same strength at a receiver.

12. SDMA • Space division multiple access (SDMA) is used for allocating a separated space to users in wireless networks. • A typical application involves assigning a optimal base station to mobile phone user. • The mobile phone may receive several base stations with different quality. • A MAC algorithm could now decide which base station is best, taking into account which frequencies (FSM), time slots (TDM) or code (CDM) are still available.

13. SDMA • Typically, SDMA is never used in isolation but always in combination with one or more other schemes. • The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the infrastructure implementing space division multiplexing (SDM).

14. FDMA • Frequency division multiple access (FDMA) comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM). • Allocation can either be fixed (e.g.-radio station) or dynamic (e.g.- demand driven). • Channels can be assigned to the same frequency at all times that is pure FDMA, or change frequencies according to a certain patter, that is FDMA combined with TDMA.

15. FDMA • The other example of many wireless systems to narrowband interference at certain frequencies known as frequency hopping. • Sender and receiver have to agree on a hopping pattern otherwise the receiver could not tune to the right frequency. • Thus hopping pattern are typically fixed, at least for a longer period. • The fact that it is not possible to arbitrarily jump in the frequency space.

16. FDMA • Example :slow hopping (e.g., GSM), fast hopping (FHSS) Frequency Hopping Spread Spectrum) • Furthermore, FDM is often used for simultaneous access to the medium by base station and mobile station in cellular networks. • Here, the two partners typically establish a duplex channel. • The two directions, Mobil station to base station and vice versa are now separated using different frequencies.

17. FDMA • This scheme is then called frequency division duplex (FDD). • The two frequencies are also known as uplink, that is from mobile station to base station or from ground control to satellite and as downlink that is base station to mobile station or from satellite to ground control. • As example for FDM and FDD show in next slide figure in that situation in a mobile phone network base on the GSM standard for 900 MHz. • The basic frequency allocation scheme for GSM is fixed.

18. FDD/FDMA - general scheme, example GSM f 960 MHz 124 200 kHz 935.2 MHz 1 20 MHz 915 MHz 124 890.2 MHz 1 t

19. TDMA • Compared to FDMA, time division multiple access (TDMA) offers a much more flexible scheme, which comprises all technologies that allocated certain time slots for communication, that is controlling TDM now tuning in a certain frequency in not necessary, that is the receiver can stay at the same frequency the whole time. • Using only one frequency and thus every simple receivers and transmitters many different algorithms exist to control medium access.

20. TDMA • As already mentioned listening to different frequencies at the same time is quite difficult, but listening to many channels separated in time at the same frequency is simple. • Now synchronization between sender and receiver has to be achieved in the time domain. • This can be done by using a fixed pattern that is allocating a certain time slot for a channel or by using dynamic allocation scheme.

21. TDMA • Dynamic allocation schemes require an identification for each transmission as this is the case for typical wired MAC schemes (e.g.-sender address) • Fixed schemes do no need an identification, but these are not as flexible considering. • Varying bandwidth requirements, there are several examples for fixed and dynamic schemes as used for wireless transmission. • Typically, those schemes can be combined with FDMA to achieve even greater flexibility.

22. Fixed TDM • The simplest algorithm for using TDM is allocating time slots for channels in a fixed pattern. • This results in a fixed bandwidth and is the typical solution for wireless phone system. • The fixed pattern can be assigned by the base station, where competition between different mobile stations that want to access the medium is solved. • TDMA scheme with fixed access patterns are used for many digital mobile phone systems like IS-54,IS-136,GSM,DECT,PHS and PACS.

23. Fixed TDM • The next slide figure these fixed TDM patterns are used to implement multiple access and a duplex channel between a base station and mobile station. • Assigning different slots for uplink and downlink using the same frequency is called time division duplex (TDD). • As shown in the figure, the base station uses one out of 12 slots for the downlink, whereas the mobile station uses one out of 12 different slots for the uplink. • Uplink and downlink are separated in time and each connection is allotted its own up and downlink pair.

24. TDD/TDMA - general scheme, example DECT 417 µs 1 2 3 11 12 1 2 3 11 12 t downlink uplink

25. What is Aloha? • Aloha, also called the Aloha method, refers to a simple communications scheme in which each source (transmitter) in a network sends data whenever there is a frame to send. • If the frame successfully reaches the destination (receiver), the next frame is sent. • There are different types of Aloha: • Classical Aloha • Slotted Aloha

26. Classical Aloha • This is exactly what the classical Aloha scheme does, a scheme which was invented at the university of Hawaii and was used in the ALOHANET for wireless connection of several stations. • Aloha neither co-ordinates medium access nor does it resolve contention on the MAC layer. • Instead each station can access the medium at any time as shown in next slide figure. • This is a random access scheme, without a central arbiter controlling access and without co-ordination, among the stations.

27. Classical Aloha • In two or more stations access the medium at the same time, a collision occurs and the transmitted data is destroyed. • Resolving this problem is to retransmission of data.

28. Classical Aloha collision sender A sender B sender C t

29. Slotted Aloha • "Slotted Aloha" reduces the chance of collisions by dividing the channel into time slots and requiring that the user send only at the beginning of a time slot. Aloha was the basis for Ethernet, a local area network protocol. • In this case, all senders have to be synchronized, transmission can only start at the begin of a time slot as shown a figure. • Under the assumption stated above, the introduction of slots raises the throughput from 18 to 36 percent that is slotting double the throughput.

30. Slotted Aloha collision sender A sender B sender C t

31. Carrier sense multiple access (CDMA) • One improvement to the basic Aloha is sensing the carrier before accessing the medium. • This is what carrier sense multiple access (CSMA) schemes generally do the sensing the carrier and accessing the medium only if the carrier is idle decreases the probability of a collision. • But as already mentioned in the introduction, hidden terminals cannot be detected. • Thus if a hidden terminal transmits at the same time as another sender, a collision might occur at the receiver.

32. Carrier sense multiple access (CDMA) • Still, this basic scheme is used in most wireless LANs. • Several versions of CSMA exist. • Non-persistent CSMA: • The stations sense the carrier and start sending immediately if the medium is idle. • If the medium is busy the station pauses a random amount of time before sensing the medium again and repeating this pattern.

33. Carrier sense multiple access (CDMA) • P-Persistent CSMA: • In systems nodes also sense the medium, but only transmit with a probability of P, with the station reschedule to the next slot with the probability P that is access is slotted in addition. • I-Persistent CSMA: • in systems all stations wishing to transmit access the medium at the same time, as soon as it becomes idle.

34. Carrier sense multiple access (CDMA) • CSMA with collision avoidance (CSMA/CD) is one of the access schemes used in wireless LANs following the standard IEEE 802.11.

35. Demand assigned multiple access (DAMA) • A general improvement of aloha access systems can also be achieved by reservation mechanisms and combinations with some (fixed) TDM patterns. • These schemes typically have a reservation period followed by a transmission period. • During the reservation period, stations can reserve future slots in the transmission period.

36. Demand assigned multiple access • While, depending on the scheme, collision may occur during the reservation period, the transmission period can then be accessed without collision or split into transmission periods with and without collision. • In general these schemes cause a higher delay under a light load, but allow higher throughput. • One basic scheme is demand assigned multiple access (DAMA) also called reservation Aloha, a scheme typical for satellite system.

37. Demand assigned multiple access • Show in this next slide figure has two modes. • ALOHA mode for reservation:competition for small reservation slots, collisions possible • Reserved mode for data transmission within successful reserved slots (no collisions possible) • During a contention phase following the slotted Aloha scheme, all stations can try to reserve future slots.

38. Demand assigned multiple access For example, different stations on earth try to reserve access time for satellite transmission. Thus collisions during the reservation phase do not destroy data transmission, but only the short requests for data transmission. If successful, a time slot in the future is reserved, and no other station is allowed to transmit during this slots.

39. Demand assigned multiple access collision t Aloha reserved Aloha reserved Aloha reserved Aloha

40. Demand assigned multiple access • Therefore, the satellite collects all successful requests and sends back a reservation list indicating access rights for future slots. • All ground stations have obey this list. • To maintain the fixed TDM pattern of reservation and transmission, the stations have to be synchronized from time to time. • DAMA is an explicit reservation scheme. • Each transmission slot has to be reserved explicitly.

41. Packet Reservation Multiple Access(PRMA) • An example for an implicit reservation scheme is packet reservation multiple access (PRMA). • Here, slots can be reserved implicitly according to the following scheme. • Show in the figure a certain number of slots forms a frame. • The frame is repeated in time, that is a fixed TDM pattern is applied. • The base station now broadcasts the status of each slot to all mobile stations.

42. Packet Reservation Multiple Access(PRMA) reservation 1 2 3 4 5 6 7 8 time-slot ACDABA-F frame1 A C D A B A F ACDABA-F frame2 A C A B A AC-ABAF- collision at reservation attempts frame3 A B A F A---BAFD frame4 A B A F D ACEEBAFD frame5 A C E E B A F D t

43. Packet Reservation Multiple Access(PRMA) • All stations receiving this vector will then know which slot is occupied and which slot is currently free. • In the example, the base station broadcasts the reservation status ‘ACDABA-F’ to all stations, here, A to F. • This means that slots one to six and eight are occupied, but slot seven is free in the following transmission. • All stations wishing to transmit for this free slot in Aloha fashion. • In the example shown, more than one station wants to access this slot, thus a collision occurs.

44. Packet Reservation Multiple Access(PRMA) • The base station returns the reservation status ‘ACDABA-F’ indicating that the reservation of slot seven failed and that nothing has changed for the other slots. • Again stations can compete for this slot. • Additionally, station D has stopped sending in slot three and station F in slot eight. • This is noticed by the base station after the second frame. • Before the third frame starts, the base station indicates that slots three and eight are now idle.

45. Packet Reservation Multiple Access(PRMA) • Additionally, station F has succeeded in reserving slot seven as also indicated by the base station. • PRMA constitutes yet another combination of fixed and random TDM schemes with reservation compared to the previous schemes. • As soon as a station has succeeded with a reservation, all future slots are implicitly reserved for this station. • This ensures transmission with a guaranteed data rate. • The slotted aloha scheme is used for idle slots only, data transmission is not destroyed by collision.

46. Access Method DAMA: Reservation TDMA • An even more fixed pattern that still allows some random access exhibited by reservation TDMA. • Show in the figure. e.g. N=6, k=2 N * k data-slots N mini-slots reservationsfor data-slots other stations can use free data-slots based on a round-robin scheme

47. Access Method DAMA: Reservation TDMA • every frame consists of N mini-slots and x data-slots • every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k). • other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic)

48. MACA - collision avoidance • MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance • RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet • CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

49. MACA-Collision Avoidance • Signaling packets contain • sender address • receiver address • packet size • Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

50. A C MACA examples • MACA avoids the problem of hidden terminals • A and C want to send to B • A sends RTS first • C waits after receiving CTS from B RTS CTS CTS B