WIRELESS NETWORKSCH3: MEDIA ACCESS CONTROL
 This chapter aims to explain why special MACs are  needed in the wireless domain and why standard  MAC schemes known fro...
 The situation is different in wireless networks. the strength of a signal decreases proportionally to the  square of th...
HIDDEN AND EXPOSED TERMINALS SCENARIO   The transmission range of A reaches B, but not C.   The transmission range of C ...
   A starts sending to B, C does not receive this    transmission.   C also wants to send something to B and senses the ...
NEAR AND FAR TERMINALS SCENARIO A and B are both sending with the same  transmission power. As the signal strength decre...
 Now think of C acts as a base station coordinating  media access. In this case, terminal B would already drown out  ter...
SDMA   Space Division Multiple Access (SDMA) is used for    allocating a separated space to users in wireless    networks...
FDMA   Frequency division multiple access (FDMA)    comprises all algorithms allocating frequencies to    transmission ch...
 The two frequencies are also known as  uplink, i.e., from mobile station to base station. downlink, i.e., from base sta...
 the base station, shown on the right side, allocates  a certain frequency for up- and downlink to  establish a duplex ch...
TDMA Compared to FDMA, time division multiple  access (TDMA) offers a much more flexible  scheme, which comprises all tec...
FIXED TDM   The simplest algorithm for using TDM is allocating time    slots for channels in a fixed pattern.   This res...
 Assigning different slots for uplink and downlink  using the same frequency is called time division  duplex (TDD). the ...
 Classical Aloha Slotted Aloha Carrier sense multiple access Demand assigned multiple access PRMA packet reservation ...
ALOHA: PURE ALOHA The basic idea of an ALOHA system is simple: let users  transmit whenever they have data to be sent. T...
   Systems in which multiple users share a common    channel in a way that can lead to conflicts are    widely known as c...
   If the first bit of a new frame overlaps with just the    last bit of a frame almost finished, both frames will    be ...
   any other frame started between t0 + t and t0 + 2t    will bump into the end of the shaded frame.
SLOTTED ALOHA Slotted ALOHA was invented to improve the  efficiency of pure ALOHA. In slotted ALOHA we divide the time i...
   Because a station is allowed to send only at the    beginning of the synchronized time slot, if a station    misses th...
• The throughput for slotted ALOHA is S = G x e -G.• The maximum throughput Sma x = 0.368 when  G = 1.
CARRIER SENSE MULTIPLE ACCESS (CSMA) To minimize the chance of collision  and, therefore, increase the performance, the  ...
   a station may sense the medium and find it idle, only    because the first bit sent by another station has not yet    ...
 Vulnerable Time : The vulnerable time for CSMA is the propagation time  Tp. This is the time needed for a signal to pro...
1-PERSISTENT The 1-persistent method is simple and  straightforward. In this method, after the station finds the line id...
NONPERSISTENT A station that has a frame to send senses the line. If  the line is idle, it sends immediately. If the lin...
P-PERSISTENT   The p-persistent method is used if the channel has time slots    with a slot duration equal to or greater ...
DEMAND ASSIGNED MULTIPLE ACCESS   A general improvement of Aloha access systems can    also be achieved by reservation me...
   demand assigned multiple access (DAMA) also called    reservation Aloha, a scheme typical for satellite systems.   DA...
PRMA PACKET RESERVATION MULTIPLEACCESS   An example for an implicit reservation scheme is    packet reservation multiple ...
   A certain number of slots forms a frame. The frame is    repeated in time   A base station, which could be a satellit...
 Additionally, station D has stopped sending in slot  three and station F in slot eight. This is noticed by the base sta...
RESERVATION TDMA In a fixed TDM scheme N mini-slots followedby  N·k data-slots form a frame that is repeated. Each stati...
MULTIPLE ACCESS WITH COLLISIONAVOIDANCE   Multiple access with collision avoidance    (MACA) presents a simple scheme tha...
 With MACA, A does not start its transmission at  once, but sends a request to send (RTS) first. B receives the RTS that...
 Still, collisions can occur during the sending of an  RTS. Both A and C could send an RTS that collides at B.  RTS is v...
MACA ALSO HELP TO SOLVE THE ‘EXPOSEDTERMINAL’ PROBLEM   With MACA, B has to transmit an RTS first containing    the name ...
   Problems with MACA is the overheads associated    with the RTS and CTS transmissions – for short    and time-critical ...
POLLING Where one station is to be heard by all others  (e.g., the base station of a mobile phone network or  any other d...
INHIBIT SENSE MULTIPLE ACCESS This scheme, which is used for the packet data  transmission service, is also known as digi...
CODE-DIVISION MULTIPLE ACCESS (CDMA) In CDMA, one channel carries all transmissions  simultaneously. CDMA differs from F...
 Let us assume we have four stations 1, 2, 3, and 4  connected to the same channel. The data from station 1 are d l , fr...
   Station 1 multiplies its data by its code to get d l . Cl    Station 2 multiplies its data by its code to get d2 . C2...
 For example, suppose stations 1 and 2 are talking  to each other. Station 2 wants to hear what station 1 is saying. It...
   CDMA is based on coding theory. Each station is    assigned a code, which is a sequence of numbers called    chips.  ...
 This is called the inner product of two equal  sequences. For example, [+1 +1-1 -1]· [+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4...
DATA REPRESENTATION   We follow these rules for encoding: If a station needs to    send a 0 bit, it encodes it as -1;   ...
   The sequence on the channel is the sum of all four    sequences as defined before Now imagine station 3, which we sai...
Wierless networks ch3 (1)
Wierless networks ch3 (1)
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Wierless networks ch3 (1)

  1. 1. WIRELESS NETWORKSCH3: MEDIA ACCESS CONTROL
  2. 2.  This chapter aims to explain why special MACs are needed in the wireless domain and why standard MAC schemes known from wired networks often fail. CSMA/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. But the sender is the one detecting collisions. This is not a problem using a wire, as more or less the same signal strength can be assumed all over the wire if the length of the wire stays within certain often standardized limits. If a collision occurs somewhere in the wire, everybody will notice it.
  3. 3.  The situation is different in wireless networks. the strength of a signal decreases proportionally to the square of the distance to the sender. The sender may now apply carrier sense and detect on idle medium. The sender starts sending – but a collision happens at the receiver due to a second sender. The sender detects no collision and assumes that the data has been transmitted without errors, but a collision might actually have destroyed the data at the receiver. Collision detection is very difficult in wireless scenarios as the transmission power in the area of the transmitting antenna is several magnitudes higher than the receiving power. So, this MAC scheme from wired network fails in a wireless scenario.
  4. 4. HIDDEN AND EXPOSED TERMINALS SCENARIO The transmission range of A reaches B, but not C. The transmission range of C reaches B, but not A. Finally, the transmission range of B reaches A and C, i.e., A cannot detect C and vice versa.
  5. 5.  A starts sending to B, C does not receive this transmission. C also wants to send something to B and senses the medium. The medium appears to be free, the carrier sense fails. C also starts sending causing a collision at B. But A cannot detect this collision at B and continues with its transmission. A is hidden for C and vice versa. While hidden terminals may cause collisions, the next effect only causes unnecessary delay. Now consider the situation that B sends something to A and C wants to transmit data to some other mobile phone outside the interference ranges of A and B. C senses the carrier and detects that the carrier is busy. C postpones its transmission until it detects the medium as being idle again. But as A is outside the interference range of C, waiting is not necessary.
  6. 6. NEAR AND FAR TERMINALS SCENARIO 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.
  7. 7.  Now think of C acts as a base station coordinating media access. In this case, terminal B would already drown out terminal A on the physical layer. C in return would have no chance of applying a fair scheme as it would only hear B. The near/far effect is a severe problem. All signals should arrive at the receiver with more or less the same strength. Otherwise a person standing closer to somebody could always speak louder than a person further away. Precise power control is needed to receive all senders with the same strength at a receiver.
  8. 8. SDMA Space Division Multiple Access (SDMA) is used for allocating a separated space to users in wireless networks. A typical application involves assigning an optimal base station to a 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 (FDM), time slots (TDM) or code (CDM) are still available. 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
  9. 9. FDMA Frequency division multiple access (FDMA) comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM) scheme. Allocation can either be fixed or dynamic. Channels can be assigned to the same frequency at all times, i.e., pure FDMA, or change frequencies according to a certain pattern i.e., FDMA combined with TDMA. 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, i.e., a channel that allows for simultaneous transmission in both directions. The two directions, mobile station to base station and vice versa are now separated using different frequencies. This scheme is then called frequency division duplex (FDD).
  10. 10.  The two frequencies are also known as uplink, i.e., from mobile station to base station. downlink, i.e., from base station to mobile station or from satellite to ground control. All uplinks use the band between 890.2 and 915 MHz, all downlinks use 935.2 to 960 MHz.
  11. 11.  the base station, shown on the right side, allocates a certain frequency for up- and downlink to establish a duplex channel with a mobile phone. Up- and downlink have a fixed relation. If the uplink frequency is fu = 890 MHz + n·0.2 MHz, the downlink frequency is fd = fu +45 MHz, i.e., fd = 935 MHz + n·0.2 MHz for a certain channel n.
  12. 12. TDMA Compared to FDMA, time division multiple access (TDMA) offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for communication, i.e., controlling TDM. 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. synchronization between sender and receiver can be achieved in the time domain by allocating a certain time slot for a channel, or by using a dynamic allocation scheme.
  13. 13. 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 systems. MAC is quite simple, as the only crucial factor is accessing the reserved time slot at the right moment. If this synchronization is assured, each mobile station knows its turn and no interference will happen. The fixed pattern can be assigned by the base station, where competition between different mobile stations that want to access the medium is solved. Fixed access patterns fit perfectly well for connections with a fixed bandwidth. Furthermore, these patterns guarantee a fixed delay – one can transmit, e.g., every 10 ms. TDMA schemes with fixed access patterns are used for many digital mobile phone systems like IS-54, IS- 136, GSM, DECT, PHS, and PACS.
  14. 14.  Assigning different slots for uplink and downlink using the same frequency is called time division duplex (TDD). 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. Up to 12 different mobile stations can use the same frequency without interference using this scheme.
  15. 15.  Classical Aloha Slotted Aloha Carrier sense multiple access Demand assigned multiple access PRMA packet reservation multiple access Reservation TDMA Multiple access with collision avoidance Polling Inhibit sense multiple access CDMA Spread Aloha multiple access
  16. 16. ALOHA: PURE ALOHA The basic idea of an ALOHA system is simple: let users transmit whenever they have data to be sent. There will be collisions, of course, and the colliding frames will be damaged. However, due to the feedback property of broadcasting, a sender can always find out whether its frame was destroyed by listening to the channel. If listening while transmitting is not possible for some reason, acknowledgements are needed. If the frame was destroyed, the sender just waits a random amount of time and sends it again. The waiting time must be random or the same frames will collide over and over, in lockstep.
  17. 17.  Systems in which multiple users share a common channel in a way that can lead to conflicts are widely known as contention systems.
  18. 18.  If the first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted later. Let t be the time required to send a frame. If any other user has generated a frame between time t0 and t0 + t, the end of that frame will collide with the beginning of the shaded one. In pure ALOHA a station does not listen to the channel before transmitting, it has no way of knowing that another frame was already underway.
  19. 19.  any other frame started between t0 + t and t0 + 2t will bump into the end of the shaded frame.
  20. 20. SLOTTED ALOHA Slotted ALOHA was invented to improve the efficiency of pure ALOHA. In slotted ALOHA we divide the time into slots of T and force the station to send only at the beginning of the time slot.
  21. 21.  Because a station is allowed to send only at the beginning of the synchronized time slot, if a station misses this moment, it must wait until the beginning of the next time slot. This means that the station which started at the beginning of this slot has already finished sending its frame. Of course, there is still the possibility of collision if two stations try to send at the beginning of the same time slot. However, the vulnerable time is now reduced to one-half, equal to Tfr.
  22. 22. • The throughput for slotted ALOHA is S = G x e -G.• The maximum throughput Sma x = 0.368 when G = 1.
  23. 23. CARRIER SENSE MULTIPLE ACCESS (CSMA) To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. In other words, CSMA is based on the principle "sense before transmit" or "listen before talk." CSMA can reduce the possibility of collision, but it cannot eliminate it. The possibility of collision still exists because of propagation delay. when a station sends a frame, it still takes time for the first bit to reach every station and for every station to sense it.
  24. 24.  a station may sense the medium and find it idle, only because the first bit sent by another station has not yet been received. At time t1, station B senses the medium and finds it idle, so it sends a frame. At time t2 (t 2 > tl), station C senses the medium and finds it idle because, at this time, the first bits from station B have not reached station C. Station C also sends a frame. The two signals collide and both frames are destroyed.
  25. 25.  Vulnerable Time : The vulnerable time for CSMA is the propagation time Tp. This is the time needed for a signal to propagate from one end of the medium to the other. But if the first bit of the frame reaches the end of the medium, every station will already have heard the bit and will refrain from sending. Persistence Methods What should a station do if the channel is busy? What should a station do if the channel is idle?
  26. 26. 1-PERSISTENT The 1-persistent method is simple and straightforward. In this method, after the station finds the line idle, it sends its frame immediately (with probability 1). This method has the highest chance of collision because two or more stations may find the line idle and send their frames immediately.
  27. 27. NONPERSISTENT A station that has a frame to send senses the line. If the line is idle, it sends immediately. If the line is not idle, it waits a random amount of time and then senses the line again. The nonpersistent approach reduces the chance of collision because it is unlikely that two or more stations will wait the same amount of time and retry to send simultaneously. However, this method reduces the efficiency of the network because the medium remains idle when there may be stations with frames to send.
  28. 28. P-PERSISTENT The p-persistent method is used if the channel has time slots with a slot duration equal to or greater than the maximum propagation time. It reduces the chance of collision and improves efficiency. In this method, after the station finds the line idle it follows these steps: 1. With probability p, the station sends its frame. 2. With probability q = 1 - p, the station waits for the beginning of the next time slot and checks the line again. a. If the line is idle, it goes to step 1. b. If the line is busy, it acts as though a collision has occurred and uses the back- off procedure.
  29. 29. DEMAND ASSIGNED MULTIPLE ACCESS 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. While, depending on the scheme, collisions may occur during the reservationperiod, the transmission period can then be accessed without collision. These schemes cause a higher delay under a light load but allow higher throughput due to less collisions.
  30. 30.  demand assigned multiple access (DAMA) also called reservation Aloha, a scheme typical for satellite systems. DAMA has two modes. During a contention phase following the slotted Aloha scheme, all stations can try to reserve future slots. For example, different stations on earth try to reserve access time for satellite transmission. 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 slot. Therefore, the satellite collects all successful requests and sends back a reservation list indicating access rights for future slots. All ground stations have to 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 bereserved explicitly.
  31. 31. PRMA PACKET RESERVATION MULTIPLEACCESS An example for an implicit reservation scheme is packet reservation multiple access (PRMA). Here, slots can be reserved implicitly according to the following scheme.
  32. 32.  A certain number of slots forms a frame. The frame is repeated in time A base station, which could be a satellite, now broadcasts the status of each slot to all mobile stations. 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. All stations wishing to transmit can now compete for this free slot in Aloha fashion. If more than one station wants to access this slot, a collision occurs. 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.
  33. 33.  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. 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.
  34. 34. RESERVATION TDMA In a fixed TDM scheme N mini-slots followedby N·k data-slots form a frame that is repeated. Each station is allotted its own mini-slot and can use it to reserve up to k data-slots. This guarantees each station a certain bandwidth and a fixed delay. Other stations can now send data in unused data- slots as shown.
  35. 35. MULTIPLE ACCESS WITH COLLISIONAVOIDANCE Multiple access with collision avoidance (MACA) presents a simple scheme that solves the hidden terminal problem, does not need a base station, and is still a random access Aloha scheme – but with dynamic reservation.
  36. 36.  With MACA, A does not start its transmission at once, but sends a request to send (RTS) first. B receives the RTS that contains the name of sender and receiver, as well as the length of the future transmission. This RTS is not heard by C, but triggers an acknowledgement from B, called clear to send (CTS). The CTS again contains the names of sender (A) and receiver (B) of the user data, and the length of the future transmission. This CTS is now heard by C and the medium for future use by A is now reserved for the duration of the transmission. After receiving a CTS, C is not allowed to send anything for the duration indicated in the CTS toward B.
  37. 37.  Still, collisions can occur during the sending of an RTS. Both A and C could send an RTS that collides at B. RTS is very small compared to the data transmission, so the probability of a collision is much lower. B resolves this contention and acknowledges only one station in the CTS. No transmission is allowed without an appropriate CTS.
  38. 38. MACA ALSO HELP TO SOLVE THE ‘EXPOSEDTERMINAL’ PROBLEM With MACA, B has to transmit an RTS first containing the name of the receiver (A) and the sender (B). C does not react to this message as it is not the receiver, but A acknowledges using a CTS which identifies B as the sender and A as the receiver of the following data transmission. C does not receive this CTS and concludes that A is outside the detection range. C can start its transmission assuming it will not cause a collision at A. The problem with exposed terminals is solved without fixed access patterns or a base station.
  39. 39.  Problems with MACA is the overheads associated with the RTS and CTS transmissions – for short and time-critical data packets, this is not negligible.
  40. 40. POLLING Where one station is to be heard by all others (e.g., the base station of a mobile phone network or any other dedicated station), polling schemes can be applied. Polling is a strictly centralized scheme with one master station and several slave stations. The master can poll the slaves according to many schemes: round robin ,randomly, according to reservations etc. The master could also establish a list of stations wishing to transmit during a contention phase. After this phase, the station polls each station on the list. Similar schemes are used, e.g., in the Bluetooth wireless LAN
  41. 41. INHIBIT SENSE MULTIPLE ACCESS This scheme, which is used for the packet data transmission service, is also known as digital sense multiple access (DSMA). Here, the base station only signals a busy medium via a busy tone on the downlink. After the busy tone stops, accessing the uplink is not coordinated any further. The base station acknowledges successful transmissions. A mobile station detects a collision only via the missing positive acknowledgement. In case of collisions, additional back-off and retransmission mechanisms are implemented.
  42. 42. CODE-DIVISION MULTIPLE ACCESS (CDMA) In CDMA, one channel carries all transmissions simultaneously. CDMA differs from FDMA because only one channel occupies the entire bandwidth of the link. It differs from TDMA because all stations can send data simultaneously; there is no timesharing. CDMA simply means communication with different codes.
  43. 43.  Let us assume we have four stations 1, 2, 3, and 4 connected to the same channel. The data from station 1 are d l , from station 2 are d2, and so on. The code assigned to the first station is c1, to the second is c2, and so on. We assume that the assigned codes have two properties. 1. If we multiply each code by another, we get O. 2. If we multiply each code by itself, we get 4 (the number of stations). With these two properties in mind, let us see how the above four stations can send data using the same common channel,
  44. 44.  Station 1 multiplies its data by its code to get d l . Cl Station 2 multiplies its data by its code to get d2 . C2 And so on. The data that go on the channel are the sum of all these terms. Any station that wants to receive data from one of the other three ,multiplies the data on the channel by the code of the sender.
  45. 45.  For example, suppose stations 1 and 2 are talking to each other. Station 2 wants to hear what station 1 is saying. It multiplies the data on the channel by cl the code of station 1. Because (c1 . c1) is 4, but (c2 . c1), (c3 . c1), and (c4 . c1) are all Os, station 2 divides the result by 4 to get the data from station 1. data =(dj . Cj + dz . Cz +d3 . C3 + d4 . c4) . C1 =d j • C1 . Cj + dz. Cz . C1 + d3 . C3 . C1 + d4 . C4 C1 =4 X d1
  46. 46.  CDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called chips. They are called orthogonal sequences and have the following properties: 1. Each sequence is made of N elements, where N is the number of stations. 2. If we multiply a sequence by a number, every element in the sequence is multiplied by that element. This is called multiplication of a sequence by a scalar. For example, 2. [+1 +1-1-1]=[+2+2-2-2] 3. If we multiply two equal sequences, element by element, and add the results, we get N, where N is the number of elements in the each sequence.
  47. 47.  This is called the inner product of two equal sequences. For example, [+1 +1-1 -1]· [+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4 4. If we multiply two different sequences, element by element, and add the results, we get O. This is called inner product of two different sequences. For example, [+1 +1 -1 -1] • [+1 +1 +1 +1] = 1 + 1 - 1 - 1 = 0 5. Adding two sequences means adding the corresponding elements. The result is another sequence. For example, [+1+1-1-1]+[+1+1+1+1]=[+2+2 00]
  48. 48. DATA REPRESENTATION We follow these rules for encoding: If a station needs to send a 0 bit, it encodes it as -1; if it needs to send a 1 bit, it encodes it as +1. When a station is idle, it sends no signal, which is interpreted as a O Encoding and Decoding We assume that stations 1 and 2 are sending a 0 bit and channel 4 is sending a 1 bit. Station 3 is silent. The data at the sender site are translated to -1, - 1, 0, and +1. Each station multiplies the corresponding number by its chip. The result is a new sequence which is sent to the channel. For simplicity, we assume that all stations send the resulting sequences at the same time.
  49. 49.  The sequence on the channel is the sum of all four sequences as defined before Now imagine station 3, which we said is silent, is listening to station 2. Station 3 multiplies the total data on the channel by the code for station 2, which is [+1 -1 +1-1], to get

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