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.
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.
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.
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.
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.
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.
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
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).
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.
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.
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.
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.
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.
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.
Systems in which multiple users share a common channel in a way that can lead to conflicts are widely known as contention systems.
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.
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 into slots of T and force the station to send only at the beginning of the time slot.
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.
• 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 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.
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.
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Problems with MACA is the overheads associated with the RTS and CTS transmissions – for short and time-critical data packets, this is not negligible.
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
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.
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.
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,
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.
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
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.
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]
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.
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