![Chip sequences
• I [+1 +1 +1 +11 I 1[+1 -1 +1 -I) I I [+1 + -I - 11 I I [+ -1 -1 +IJ I
• Each sequence is made of N elements, where N is the number of
stations.
• 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,
[+1 +1-1-1]=[+2+2-2-2]
• 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 -n· [+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4
Multiple Access Control 21](https://image.slidesharecdn.com/computernetworks-200102061919/85/Computer-networks-Channelization-21-320.jpg)
![• 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
• 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]
Multiple Access Control 22](https://image.slidesharecdn.com/computernetworks-200102061919/85/Computer-networks-Channelization-22-320.jpg)
Computer networks - Channelization
- 1. COMPUTER NETWORKS MULTIPLE ACCESS - CHANNELIZATION Multiple Access Control 1
- 2. OVERVIEW • Multiple Access • Random access protocol • Controlled access protocol • Channelization protocol • FDMA • TDMA • CDMA • Reference Multiple Access Control 2
- 3. Multiple Access • If there is a dedicated link between the sender and the receiver then data link control layer is sufficient, however if there is no dedicated link present then multiple stations can access the channel simultaneously. • Decrease collision and avoid crosstalk. • For example, In a classroom full of students, when a teacher asks a question and all the students (or stations) start answering simultaneously (send data at same time) then a lot of chaos is created( data overlap or data lost) then it is the job of the teacher (multiple access protocols) to manage the students and make them answer one at a time. Multiple Access Control 3
- 4. TAXONOMY Fig 1: Taxonomy of Multiple access protocol Multiple Access Control 4
- 5. RANDOM ACCESS PROTOCOL • No station is superior to another station • None is assigned the control over another • At any instance, a station sends data by following a protocol to make decision whether to send or not • It depends on state of medium (idle or busy) • As there is no scheduled time for station to transmit and as transmission is among stations, this method is called random access Multiple Access Control 5
- 6. CONTROL ACCESS PROTOCOL • The stations consult one another to find station has right to send • A station cannot send unless it has been authorized by other stations. Multiple Access Control 6
- 7. CHANNELIZATION • A multiple access method in which the available bandwidth of a link is shared in time, frequency or through code, between different stations • Three Channelization protocol Frequency division multiple access Time division multiple access Code division multiple access Multiple Access Control 7
- 8. Frequency Division Multiple Access(FDMA) • Available bandwidth is divided into frequency bands • Each station is allocated with a band to send its data • Each band is reserved for specific station all the time • Each station uses a bandpass filter to confine transmitter frequencies • In order to avoid station interferences, allocated bands are separated from one another by small guard bands. Multiple Access Control 8
- 9. • FDMA specifies a predetermined frequency band for entire period of communication • Stream data (a continuous flow of data that may not be packetized) can be easily used with FDMA Fig 2: Frequency division multiple access Multiple Access Control 9
- 10. FDM FDMA Is a physical layer technique Is a data link layer technique Combine load from low bandwidth channels and transmits them by using a high bandwidth channel Each station tells its physical layer to make a bandpass signal from the data passed to it The channels that are combined the low pass The signal must be created in the allocation band. The multiplexer modulates the signal combines them and creates a bandpass signal There is no physical multiplexer at the physical layer. the signal created at each station are automatically bandpass filtered. The bandwidth of each channel is shifted by the multiplexer The bandwidth are mixed when they are sent to a common channel Multiple Access Control 10
- 11. TIME DIVISION MULTIPLE ACCESS(TDMA) • Time division multiple access is a channel access method for shared medium networks. • It allows several users to share the same frequency. • Each station is allocated a time slot during which it can send data. • Each station transmits its data in is assigned time slot. Multiple Access Control 11
- 12. Fig 3: Multiple Access Control 12
- 13. CONTD,.. • The main problem with TDMA is lies in achieving synchronization between the different stations. • Each station needs to know the beginning of its slot and the location of its slot. • This is difficult because of propagation delays introduced in the system if the stations are spread over large area. • To compensate this we insert guard times. Multiple Access Control 13
- 14. • The process uses a physical multiplexer that interleaves data units from each channel. • TDMA, on the other hand, is an access method in the data link layer. • The data link layer in each station tells its physical layer to use the allocated time slot. There is no physical multiplexer at the physical layer. CONTD,.. Multiple Access Control 14
- 15. EXAMPLES OF TDMA • IS-136 • Personal digital cellular(PDC) • Integrated digital enhanced network • 2G-second generation 3G based on CDMA • Universal terrestrial radio access(UTRA) Multiple Access Control 15
- 16. CODE-DIVISION MULTIPLE ACCESS (CDMA) • Code-division multiple access (CDMA) was conceived several decades ago. Recent advances in electronic technology have finally made its implementation possible. • 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
- 17. ANALOGY • CDMA simply means communication with different codes. • For example, In a large room with many people, two people can talk in English if nobody else understands English. Another two people can talk in Chinese if they are the only ones who understand Chinese, and so on. In other words, the common channel, the space of the room in this case, can easily allow communication between several people, but in different languages (codes).
- 18. Idea • Let us assume we have four stations 1, 2, 3, and 4 connected to the same channel. The data from station 1 are dl , from station 2 are d2, and so on. The code assigned to the first station is cI, 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). Multiple Access Control 18
- 19. • Station 1 multiplies its data by its code to get dl . 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 term. 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 I is saying. It multiplies the data on the channel by cl' the code of station 1. • Because (cl . cl) is 4, but (c2 . cI), (c3 . cI), and (c4 . cl) are all Os, station 2 divides the result by 4 to get the data from station 1. Fig 4: Simple idea of communication with code data =(d) . Cj + dz . Cz +d3 . C3 + d4 . c4) . Cl =d j . Cl . Cj +dz.Cz . Cl + d3 . C3 . Cl + d4 . C4' CI =4 X d1 Multiple Access Control 19
- 20. CHIPS • CDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called chips. Multiple Access Control 20
- 21. Chip sequences • I [+1 +1 +1 +11 I 1[+1 -1 +1 -I) I I [+1 + -I - 11 I I [+ -1 -1 +IJ I • Each sequence is made of N elements, where N is the number of stations. • 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, [+1 +1-1-1]=[+2+2-2-2] • 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 -n· [+1 +1 -1 -1] = 1 + 1 + 1 + 1 = 4 Multiple Access Control 21
- 22. • 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 • 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] Multiple Access Control 22
- 23. DATA REPRESENTATION Rules of encoding • If station sends a 0 bit, it encodes it as -1 • If station sends a 1 bit, it encodes it as +1 • When a system is idle, it send no signal i.e. 0 Fig 5: Data representation Multiple Access Control 23
- 24. ENCODING AND DECODING Consider station3 is silent , and its listening to station2. Station 3 multiplies the total data on the channel by code for station2 Fig 6: Sharing channel in CDMA 0 Multiple Access Control 24
- 25. SIGNAL LEVEL • The total data on channel are multiplied by the signal representing station 2 chip code to get a new signal • The station then integrates and adds the area under signal to get the value -4, which divided by 4 and interpreted as bit0 Multiple Access Control 25
- 26. Multiple Access Control 26
- 27. SEQUENCE GENERATION • Walsh table is used, which is two dimensional table (n*n matrix) • Each row is a sequence of chips • Values assigned are either +1 or -1 Multiple Access Control 27
- 29. REFERENCES • Forouzan A Behrouz, Data Communications and Networking. New York,2007,pp 363-390 Multiple Access Control 29