Multiplexing
Contents FDM Bell Systems s FDM Synchronous TDM T1, T3 Statistical TDM Multiple Access: FDMA, TDMA, CDMA 2
Multiplexing/Demultiplexing Multiplexing is the process of combining two or more signals and transmitting them over a single transmission link Demultiplexing is the reverse process of separating the multiplexed signals at the receiving end of the transmission link 3
Multiplexing Two approaches to achieve greater efficiency in the use of transmission services: Multiplexing: several information sources share a large transmission capacity Compression: reduces the number of bits required to represent a given amount of information 4
Why multiplexing? The higher the data rate, the more cost-effective the transmission facility Data communication devices require relatively modest data rate support 5
Multiplexing Example Long-distance telephone networking Traffic from subscribers is routed and switched from source to destination, with each trunk carrying multiplexed traffic from a variety of sources By multiplexing, tens of voice channels can be combined on pairs of wire, hundreds on coaxial cable and thousands on coaxial cable, microwave and fiber optics 6
FDM/TDM 7
FDM A number of signals can be carried simultaneously if each signal is modulated onto a different carrier frequency The carrier frequencies are sufficiently separated so that the bandwidths of the signals do not overlap Each modulated signal requires a certain bandwidth centered around its carrier frequency, referred to as a channel The greater the bandwidth, the greater the number of signals that can be multiplexed 8
FDM 9
Hierarchy of the Bell Systems s FDM FDM of 4 khz voice signals Multiple TV signals can be frequency-division multiplexed on a cable, each with a bandwidth of 6 MHz Dozens of video signals can be simultaneously carried using FDM on a coaxial cable (500 MHz) 10
Hierarchy of the Bell Systems s FDM
Hierarchy of the Bell Systems s FDM 12
Hierarchy of the Bell Systems s FDM 13
Synchronous TDM PCM (Pulse Code Modulation): An example of TDM signals Analog signals from several sources are digitized and interleaved to form a PCM signal The time-division multiplexed PCM signal is then transmitted onto a single frequency channel 14
Time Division Multiplexing (also called STDM Synchronous Time Division Multiplexing) n links rate r bps each Multiplexer 1 link, rate nr bps Frame: Time slots are reserved 15
TDM PCM Signal 16
PCM Signals 17
Digital Multiplex Hierarchy The basis of the TDM hierarchy is the DS-1 transmission format which multiplexes 24 channels: T-1 Carrier T-1 facility was first introduced by AT&T in the 1960s The Bell Operating Companies began to tariff T-1 services in 1984 The most common use of T-1 facilities is for leased dedicated transmission between customer premises 18
Digital Multiplex Hierarchy 19
Digital Multiplex Hierarchy 20
Digital Multiplex Hierarchy: T-1 T-1 carrier provides a data rate of 1.544 Mbps and is capable of supporting the DS-1 multiplex format Each channel contains one word (8 bits) of digitized voice data Each frame contains 8 bits per channel plus a framing bit for 24*8+1=193 bits Each frame repeats 8000 times per second for a data rate 8000*193=1.544 Mbps 21
Digital Multiplex Hierarchy: T-1 22
Digital Multiplex Hierarchy: T-1 Superframe: T1 carrier frames are tx in groups of 12 The data transmitted by a synchronous TDM are organized into frames, each of which contains a cycle of time slots The set of time slots dedicated to one source, from frame to frame is called a channel Synchronous TDM is called synchronous not because synchronous transmission is used but because the time slots are preassigned to sources and are fixed 23
Digital Multiplex Hierarchy: T-1 Bit robbing: LSB of every 6 th and 12 th frame To tx supervisory information: on-hook/off-hook condition, dialing, ringing, busy status 24
Digital Multiplex Hierarchy: T-3 T-3 Facilities Each higher level of the TDM hierarchy is formed by multiplexing signals from a lower level or by combination of those signals plus input at the appropriate data rate from other sources Almost 30 times the capacity of T-1 lines, 44.7 Mbps (DS 3) T-3 facilities are attractive for building backbone wide-area networks for high-volume users 25
Statistical TDM The use of synchronous TDM for a group of devices is extremely inefficient In statistical multiplexer, there are more attached devices than time slots available within a frame for transmission Each device (I/O line) has a buffer associated with it 26
Statistical TDM 27
Statistical TDM For input, the multiplexer scans the input buffers, collect the data until a frame is filled, and then send the frame For output, the multiplexier receives a frame and distributes the slots of data to the appropriate output buffers 28
Statistical TDM The statistical multiplexer does not send empty slots when there are no data to send Since data arrive from and are distributed to I/O lines unpredictably, address information is required to ensure proper delivery Although the average aggregate input may be less than the multiplexed line capacity, there may be peak periods when the input exceeds capacity The solution to this problem is to include a buffer in the multiplexer to hold temporary excess input 29
Statistical TDM There is a trade-off between the size of the buffer and the data rate of the line The trade-off is one between system response time and the speed of the multiplexed line Overflow probability is a function of buffer size Synchronous multiplexers still have an important role in long distance private and public networks 30
Multiple Access How to organize the channels for multiple users? Channelization FDMA u1 TDMA u2 SSMA: Spread Spectrum u3 SDMA 31
FDMA f CH N.. CH 3 CH 2 CH 1 t FDMA works best for analog signal Continuous transmission scheme 32
TDMA f CH 1 CH 2... CH N CH 1 CH 2 TDMA system tx data in a buffer-and-burst method tx is not continuous: low battery consumption TDMA works for digital systems GSM: TDMA 200kHz tx f CH 1... CH CH 8 1 Frame 1 Frame 2... CH CH 8 1 t t rx f CH 1 CH 1 CH 1 33 t
Basic Structure of time slots USDC 25 frames/sec 6 slots/frame GSM 26 frames/120msec 8 slots/frame TS 1 TS 2 TS t TS N Sync bit Signaling or Control Equalization bit INFO Guard/ Ramp Sync bit: receivers need to be synchronized for each data burst Equalization bit: training bits Guard (blank bits): buffer between TSs to compensate for the time delay between the mobile and the base station Ramp (blank bits): start/stop (power up/down) bits, only at uplink 34
Summary Bell Systems s FDM (Channel group, Supergroup): analog process Synchronous TDM (T1, T3): digital process Statistical TDM Cellular Network: FDMA, TDMA 35