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UNIT – III II: Multiplexing & T-Carriers

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UNIT – III II: Multiplexing & T-Carriers. MULTIPLEXING: Multiplexing is the transmission of information from more than one source to more than one destination over the same transmission medium (facility).

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II: Multiplexing & T-Carriers

  • Multiplexing is the transmission of information from more than one source to more than one destination over the same transmission medium (facility).
  • Although transmissions occur on the same facility, they do not necessarily occur at the same time.
  • The transmission medium may be a metallic wire pair, a coaxial cable, or a satellite microwave system.
  • There are many domains in which multiplexing can be accomplished including space, time, frequency and wavelength.
  • The most predominant methods are time-division multiplexing (TDM), frequency-division multiplexing (FDM), wavelength-division multiplexing (WDM).


  • TDM allows transmission from multiple sources on the same facility but not at the same time.
  • Transmissions from various sources are interleaved in the time domain.
  • PCM is the most common type of modulation used with TDM (PCM-TDM system).In TDM, each signal occupies a subset of the transmission facility bandwidth for a slice of time (time slot)

TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one.


In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter.


The fundamental building block for any TDM system is DS-0.

The DS-0 channel occupies a 64 kbps bandwidth

  • --- ---
  • --- ---
  • In this modulation technique, an analog signal is digitized, and interleaved with other digitized voice signal to create a single bit stream.
  • At the receiving end, the bit stream is decomposed into separate digital streams of lower frequencies, each stream is then converted back into what resembles the original voice signal.
The sampling rate used for voice = 8000 samples/sec
  • Therefore, Sampling Interval = 1/8000 = 125µs
  • This means that the time between two consecutive samples (from the same source) is 125µs.
  • TDM systems exploit this fact and utilize this interval to sample signals from other subscribers. In T1 systems the signals from 24 subscribers is sampled in 125µs.
  • The samples are quantized and then converted into a bit-stream for transmission over the channel.
  • The PCM code for each channel occupies a fixed time slot (epoch) within the total TDM frame0
T1 Digital Carrier System
  • The T1 carrier system multiplexes binary code-words corresponding to samples of each of the 24 channels in a sequence.
  • A segment containing one codeword (corresponding to one sample) from each of the 24 channels is called a FRAME.

Each frame has 24x8 = 192data bits and takes 125µs.

  • As mentioned previously, sampling rate used for voice = 8000 samples/sec

Every sample is represented by 8 bits


Data rate of 1 voice channel = 8x8000 = 64kbps

  • In the T1 system 24 voice channels are multiplexed in time

Data rate of a T1 stream should be = 24x64kbps = 1.536 Mbps

  • At the receiver it is also necessary to know where a frame starts in order to separate information bits correctly. For this purpose, a Framing bit is added at the beginning of each frame.

Framing Bits: Indicate start of frames.

Total number of bits/ frame = 193

the actual data rate = 1.544Mbps

D-type channel banks
  • Early TDM systems used D1 digital channel banks (PCM encoders) with a seven bit sign magnitude only PCM code, analog companding, and a µ=100.
  • The framing bit sequence was simply a 1/0 pattern with the early digital channel banks.
  • Later version digital channel banks (D2 and D3) added an eight bit called the signaling bit to each PCM code for the purpose of interoffice signaling (On-Hook/ off-Hook signals, Alarm signals).
  • Modern versions use digitally companded, eight-bit sign magnitude compressed PCM codes, with a µ=100
Superframe TDM format

Each frame has 24x8 = 192data bits and takes 125µs.

Total number of bits/ frame = 193

The actual data rate = 1.544Mbps

Composite frame alignment

  • Framing bit sequence for the T1 super frame format using D2 and D3 channel banks
  • Another framing format recently developed for new designs of T1 carrier systems is the extended super frame format. It consists of 24 frames.
  • Each frame consists of 24 DS-0 64 kbps channels
  • Each frame has a framing bit (f-bit)
  • Each frame consists of 193 bits
  • The ESF consists of 193 x 24 = 4632 bits of which 24 are framing bits

4632bits/1.544mbps = 3ms

However only 6 out of the 24 f-bits are used for synchronization (frames: 4, 8, 12, 16, 20 and 24)

Framing bit sequence = 0 0 1 0 1 1
  • Another 6 of the framing bits are used for cyclic check redundancy (CRC-6).
  • The CRC-6 bits occur in frames: 1, 5, 9, 13, 17, 21
  • CRC-6 is used for error detection
  • The 12 remaining framing bits occur in frames:

2, 3, 6, 7, 10, 11, 14, 15, 18, 19, 22, 23

  • These 12 framing bits provide for a management channel called the facilities data link (FDL).
  • In ESF: Signaling bit in 6th frame  a-bit

Signaling bit in 12th frame  b-bit

Signaling bit in 18th frame  c-bit

Signaling bit in 24th frame  d-bit

These signaling bit streams are sometimes called the a,b,c and d signaling channels or signaling highways

Fractional T Carrier Service


(Users 1,2,3,and 4)

  • T1 carriers provide a higher bit rate than most users require.
  • Fractional T1 systems distribute the channel in a standard T1 system among more than one user, allowing several subscribers to share one T1 line.
  • Bit rates offered are 64 kbps (1 channel), 128 kbps (2 channels), 256 kbps (4 channels), 384 kbps (6 channels), 512 kbps (8 channels) 768 kbps (12 channels) being the most common.
  • The minimum data rate necessary to propagate video information is 384 kbps.
  • The data service unit/channel service unit (DSU/CSU) is a digital interface that provides the physical connection to a digital carrier network.
  • User 1 is allocated 128 kbps, 256 kbps for user 2, 384 kbps for user 3, and 768 kbps for user 4 for a total of 1.536 kbps (8 kbps is reserved for the framing bit).
North American Digital Multiplexing Hierarchy
  • The American Telephone and Telegraph Company’s (AT&T’s) North American Digital Hierarchy for multiplexing digital signals into a single higher-speed pulse stream suitable for transmission on the next higher level of the hierarchy.
  • To upgrade from one level in the hierarchy to the next higher level, a special device called muldem (multiplexer/ demultiplexer) is required.
  • Muldems can handle bit-rate conversions in both directions. The muldem designations (M12, M23, and so on) identify the input and output digital signals associated with that muldem.
  • The DS-1 may be further multiplexed or line encoded and placed on specially conditioned cables called T1 lines.
  • The DS-2, DS-3, DS-4 and DS-5 digital signals are placed on T2, T3, T4M, or T5 lines, respectively.
  • Digital signals are routed at central locations called digital cross-connects.
  • A digital cross-connect (DSX) provides a convenient place to make patchable interconnects and to perform routine maintenance and troubleshooting.
  • Each type of digital signals (DS-1, DS-2, and so on) has its own digital switch (DSX-1, DSX-2, and so on).
  • The output from a digital switch may be upgraded to the next higher level of multiplexing or line encoded and placed on its respective T lines (T1, T2, and so on).
Digital Line Encoding
  • Digital line encoding involves converting standard logic levels(TTL,CMOS) to a form more suitable to telephone line transmission. Essentially six primary constants are considering when selecting line encoding format
  • 1) Transmission voltages and dc component
  • 2) Duty cycle
  • 3) Bandwidth considerations
  • 4) Clock and framing bit recovery
  • 5) Error detection & Ease of detection and decoding
  • 6) Digital biphase.
  • Transmission voltages and dc component: Transmission voltages or levels can be categorized as being UP or BP.
  • Unipolar transmission of binary data involves the transmission of only a single non zero voltage level(either a +ve or –ve voltage for a logic 1 and 0v for a logic 0).
  • In bipolar transmission, two nonzero voltages are involved(a +ve voltage for a logic 1 and equal magnitude negative voltage for a logic 0 or vice versa)
  • Digital Biphase: It is sometimes called the Manchester or diphase, which is a popular type of line encoding that produces a timing component for clock recovery and does not cause DC wandering.
  • Here equal probability of 1s and 0s are taken, then the average dc voltage is ov, and there is no wandering .
  • A disadvantage of biphase is that it contains no means of error detection
T Carrier Systems
  • T carriers are used for the transmission of PCM-encoded time division multiplexed digital signals

T1 Carrier System

  • Transmission of 24, 64-kbps channels, T1 line speed 1.544 Mbps.
  • Lengths from about 1 mile to over 50 miles.
  • Binary eight zero substitution, B8ZS--- (+-0-+000) or (-+0+-000)

T2 Carrier System

  • Transmission of 94, 64-kbps channels, T2 line speed 6.312 Mbps.
  • Lengths up to 500 miles.
  • Binary six zero substitution, B6ZS--- (0-+0+-) or (0+-0-+)

T3 Carrier System

  • Transmission of 672, 64-kbps channels, T3 line speed 44.736 Mbps.
  • Binary three zero substitution, B3ZS

T4M Carrier System

  • Transmission of 4032, 64-kbps channels, T4 line speed 274.176 Mbps.
  • Lengths up to 500 miles

T5 Carrier System

  • Transmission of 8064, 64-kbps channels, T5 line speed 560.160 Mbps
European Time-Division Multiplexing
  • In Europe, a different version of T carrier lines is used called E lines.
  • A high speed digital communications link that enables the transmission of voice, data, and video signals at a rate of 2.048 Mbps

a) Initially designed for transmission of 30 telephone channels

b) Basis for design: PCM voice digitizing using 64 kbps for each channel.

The E1 frame consists of 328-bit channels (timeslots)

32 time slots X 8 bits = 256 bits/frame

frame time slots

E1 frames are transmitted at the rate of 8,000 frames/s

256 bits X 8,000 frames = 2,048 kbps or 2.048 Mbps

frame second

Statistical Time Division Multiplexing (STDM)
  • STDM is designed to make use of the idle time created when terminals are not using the multiplexed circuit.
  • Like regular TDM, STDM uses time slots, but the time slots are not fixed. Instead, they are used as needed by the different terminals on the multiplexed circuit.
  • Since the source of a data sample is not identified by the time slot it occupies, additional addressing information must be added to each sample.
  • If all terminals try to use the multiplexed circuit intensively, response time delays can occur. The multiplexer also needs to contain memory to store data in case more data samples come in than its outgoing circuit capacity can handle.

Statistical TDM frame one source per frame

Multiple sources per frame

Frame synchronization
  • To acquire frame synchronization, a certain amount of overhead must be added to the transmission

1) Added-Digit Framing:

  • Initial frame synchronization depends on the total frame time, the number of bits per frame, and the period of each bit

synchronization time = 2NT = 2 N2tb

N = number of bits per frame

T = frame period of N * tb

tb = bit time

  • For T1 carrier, N= 193, T=125µs, and tb=0.468µs; for 74,498 bits, maximum average synchronization time is 48.25ms
  • 2) Robbed-Digit Framing
  • -- When a short frame is used, added-digit framing is inefficient.
  • -- This occurs with single-channel PCM systems
  • -- Replace the least significant bit of every nth with a framing bit called robbed-
  • digit framing
  • -- For n=10, the SQR is impaired by only 1 dB
3) Added-Channel Framing

-- Same as added-digit framing except that digits are added in groups or words instead of as individual bits

-- The average number of bits to acquire frame synchronization using added-channel framing is

Number of synchronization bits = N2 N= number of bits per frame

2(2k-1) K= number of bits in the synchronizing word

4) Statistical Framing

--The second bit is a logic 1 in the central half of the code range and a logic 0 at the extremes

-- Second digit of a given channel can be used for the framing bit

5) Unique-Line Code Framing

-- Some property of the framing bit is different from the data bits

-- The framing bit is either made higher or lower in amplitude or with different time duration

-- Either added-digit or added-word framing can be used, or specified data bits can be used to simultaneously convey information and carry synchronizing signals

Frequency Division Multiplexing

--With FDM, multiple sources that originally occupied the same frequency spectrum are each converted to a different frequency band and transmitted simultaneously over a single transmission medium

--Each narrowband channel is converted to a different location in the total frequency spectrum

--Channel 1 signals amplitude modulate a 100-kHz carrier in a balanced modulator, which inherently suppresses the 100-kHz carrier

--The output of the balanced modulator is a double-sideband suppressed carrier waveform with a bandwidth of 10 kHz

--The double-sideband waveform passes through a band pass filter (BPF) where it is converted to a single-sideband signal

--The lower sideband is blocked for this example, the output of BPF occupies the frequency band between 100kHz and 105 kHz.

--The total combined bandwidth is equal to 20 kHz and each channel occupies a different 6-kHz portion of the total 20-kHz bandwidth.

--The applications for FDM are commercial FM, television broadcasting, high-volume telephone and data communications systems, cable television and data distribution networks.


FDM is an analog multiplexing technique that combines analog signals.

Message Channel: The message channel is the basic building block of the FDM hierarchy. It was originally intended for analog transmission of voice signals, although it now includes any transmissions that utilize voce-band frequencies (0 kHz to4 kHz) such as data transmission using voice-band data modems

Basic group: A group is the next higher level in the FDM hierarchy above the basic message channel and consequently is the first multiplexing step for combining message channels. Here Twelve 4 kHz voice-band channels occupy a combined bandwidth of 48 kHz (4 * 12)

Basic supergroup: The next higher level in the FDM hierarchy is the super group, formed by frequency-division multiplexing five groups containing 12 channels each, for a combined bandwidth of 240-kHz (5 groups * 48 kHz/group)

Basic mastergroup-- The next higher level of multiplexing, is the master group, which is formed by frequency-division multiplexing 10 super groups together for a combined capacity of 600 voice-band message channels occupying a bandwidth of 2.4 MHz (600 channels * 4 kHz/channel)

  • Three master groups are frequency-division multiplexed together and placed on a single microwave or satellite radio channel. The capacity is 1800 VB channels utilizing a combined bandwidth of 7.2 MHz
  • (3master groups * 600 channels/master group)
  • Master groups can be further multiplexed in master group banks to form jumbo groups (3600 VB channels), multi jumbo groups (7200 VB channels) and superjumbo groups (10,800 VB channels).
Wavelength-Division Multiplexing

-- Different wavelengths carry separate signals

-- Multiplex into shared optical fiber

-- Each wavelength like a separate circuit

-- The wavelength spectrum used is in the region of 1300 nm or 1500 nm

-- WDM is a process in which different sources of information (channels) are propagated down an optical fiber on different wavelengths where the different wavelengths do not interfere with each other

Synchronous Optical Network
  • The Synchronous Optical Network (SONET) is a multiplexing system similar to conventional TDM except SONET was developed to be used with optical fibers
  • SONET has some standards