Lecture 5 Multiplexing

Time Division Multiplexing

TDM: Time Slots and Frames In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter

TDM: Example 1 Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find (a) the duration of 1 bit before multiplexing, (b) the transmission rate of the link, (c) the duration of a time slot, and (d) the duration of a frame? a) The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms). b) The rate of the link is 4 Kbps. c) The duration of each time slot 1/4 ms or 250 μs. d) The duration of a frame 1 ms.

Interleaving Interleaving can be done by bit, by byte, or by any other data unit

Empty Slots Synchronous TDM is not efficient in many cases Statistical TDM can improve the efficiency by removing the empty slot from the frame

Data Rate Management Multiple-slot allocation / Pulse stuffing

Frame Synchronizing Synchronization between the multiplexing and demultiplexing is a major issue in TDM

TDM: Example 2 We have four sources, each creating 250 characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (a) the data rate of each source, (b) the duration of each character in each source, (c) the frame rate, (d) the duration of each frame, (e) the number of bits in each frame, and (f) the data rate of the link. 1. The data rate of each source is 2000 bps = 2 Kbps. 2. The duration of a character is 1/250 s, or 4 ms. 3. The link needs to send 250 frames per second. 4. The duration of each frame is 1/250 s, or 4 ms. 5. Each frame is 4 x 8 + 1 = 33 bits. 6. The data rate of the link is 250 x 33, or 8250 bps

Synchronous Time Division Multiplexing the multiplexer allocates exactly the same time slot to each device at all times, whether or not a device has anything to transmit. Many types popular today: T1 multiplexing E1 multiplexing ISDN multiplexing SONET (Synchronous Optical NETwork) SDH (Synchronous Digital Hierarchy)

Synchronous Time Division Multiplexing (T1) The T1 (1.54 Mbps) multiplexor stream is a continuous series of frames of both digitized data and voice channels. 24 separate 64Kbps channels

T-1 Line for Multiplexing Telephone Lines Synchronous Time Division Multiplexing (T1) T-1 Line for Multiplexing Telephone Lines

Synchronous Time Division Multiplexing (T1) T-1 Frame Structure

DS and T Line Rates

Synchronous Time Division Multiplexing (E1)   Synchronous Time Division Multiplexing (E1) E1 Frame description Frame time length 125 microsec Frame Length 256 bits = 30*voice channel + 1 synch + 1 signaling channel Rate 256 bits/frame x 8000 frames/second = 2.048 Mbps  

E Line Rates European use a version of T lines called E lines

Synchronous Time Division Multiplexing (ISDN) The ISDN multiplexer stream is also a continuous stream of frames. Each frame contains various control and sync info.

Synchronous Time Division Multiplexing (SONET) Synchronous Optical Network (SONET) Framing STS (Synchronous Transport Signal) frame consists of: 90 columns x 9 rows x 8 bits/byte x 8000 frames/sec = 51.84 Mbps

Synchronous Time Division Multiplexing (SDH ) Synchronous Digital Hierarchy (SDH) Framing The STM-1 frame consists of 9 rows of 270-column bytes, for a total of 2430 bytes. 270 columns x 9 rows x 8 bits/byte x 8000 frames/sec = 155.52 Mbps Synchronous Payload Envelope (SPE) for an actual data rate of 148.608 Mbps. 258 columns x 9 rows x 8 bits/byte x 8000 frames/sec = 148.608 Mbps

SDH / SONET – Higher level of multiplexing

Statistical TDM

Statistical TDM Addressing is required in Statistical TDM Slot size: the ratio of the data size to address size must be reasonable to make transmission efficient No synchronization bit: no need for frame-level sync. Bandwidth: normally less than the sum of the capacities of each channel This type of TDM is used in Asynchronous Transfer Mode (ATM) networks.

OFDM (Orthogonal Frequency Division Multiplexing) Different symbols are transmitted over different subcarriers

OFDM (Orthogonal Frequency Division Multiplexing) Transmission of QAM symbols on parallel subcarriers Overlapping, yet orthogonal subcarriers Orthogonal Frequency Division Multiplexing (OFDM) is special form of multi-carrier modulation, patented in 1970. It is particularly suited for transmission over a dispersive channel. In a multipath channel, most conventional modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel. OFDM is significantly less sensitive to intersymbol interference than conventional modulation such as BPSK or QAM, because a special set of signals is used to build the composite transmitted signal. The basic idea is that each bit occupies a frequency-time window which ensures little or no distortion of the waveform. In practice, it means that bits are transmitted in parallel over a number of frequency-nonselective channels.

OFDM (Orthogonal Frequency Division Multiplexing) Multi-carrier Modulation OFDM (Orthogonal Frequency Division Multiplexing) Basic FFT,OFDM transmitter and receiver

Asymmetric Digital Subscriber Line (ADSL) IFFT

Bandwidth Division in ADSL There is no set way that the bandwidth is divided Channel 0 for voice Upstream 24 channels x 4 kHz x 15 bits/Hz = 1.44 Mbps (ch 6:ch 30) Downstream 224 channels x 4 kHz x 15 bits/Hz = 13.4 Mbps (ch 31:ch 255)

DSL: Actual Bit Rate Because of the high signal/noise ratio Upstream Normally below 500 kbps Downstream Normally below 8 Mbps

Asymmetric Digital Subscriber Line (ADSL)

Asymmetric Digital Subscriber Line (ADSL)

Customer Site: ADSL Modem

Telco Site: DSLAM Digital subscriber line access multiplexer