Multiplexing Techniques in PSTN

1. Multiplexing1.1 Frequency Division Multiplexing (FDM)1.2 Time Division Multiplexing

2. Standardization3. Pulse Code Modulation (PCM)4. PDH5. SDH

1. Multiplexing

Multiplexing allows the transmission of multiple communications over a single line. A special device called multiplexer combines the incoming signals into a single signal to be transmitted over, for instance, an optical fibre or a coaxial cable. The single signal at the end of the transmission is demultiplexed: each original signal is separated and sent where appropriate.

1.1 Frequency Division Multiplexing

Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communication line or channel. Each signal is assigned a different frequency (sub channel) within the main channel.

Example

In the case of telephony, a single telephone circuit has a bandwidth of 4kHz. If we consider 6 telephone circuits, they can be multiplexed onto a single carrier with a bandwidth of 24 kHz. The signal in each circuit is combined with a different carrier frequency (f ) by 4 kHz. A different carrier frequency is used for each circuit. (Figure 1)

It is now possible to multiplex composite 24kHz signals, with 4 new carriers, to form a higher order signal with a bandwidth of 96kHz. This is shown in the stage 2 of the figure1.In this way it is theoretically possible to continue multiplexing by adding more and more multiplexing stages. Later, you will see that this characteristic is shared by PDH and SDH, where the final limitation is the bandwidth of the transmission medium.

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Figure 1

1.2 Time Division Multiplexing

This is a type of digital or analog multiplexing which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but physically are taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample, byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel. After the last sub-channel the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc. You can see this in Figure 2. Here the interleaving of signals can be on bit-by-bit basis (bit interleaving) or byte-by-byte basis (byte interleaving).

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Pulse Code Modulation

PCM is a technique which transforms an analog telephone circuit into a digital signal, and involves 3 consecutive processes, called sampling, quantizing and encoding. Figure 3 depicts these 3 processes applied to a speech waveform.

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First the analog voice signal is sampled and the sampling frequency is decided according to the Shannon’s Sampling Theorem. i.e. Sampling frequency = 2 x maximum frequencyTherefore for a voice channel, the sampling rate is selected as 8 kHzYou can see these samples marked in vertical dotted lines. Grey horizontal lines indicate the sampled voltage levels. The vertical axis is divided into some quantizing levels and the actual voltage level of each sample is approximated to the nearest quantizing level and this process is called quantization. Each quantizing level is then assigned with a particular code and thereby each quantized sample of the signal is allocated a digital code (encoding).

The bit rate of a PCM signal is the product of the number of bits used to encode each sample and the number of times a waveform is sampled in one second. In current PCM telephone systems the sampling rate is 8 kHz and 8 bits are used to encode each sample.

PCM and TDM

In practical applications telephone channels are combined by time division multiplexing to form an assembly of 24 or 30 channels. This is known as the primary multiplex group.It is also used as a building block for assembling larger numbers channels in higher-order multiplex systems. Figure 4 and 5 shows the frame structures of primary multiplexer. The length of the frame is 125 which correspond to the sampling interval. This frame contains one speech sample from each channel and additional digits are allocated for synchronizing and signaling. There are two frame structures widely used: 30-channel system used in Europe (Fig. 4) and 24-channel system used in North America and Japan (Fig. 5). Both these systems use 8-bit coding but companding methods are different. 30-channel system uses A-law companding and 24-channel system uses the -law.

According to these frame structures we can calculate the total bit rate of each system. In 30-channel system, the frame is divided into 32 time slots and each contains 8 digits. Thus the total bit rate is 8kHz x 8 x 32 = 2.048 Mbit/s. As marked in the figure 4, time slots 1 to 15 and 17 to 31 are each allocated to speech channels, time slot 0 is allocated for frame alignment and time slot 16 is allocated for signaling.

The fame format for 24-channel system is shown in figure 5. Here the basic frame consists of 193 bits and the first bit is used for framing and is called the F bit. The others form 24 8-bit time slots for speech channels.

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Figure 4 : 30-channel PCM frame format

Figure 5 : 24-channel PCM frame format

Plesiochronous Digital Hierarchy (PDH)

In higher order multiplex systems, PCM frames are multiplexed together, in stages, to form a hierarchy of bit rates. At each level in the hierarchy, several bit streams, known as tributaries, are combined by a multiplexer. The output from a multiplexer may serve as a tributary to a multiplexer at the next higher level in the hierarchy or it may be sent directly over a line or radio link. In the first generation of higher order digital multiplex systems, the inputs to a digital multiplexer are not exactly synchronous and this is called as Plesiochronous Digital Hierarchy (PDH). PDH was first introduced in 1960 and since then it provided reliable digital transmission in communication networks. It also caused to make the following developments in equipments as well as in technical side:

High speed optical transmission systems Increased use of integrated circuits Increased use of software implemented directly on circuit boards

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The accumulation of knowledge, on error performance and maintenance events, through practical experience with PDH

The laying of optical fiber cables that span the Atlantic and Pacific Oceans The digitization of satellite communication using time division multiple access

(TDMA) An acceleration in the agreement of international standards

Table 1 shows the international PDH hierarchies used in 3 main regions. You will see there is a wide variety in bit rates from region to region. This was a great deficiency in PDH as it shows incompatibilities among regions. Another significant problem with PDH was its lack of inbuilt facilities for monitoring the transmission path for errors or interruptions.

Europe(kbits/s)

North America(kbits/s)

Japan(kbits/s)

64 64 642,048 1,544 1,5448,448 6,312 6,31234,368 44,736 32,064139,264 139,264 97,728

Table 1

. Figure 6 shows the multiplexing stages in the European PDH hierarchy as a series of steps. Here, ‘up’ steps are multiplexing signals and the ‘down’ steps are de-multiplexing the same signals. There are 3 multiplexing stages: from 2 to 8, 8 to 34 and 34 to 140 Mbits/s. There are 4 inputs in each multiplexer and they are combined together with overhead and control bits to give a single bit rate. All these layers use bit interleaving. Clock signals are marked at the bottom of each multiplexer. The frame length is the same as for the primary multiplex, i.e. 125 , since this is determined by the basic channel sampling rate of 8 kHz. However when N tributaries are combined, the number of digits contained in the higher order frame is greater than N times the number of digits in the tributary frame. This is because it is necessary to add extra overhead digits.

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Figure 6

Clock synchronization

Plesiochronous means “almost synchronous”. As its name implies PDH hierarchy is almost synchronous as extra bits are inserted into the digital signal stream to bring them to a common rate.

You will see in PDH, the bit rate at the output of each multiplexer is greater than the sum of the bit rates at the input. These additional bits are called as overhead bits and they are required to maintain bit synchronization and frame synchronization. Bit synchronization is the requirement to ensure that the receiver can ‘read’ all the bits. In PDH separate clocks are used to time each multiplexer and there by it can cope with tributaries that have variations in their bit rates within controllable limits.

In PDH the variation in timing, between the tributaries is handled by a process called justification which can add extra bits to match the bit rate of each tributary to the aggregate bit rate. The control code for the justification is embedded in the multiplexed bit stream. This process is repeated at the each stage of the multiplexing.In PDH a frame alignment word is used to mark the beginning of each frame to identify the location of data blocks (Frame synchronization).

Disadvantages:

PDH has the burden of providing chains of multiplexers (multiplex mountain problem)

PDH multiplexers need to be interconnected using manual interface racks

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Synchronous Digital Hierarchy (SDH)

When the networks become fully digital, they operate synchronously, using high capacity optical fibre transmission systems and time division switching. Therefore it is advantageous for the multiplexers used in these networks to be compatible with the switches used at the network nodes. SDH (Synchronous Digital Hierarchy) was introduced to fulfill this requirement.

In 1988 SDH (Synchronous Digital Hierarchy) was defined by CCITT and in USA this is called SONET (synchronous optical Networks) since the multiplexers use optical interfaces.SDH was developed to allow streams 1.544 Mbit/s and above to be multiplexed, so as to create larger SDH frames known as Synchronous Transport Modules (STM). The basic SDH signal, STM-1 consists of smaller streams that are multiplexed to create a 155.52 Mbit/s frame. SDH can also multiplex packet based frames such as Ethernet, PPP and ATM.

When do we use SDH ? When networks need to increase capacity , SDH simply acts as a means of increasing transmission capacity.

When networks need to improve flexibility, to provide services quickly or to respond to new change more rapidly.

When networks need to improve survivability for important user services.

When networks need to reduce operation costs, which are becoming a heavy burden.

SDH network works with a single central clock that synchronizes all the elements in the network. i.e. SDH is synchronous as its name implies.

The SDH protocol enables transmitting any of the PDH bit rates directly by mapping it to the STM-n frame, that gives the user the flexibility to transmit any configuration of tributary rates using only one multiplexing element.

The most important main standards are STM-1, STM-4 and STM-16. The fundamental SDH frame is known as STM1 (synchronous transport module); its Sonet counterpart is OC3 (optical container). Each provides a bit rate of 155 Mbit/s with a total frame size of around 20 kbits.

The SDH standards used in Europe are

STM-1 which provides 155 Mbits/secSTM-2 which provides 310 Mbits/secSTM-3 which provides 465 Mbits/sec

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STM-4 which provides 620 Mbits/sec etc (increments of 155 Mbits/sec ) The ability to multiplex any of the standard bit rates into the STM-n frame is possible due to the complicated container structure of the STM-n frame as depicted below.

Let’s see how the STM-1 is formed as it is the most simple structure to understand SDH.

Figure 7 : SDH frame structure (STM-1)(a) Outline frame structure(b) Frame structure shown in rows and columns

The SDH frame has a two-dimensional structure, as shown in the diagram. Each frame is physically transmitted through the fiber row by row, each row from left to right. In practice each frame is transmitted as a continuous sequence of bits, with the last bit of one frame being immediately followed by the first bit of the next. Each block can be read by starting with the top left byte reading along each horizontal row from left to right, and ending with the bottom right byte (byte 2430). Each frame is transmitted in 125 s.

The basic SDH signal, STM-1 (Fig. (a)) has 9 equal segments, with overhead bytes at the start of each. The remaining bytes contain a mixture of traffic and overheads, depending on the type of traffic carried. The total length is 2430 bytes, with each using 9 bytes. Therefore, the overall bit rate is 155,520 kbits/s, which is usually called ‘155Mbits/s’.

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An STM1 frame is usually represented as nine rows and 270 columns of 8-bit bytes, as shown in Fig. (b). The first nine columns form what is known as the "section overhead" or SOH, which provides a comprehensive range of facilities such as error monitoring, network management, and automatic switching between fiber links should one be unavailable (known as "protection switching").

The remainder of the frame is termed a "virtual container" or VC, and in the case of an STM1 is known as a VC4. This contains the data – except for the first column, which is the "path overhead" or POH, whose function is to monitor the quality of the link and indicate the type of data payload it is carrying.

The organization of the STM-1 frame has been specifically chosen to transport the full range of PDH bit rates and provide sufficient flexibility to transport future services based around different bit rates. All bit rates must be supported in such a way that there is scope for mixing and matching; for example, a single STM-1 needs to be able to transport both 2 Mbit/s and 34 Mbit/s, and any bit-rate transported should be able to be inserted and dropped (add/drop) without de-multiplexing the entire STM-1. All of these features are easily provided through organization of the columns. In SDH, each bit rate is transported, in a container, by using a fixed number of columns. E.g. each 2 Mbit/s PDH signal is transported in four columns.

SDH is able to transport containers of different sizes by using different numbers of columns. But and Frame synchronization are necessary in SDH.

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