Delivery of services like telephony, voice-over-IP, video streaming, telemedicine, broadcasting of TV programmes, high-speed file sharing, online banking, online video gaming, online education and shopping, needs a transmission network capable of very high data-rate transport capabilities. Innovation in the telecom industry has led to a new kind of multiplexing and de-multiplexing technique called plesiochronous digital hierarchy (PDH).
The term plesiochronous is derived from Greek plēsios, meaning near, and chronos, time. This ensures that PDH network elements run in nearly time synchronised manner. Multiplexing of several low data-rate channels is done to utilise the high data-rate transmission capacity of transmission media. The bit rates start with the basic multiplex rate of 2Mbit/s with further stages of 8, 34 and 140Mbit/s. Such a hierarchy is shown in Fig. 1. PDH networks provide circuits to switched public networks and may also be used to build synchronisation networks.
Evolution of PDH
With the introduction of pulse code modulation (PCM) technology in the 1960s, communication networks were gradually converted to digital technology over the next few years. PCM allows multiple use of a single line by means of digital time-domain multiplexing (TDM). The PCM involves sampling, quantisation and encoding. As a standard voice telephone signal has a bandwidth of 4kHz, it is sampled at twice the 4kHz frequency, that is, at 8kHz (Nyquist rate). Each sample is then converted to an 8-bit binary number. This occurs 8000 times per second.
Thus, if we multiply 8k samples/s × 8 bits/sample, we get the standard bit rate (= 64kbps) for a single voice channel. A transmission rate of 2048kbit/s (= 64kbps x (30+2)) results when 30 such coded channels are collected together into a frame along with the necessary signalling information (equivalent to two time slots). This so-called primary rate, 2048kbit/s, is used in most parts of the world.
The growing demand for more bandwidth meant that more stages of multiplexing were needed throughout the world. In order to meet the demand for higher bit-rates, the digital multiplex hierarchy PDH evolved in various parts of the world, differing with one another. PDH hierarchies developed and adopted by Europe, North America and Japan used different tributaries for multiplexing and de-multiplexing (Fig. 2).
In Europe, PDH adopts bit rates starting with the basic multiplex rate of 2048kbit/s ≅ 2 Mbit/s with further stages of 8, 34 and 140 Mbit/s. In this system, 30 channels are multiplexed together that results in 2.048Mbps basic rate, which is designated as E1. If four such lines are multiplexed together, and taking some framing bits, we get 4×30 channels = 120 channels = 8.448Mbps = E2 designation.
Multiplexing four E2 lines together we get 4×120 channels = 480 channels = 34.368 Mbps = E3 designation. Again, multiplexing four such E3 lines results in 4 x 480 channels = 1920 channels = 139.264Mbps = E4 designation. Further multiplexing such four E4 lines results in bit rate = 4 × 1920 channels = 7680 channels = 564.992Mbps = E5 designation.
The basic digital multiplexing standard established in the United States, called the Bell System Level 1 PCM Standard, or the Bell T1 Standard, multiplexes 24 separate voice channels together, resulting in a primary rate of 1544kbit/s. This basic rate is also being followed by Canada and Japan. In the USA’s (North American) DS1 System each voice channel is 64kbps and is designated as digital signalling Level 0, or DS-0. Each frame in the 24-channel multiplexer consists of 8 bits/channel × 24 channels + 1 framing bit = 193 bits. The total data rate when transmitting 24 channels is determined by 193 bits/frame × 8000 frames/s = 1.544Mbps = T1, or DS-1 designation.
If four T1 lines are multiplexed together, we get 4 × 24 channels = 96 channels = 6.312Mbps = T2, or DS-2 designation. Multiplexing seven T2 lines together we get 7 × 96 channels = 672 channels = 44.736Mbps = T3, or DS-3 designation. If six T3 lines are multiplexed together, we get 6 × 672 = 4032 channels = 274.176 Mbps = T4, or DS-4 designation.
Similar hierarchy has also been adopted by Japan, but with slight deviations at higher order multiplexing/de-multiplexing levels. Japanese PDH system multiplexes 24 channels together, which results in basic rate of 1544kbps. At the second order, four such lines are multiplexed together. Taking some framing bits, we get 4×24 channels = 96 channels = 6.312Mbps. At the third order level, five second order lines are multiplexed together resulting in 5 × 96 channels = 480 channels = 32.064Mbps. Again, multiplexing three such third order lines results in 3 × 480 channels = 1440 channels = 97.728Mbps. Multiplexing such four fourth order lines results in bit rate = 4 × 1440 channels = 5760 channels = 397.2Mbps.
General structure of PDH signals
The PDH signal is a serial signal stream with a frame structure formed by bit-interleaving the various signals carried within its structure. A general frame structure for a 2.048Mbps bit steam is shown in Fig. 3. Each frame consists of 30 channels in channel-associated signalling (CAS) scheme or 31 channels in common-channel signalling (CCS) scheme. Sampling rate of each channel is 8000 samples/s (frame duration 125 microseconds) and there are 8 bits/sample. Thus, the basic speed of each channel is 8000 samples/s × 8 bits/sample = 64 kbps. In total, there are 32 time slots in each frame (designated as TS0, TS1, …. , TS31) resulting in 32 × 64kbps = 2.048Mbps speed.
In CCS scheme, TS0 and TS16 time slots are used for synchronisation, bit-error detection, alarm indication, frame alignment, etc. Cyclic redundancy check (CRC-4) bits allow the detection of errors. In TS0 time slot, frame alignment supervision (FAS) bits allow targeting of synchronisation to find the beginning of the frame. FAS bits are only transmitted on odd frames. Non-frame alignment supervision (NFAS) is used to manage alarms and errors, such as loss of signal (LOS) indication in the event of link failure or frame loss. NFAS uses a bit equal to ‘1’ to avoid coincidences. In TS16, bit A is used for remote alarm indication for such instances as a power fault, loss of incoming signal, or loss of multi frame alignment. Bit S is used for maintenance or performance monitoring. Multi frame alignment signal (MFAS) is used to synchronise the channel-associated signalling.
Difficulties with PDH
Traditionally, PDH system is plesiochronous, which means the network elements work more or less in time-synchronised manner, not in exact sync. Slight differences in timing signals mean that justification or stuffing is necessary when forming the multiplexed signals. Inserting or dropping an individual 64kbit/s channel to or from a higher digital hierarchy requires a considerable amount of complex multiplexer/de-multiplexer equipment.
One of the major hurdles with PDH system is adoption of different standards around the world. Various geographies of the world use different hierarchies, which lead to problems of international interworking. For example, between those countries using 1.544Mbit/s systems (USA and Japan) and those using the 2.048Mbit/s system (Europe), specialised interface equipment is required to interwork the two hierarchies.
To recover a 64kbit/s channel from a 140Mbit/s PDH signal, it is necessary to de-multiplex the signal all the way down to the 2Mbit/s level before the location of the 64kbit/s channel can be identified. PDH requires ‘steps’ (140-34, 34-8, 8-2 de-multiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual speech or data channel (Fig. 1).
Channel cross-connection is also an issue with PDH. In this system, identification of individual channels in a higher-order bit stream is not possible. Most PDH network equipment is proprietary. Vendors use their own line coding, optical interfaces, etc. Moreover, PDH systems lack in network management and monitoring capabilities too. In most parts of the world, there is no standardised definition of PDH bit rates greater than 140Mbit/s. Besides, each multiplexing section has to add overhead bits for justification, which is again not an efficient use of the available transmission bandwidth. PDH network architecture does not allow ring structure and works in point-to-point topology.
A PDH transport system provides the technical means to transfer large quantities of data between two network nodes. But it is very difficult to interoperate different PDH networks as it is a specific vendor proprietary based technology that lacks in global standardisation. Different parts of the world use different hierarchies for multiplexing and de-multiplexing. Moreover, PDH does not allow direct multiplexing, which means individual tributary signals cannot be inserted or removed into the PDH multiplexed signal without intermediate multiplexing and de-multiplexing steps. Supervision and maintenance functions are limited. This makes PDH an inefficient signal transport technology.
The author, working with BSNL, holds a PhD degree from IIT (BHU), Varanasi. He holds senior research fellowship of UGC at Centre of Advanced Study, Department of Electronics Engineering, IT-BHU in Varanasi. His current research interests include wired and wireless technologies for high-speed Internet access