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Definition and Description of Digital Hierarchies

1.0 INTRODUCTION AND DEFINITION: Digital Hierarchies

The term “digital hierarchy” has been created when developing digital transmission systems. It was laid down when by multiplexing a certain number of PCM primary multiplexers were combined to form digital multiplexers of higher order (e.g. second-order multiplex equipments).

Consequently, a digital hierarchy comprises a number of levels. Each level is assigned a specific bit rate which is formed by multiplexing digital signals, each having the bit rate of the next lower level. In CCITT Rec. G.702, the term “digital multiplex hierarchy” is defined as follows :

“A series of digital multiplexes graded according to capability so that multiplexing at one level combines a defined number of digital signals, each having the digit rate prescribed for the next lower order, into a digital signal having a prescribed digit rate which is then available for further combination with other digital signals of the same rate in a digital multiplex of the next higher order”.

• WHY HIERARCHIES ?

  • Before considering in detail the digital hierarchies under discussion we are going to recapitulate in brief, why there are several digital hierarchies instead of one only. It has always been pointed out that as far as the analogue FDM technique is concerned, the C.C.I.T.T. recommends the world wide use of the 12-channel group (secondary group). Relevant C.C.I.T.T. Recommendation exists also for channel assemblies with more than 60 channels so that with certain exceptions – there is only one world-wide hierarchy for the FDM system (although the term “hierarchy” is not used in the FDM technique).
  • In the digital transmission technique it was unfortunately not possible to draw up a world-wide digital hierarchy. In practice, equipment as specified in C.C.I.T.T. Recommendation G.732 and 733, they do not only differ completely in their bit rates, but also in the frame structures, in signalling, frame alignment, etc. Needless to say that, as a consequence, the higher order digital multiplexers derived from the two different PCM primary multiplexers and thus the digital hierarchies differ as well.
  • Since these two PCM primary multiplexers are available, two digital heirarchies only would have to be expected. In reality, however, two digital hierarchies with several variants are under discussion because the choice of the fundamental parameters of a digital hierarchy depends not only on the PCM primary multiplex, which forms the basic arrangement in that hierarchy, but on many other factors such as :
• the bit rate of the principal signal sources.
• traffic demand, network topology, operational features, flexibility of the network.
• time division and multiplexing plant requirements.
• compatibility with analog equipment.
• characteristics of the transmission media to be used at the bit rates for the various levels of the hierarchies.

Since today these factors which are essential for forming digital hierarchies vary from country to country, it is no wonder that we now have to consider more than two proposals for digital hierarchies.

3.0 DIGITAL HIERARCHIES BASED ON THE 1544 KBIT/S PCM PRIMARY MULTIPLEX EQUIPMENT

It was around 1968 that Bell labs. proposed a digital hierarchy based on the 24-channel PCM primary multiplex at the various levels of the hierarchy :

Level in hierarchy	Bit rate	Trans. line
First level	1544 kbit/s	T1
Second level	6312 kbit/s	T2
Third level	46304 kbit/s	L5 (Jumbo Grp)
Fourth level	280000 kbit/s	WT4 (Wave guide)
Fifth level	568000 kbit/s	T5

This proposal was modified during the following years. At the end of the study period 1968/72, the following digital network hierarchy was finally proposed as given in Fig.1.

• For the various bit rates at the higher levels of the two proposals, different reasons have been indicated. The bit rate of 44736 kbit/s was selected to provide a flexibility point for circuit interconnection and because it was a suitable coding level for the 600 channel FDM mastergroup.
• It is also an appropriate bit rate for inter-connection to radio-relay links planned for use at various frequencies.
• At the same time, N.T.T. published its PCM hierarchy are concerned (1554 and 6112 kbit/s, respectively), these two proposals are identical. They differ, however, in the higher levels as shown in Fig.2.

• In the N.T.T. proposal the bit rate of 32064 kbit/s at the third level of the proposed hierarchy might be considered a suitable bit rate to be used on international satellite links perhaps for administrations operating different PCM primary multiplex equipments. It is also a convenient bit rate for encoding the standardized 300-channel FDM mastergroup. Delta modulation and differential PCM for 4 MHz visual telephone are also suitable for this bit rate. Transmission of 32064 kbit/s via a special symmetrical cable of new design is also possible.
• The above fact shows that the differing bit rates of the third level indicated in the two hierarchy proposals can, therefore, be justified by technical arguments. As far as the differing bit rates of the fourth level are concerned, only a few technical reasons are included in the two proposal. In both cases coaxial cables are used as a transmission medium so that the medium does not call for different bit rates.
• Moreover, it seems that at present the specifications of the fourth level (and higher ones) in the two proposed hierarchies is not yet considered so urgent. For the time being the third level seems to be more important.
• The C.C.I.T.T. faced with this situation has reached finally the solution which is covered by CCITT recommendation G.752 as one can see from this recommendation, two different hierarchical levels are existing in the third level of this hierarchy, namely 32064 kbits/s and 44736 kbit/s respectively. Higher level have not been specified so far.

4.0 DIGITAL HIERARCHY BASED ON THE 2048 KBIT/S PCM PRIMARY MULTIPLEX EQUIPMENT

For this digital hierarchy, two specifications have at present been laid down only for the first level at 2048 kbit/s and for the second level at 8448 kbit/s.

As for the higher levels, the situation is just contrary to that existing in the case of digital hierarchies derived from 1544 kbit/s primary multiplex, i.e. general agreement has more or less been reached on the fourth level having a bit rate of 139264 kbit/s. 5th order system where bit rate of 565 Mb/s have also been planned now.

4.1 The critical point in this hierarchy is whether or not the third level at 34368 kbit/s should exist.

4.2 The C.C.I.T.T. has agreed after long discussions on the following (Recommendation G.751) “that there should be a 4^th order bit rate of 139264 kbit/s in the digital hierarchy which is based on the 2^nd order bit rate of 8448 kbit/s”.

There should be two methods of achieving the 4^th order bit rate :

Method 1 by using a 3^rd order bit rate of 34368 kbit/s in the digital hierarchy.

Method 2 by directly multiplexing sixteen digital signals at 8448 kbit/s. The digital signals at the bit rate of 139264 kbit/s obtained by these two methods should be identical.

The existence of the above two methods implies that the use of the bit rate of 34368 kbit/s should not be imposed on an Administration that does not wish to realize the corresponding equipment.

4.3 In accordance with the above two methods the following realizations of digital multiplex equipments using positive justification are recommended :

Method 1 : Realization by separate digital multiplex equipments : one type which operates at 34368 kbit/s and multiplexes four digital signals at 8448 kbit/s; the other type which operates at 139264 kbit/s and multiplexes four digital signals at 34368 kbit/s.

Method 2 : Realization by a single digital multiplex equipment which operates at 139264 kbit/s and multiplexes sixteen digital signals at 8448 kbit/s.

Method 1 has been put into practice.

• Where the fifth level is concerned, some preliminary proposals (e.g. 565148 kbit/s) have been submitted which were not discussed in detail.

Therefore, the present structure  of this digital hierarchy is as given in Fig.3.

• Most of the administrations favour the specification of a third level at 34368 kbit/s, mainly as a suitable flexibility point for the operation of the network and as an adequate bit rate for digital line systems which are to be set up either on new cables (screened symmetrical or micro-coaxial cables) or an radio-relay links. Other administrations do not consider the specification of a third level to be advantageous for their networks. On the contrary they regard it to be more economical to go directly from the second level at 8448 kbit/s so the fourth level at 139264 kbit/s, is also achieved by multiplexing four digital signals at 34368 kbit/s, each of which is obtained by multiplexing first four digital signals at 8448 kbit/s. However, this is a matter of internal multiplexing only, i.e. digital multiplex equipment of this type has no external input or output at 34368 kbit/s.

All administrations interested in the third level at 34368 kbit/s would thus be offered the possibility of using this level. Their digital multiplex equipment which multiplexes in the same way each of the four digital signals at 8448 kbit/s has to provide external outputs for the resulting signal at 34368 kbit/s. The digital multiplex equipment which multiplexes each of the four digital signals at 34368 kbit/s has to provide four inputs for these bit rates and one output for the resulting bit rate of 139264 kbit/s.

5.1 Outlook

The above context indicates that at the moment the discussion of digital hierarchies is still underway and is mainly concentrated on the third and fourth levels. Although certain trends are evident the specification of these and higher levels will take some time. In the interest of a comprehensive specification of the digital hierarchies to be drawn up as soon as possible, it is to be hoped that all parties concerned perform their studies with high priority.

All digital multiplexes and hierarchies proposed till date are operating in an asynchronous mode (positive justification, “positive stuffing”, bit-interleaved). It is likely that in the future, synchronous digital multiplex equipment has to be considered when setting up digital hierarchies. For various digital line systems being developed in many countries non-hierarchical bit rates have provisionally been adopted with due regard to the characteristics of the transmission media used. These non-hierarchical bit rates for digital line systems have also to be born in mind when defining the digital hierarchies and may affect the hierarchical bit rates.

6.0 CCITT Recommendations

6.1 Second order digital multiplex equipment  operating at 8448 kbit/s and using positive justification CCITT Rec. G 742.

1. This 2^nd order digital multiplex equipment using positive justification is intended for use on digital paths using 2048 kbit/s primary multiplex equipments.
2. Bit rates : The nominal bit rate should be 8448 kbit/s. The tolerance on this rate should be +30 PPM.
3. Frame Structure :

Frame Structure	Bit No.
Frame alignment word (1111010000)	1 to 10
Alarm to remote Tml	11 Set I
National use	12 Set I
Bits from tributaries	13 to 212  Set I
Justification Control bits	1 to 4  Set II
Bits from tributaries	5 to 212  Set II
Justification Control bits	1 to 4  Set III
Bits from tributaries	5 to 212 Set III
Justification Control bits	1 to 4  Set IV
Bits for tributaries available for justification	5 to 8  Set IV
Bits from tributaries	9 to 212 Set IV
Frame Length bits/tributary	848 bits 206 bits

4. Loss Recovery of Frame alignment and consequent action.

Loss of frame alignment should be assumed to have taken place when four consecutive  frame alignment signals have been incorrectly received in their predicted positions.

 

When frame alignment is assumed to be lost, the frame alignment device should decide that, such alignment has effectively been recovered, when it detects the presence of three consecutive frame alignment signals.

The frame alignment device having detected the appearance of a single correct frame alignment signal, should begin a new search for the frame alignment signal in one of the two following frames.

5. Multiplexing Method

Cyclic bit inter-leaving in the tributary numbering order and positive justification is recommended. Positive justification should be indicated by the signal 111, no justification by the signal 000. Majority decision is recommended.

• Third order digital multiplex equipment operating at 34368 kbit/s.

1. Bit rates :

The nominal bit rate should be 34368 kbit/s. The tolerance on the rate should be + 20 PPM

2. Frame Structure (Fig. 5)

Frame Structure	Bit No.
Frame alignment word (1111010000)	1 to 10  Set I
Alarm to indication to the remote TML	11 Set I
National use	12  Set I
Bits from tributaries	13 to 384  Set I
Justification Control bits	1 to 4  Set II
Bits from tributaries	5 to 384  Set II
Justification Control bits	1 to 4  Set III
Bits from tributaries	5 to 384 Set III
Justification Control bits	1 to 4  Set IV
Bits for tributaries available for justification	5 to 8  Set IV
Bits from tributaries	9 to 384
Frame length bits/tributary	1536 bits 378 bits

3. Loss and Recovery of Frame alignment

Same as the 2^nd order digital MUX system.

4. Multiplexing Method

            Same as the 2^nd order digital MUX system.

6.3  34368 Kb/s Multiplexing Frame Structure

• Fourth order digital multiplex equipment operating at 139264 kbit/s.

1. Bit rates :

The nominal bit rate should be 139264 kb/s. The tolerance on the rate should be +15 PPM

2. Frame Structure

Frame Structure	Bit No.
Frame alignment word (111110100000)	1 to 12
Alarm to indication to the remote digital MUX tml	13  Set I
Bit reserved for National use	14 to 16
Bits from tributaries	17 to 488
Justification Service bits	1 to 4
Bits from tributaries	5 to 488  Set II to V
Justification Control bits	1 to 4
Bits for tributaries available for justification	5 to 8  Set VI
Bits from tributaries	9 to 488
Frame length bits/tributary	2928 bits 723  bits

3. Loss and Recovery of Frame alignment

Same as the 2^nd and 3^rd order digital MUX system.

4. Multiplexing Method

        Same as the 2^nd and 3^rd order digital MUX system.

7.0 INTERFACES

7.1 Specification for Interfaces at 2048 kb/s.

1. General characteristics :

Bit rate : 2048 kb/s + 50 PPM

Code :  HDB3.

2. Specification at Output Port

Pair(s) in each direction	One Coaxial Pair	One Symmetrical Pair
Test Load Impedance	75 ohm (rest.)	120 ohm (rest.)
Nominal peak voltage of a mark (pulse)	2.37 V	3 V
Peak voltage of a space (no pulse)	0+0.237 V	0+0.3 V
Nominal pulse width	244 ns	244ns
Ratio of amplitude of +ve and –ve pulses at the centre of pulse interval	0.95 to 1.05	0.95 to 1.05
Ratio of widths of +ve and –ve pulses at the nominal half amplitude	0.95 to 1.05	0.95 to 1.05

3. Specification at I/P Ports

The digital signal presented at the i/p port shall be modified by the characteristics of the interconnecting pair. The attenuation of this pair shall be assumed to follow f law and the loss at a frequency of 1024 KHz shall be in the range of 0 to 6 dB. This attenuation should take into account any losses incurred by the presence of a digital distribution frame between the equipments.

The input port shall be able to tolerate a digital signal with these electrical characteristics but modulated by sinusoidal jitter.

• Specification for Interfaces at 8448 kb/s

1. General Characteristics :

Bit rate : 8448 kb/s + 30 ppm

Code : HDB3

2. Specification at o/p Port :

Pair(s) in each direction	One Coaxial Pair
Test Load Impedance	75 ohm (rest.)
Nominal peak voltage of a mark (pulse)	2.37 V
Peak voltage of a space (no pulse)	0 + 0.237 V
Nominal pulse width	59 ns
Ratio of amplitude of +ve and –ve pulses at the centre of pulse interval	0.95 to 1.05
Ratio of widths of +ve and –ve pulses at the nominal half amplitude	0.95 to 1.05

3. Specification at I/P Ports

The  digital signal presented at the i/p port shall be modified by the characteristics of the interconnecting cable. The attenuation of this cable shall be assumed to follow a f law and a loss at a frequency of 4224 KHz shall be in the range of 0 to 6 dB. This attenuation should take into account only losses incurred by the presence of a digital distribution frame between the equipments.

The i/p port shall be able to tolerate digital signal with these electrical characteristics but modulated by sinusoidal jitter.

• Interface at 34368 kb/s

1. General Characteristics

Bit rate : 3436 kb/s + 20 ppm

Code : HDB–3.

2. Specification at o/p Port

Pair(s) in each direction	One Coaxial Pair
Test Load Impedance	75 ohm (rest.)
Nominal peak voltage of a mark (pulse)	1.0 V
Peak voltage of a space (no pulse)	0 + 0.1V
Nominal pulse width	14.55
Ratio of amplitude of +ve and –ve pulses at the centre of pulse interval	0.95 to 1.05
Ratio of widths of +ve and –ve pulses at the nominal half amplitude	0.95 to 1.05

3. Specification at I/P Ports

The digital signal presented at the i/p port shall be modified by the characteristics of the interconnecting cable. The attenuation of this pair shall be assumed to follow f law and the loss at a frequency of 17184 KHz shall be in the range of 0 to 12 dB.

The i/p port shall be also to tolerate a digital signal with these electrical characteristics but modulated by sinusoidal jitter.

• Interface at 139264 kb/s

1. General characteristics

Bit rate : 139264 kb/s + 15 ppm

Code : CMI

2. Specification at o/p Port

Pair(s) in each direction	One Coaxial Pair
Test Load Impedance	75 ohm (rest.)
pk. to pk. voltage	1 + 0.1 V
Rise time between 10% and 90% amplitude of measured amplitude	< 2 ns
Return loss	> 15 dB for 7 MHz to 210 MHz

3. Specification at I/P Ports

The digital signal presented at the i/p port shall be modified by the characteristics of the interconnecting coaxial pair shall be assumed to follow f law and have a maximum insertion loss of 12 dB at frequency of 70 MHz.

The Return loss characteristic should be same as o/p port. The input port should be able to tolerate a digital signal with these electrical characteristics but modulated by sinusoidal jitter.