Switched Multimegabit Data Service (SMDS)

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Switched Multimegabit Data Service (SMDS)

Like Frame Relay, SMDS is another broadband wide area service aimed at achieving economies in transmission bandwidth by statis­tical multiplexing of bursty data traffic. Rather than being a standard in its own right, SMDS refers to a service which is delivered over a wide area network, which employs the DQDB standard for its access protocol. Originally it was targeted at rather higher hand-widths than Frame Relay i.e. l.5Mbit/s to 45Mbit/s, but now, it too is being repositioned to serve the LAN interconnect market. SMDS encapsulates the customer's data in trains of fixed sized cells, which are then relayed, via the SMDS switches to their destinations. Like Frame Relay, SMDS needs a high quality layer 1 service, in order that the number of cell corruptions is kept to a minimum. Once again, there is a technical sybiosis between SMDS and SDH, and they will only be in direct conflict if there are inconsistencies in some tariff structures.

Fibre Distributed Data Interface (FDDI)

FDDI is a high speed LAN that was designed for private operators who were able to install their own cables. However, the extension of the original interface definitions to incorporate single mode, as well as multimode optical fibre has increased the maximum ring circumference to 100km. This potentially brings it into conflict with PTO provided MANs e.g. SMDS. This is even more likely now that there are proposed mappings of the full FDDI 125Mbit/s signal into a VC4, so that PTO transmission facilities can be used to bridge those spans where the private operator cannot run his own fibre. Unfortunately, there are some problems, relating to the maximum delay that an FDDI ring can withstand. As it is this minimum ring delay which limits the size of an FDDI ring, care must be taken in the routing of the VC4 in order that this extra delay does not significantly reduce the maximum ring size. In short, because of the self contained nature of the FDDI standard, together with its rather narrow targeting as a high-speed LAN, as opposed to MAN or WAN, we expect no competition at all between SDH and FDDI.

Future technologies

Equipment which implements the SDH standards will be strongly influenced by the capabilities and cost of the enabling technologies. This section reviews the impact on both equipment, and the stand­ards themselves, of some of these developments.

Integrated circuits

The unrelenting trend to faster, smaller and cheaper traffic handling ICs (ASICs), obviously leads to cheaper network element hardware. This, in turn, will eventually lead to the introduction of SDH equipment onto the premises of business customers. The trend to higher functional integration will probably lead to changes in PTO premises design because of increased heat dissipation in a given volume of rack space.

Optical interfaces

Very low cost SDH optical interfaces are expected to produce the long awaited shift in PTO station cabling from coaxial copper to optical fibres. There are several advantages to optical interconnec­tion e.g. relatively long range, no crosstalk, physically small calling volume. However, perhaps the biggest advantage is the future proof­ing which results from the fact that an optical interconnect cable can, within reasonable limits, carry any hit rate. Besides enabling the reuse of cables that would be difficult to reuse otherwise, it also reduces the problem of successive layers of interconnect cables physically preventing the withdrawal of the older, disused, cables that they are burying.

Optical amplifiers

Erbium doped fibre amplifier technology has made great strides in the last three years. By using such amplifiers at both ends of an optical link, it is possible to greatly increase the maximum distance between transmitter and receiver, with distances up to 300km being predicted for links that employ low loss optical fibre cables, (c.f. present day optical inks at 565Mbit/s which can typically span 40-70km). This great increase in span capability holds out the prospect of interconnecting the majority of large centres of popula­tion in Europe by unrepeatered line systems i.e. the line systems consist of two LTEs plus cable, without any intervening simple repeaters. (Drop and insert repeaters are a different case, as they are deployed to drop and insert traffic, rather than solely to boost the amplitude of the optical signals). The resulting extinction of con­ventional line system repeaters means that the SDH RSOH in the STMs will eventually become redundant for terrestrial systems.

In addition to this, there is also the future proofing that results from replacing a conventional, electronically regenerating repeater, with a repeater that merely amplifies the transiting optical signals. Optical amplifiers are largely hit rate independent, hence it should he possible to upgrade the capacity of a line system by merely changing the LTEs, and not the repeaters. This idea is particularly exciting for undersea line systems where a major part of the total system cost lies in the repeaters. Up till now, these have always been constrained to operate at a fixed bit rate.

Optical switching

The use of optical switching, together with various forms of Wave­length Division Multiplexing (WDM), will lead to a requirement for yet another layer in the SDH hierarchy beyond the STM section layer. Real high speed optical switching, together with wavelength conversion is still some way in the future, hence there is no pressure to extend SDH at present.

Memory and processing power

As soon as it becomes economically feasible, more memory and processing power will be installed in individual SDH network elements. The use of this capability to support ever larger blocks of software will results in the average software download action trans­ferring ever more bytes, thus putting a strain on the Embedded Communication Channels (ECCs) that are built into the STM SOHs. There is also the chance that larger quantities of more sophisticated element control software will result in more management traffic between network elements and between elements and their control­lers, further increasing the load on the existing ECCs. Thus, at some point in the future is very likely that the SDH standards will have to be altered to expand the capacity of the ECCs beyond their current rates of 192 and 576kbit/s.


SDH is here to stay as the dominant public network transmission standard of this decade. However, the effects of the full range of SDH standards will probably be felt over a much wider area of telecommunications, because of their general applicability. For example the mere existence of the SDH optical interface specifica­tions will probably lead to their use in a variety of non-SDH applications, simply because there is a dearth of competing stand­ards. Another example is that of the SDH network recommendations, G.SNA1/2, which, with relatively little modification, are applicable to a wide variety of non-SDH networks, in particular, plesiochronous networks.


1. Learn the following technical words and word-combinations:



1. transfer mode режим передачи (переноса)
2. traffic load информационная нагрузка (трафик)


3. o supercharge перегружать


4. inconsistency противоречивость, несогласованность


5. LAN-local area network локальная вычислительная сеть
6. circumference окружность, длина окружности
7. MAN – metropolitan area network) (обще-) городская сеть
8. WAN – wide-area network глобальная (вычислительная) сеть


9. heat dissipation теплоотдача


10. cabling накладка кабеля; система кабелей
11. calling volume интенсивность потока вызовов


12. LTE – line termination equipment оконечная аппаратура линии


13. WDM – wavelength division multiplexing спектральное разделение; спектральное уплотнение


14. EEC –1. error correcting code; 2. embedded communication channels 1.код с исправлением ошибок 2.встроенные каналы вязи
15. processing power вычислительная мощность


Exercise 2 Read the text 42.7- 42.9


Exercise 3 Find the Russian equivalents for the following English technical word-combinations:

1. packet transfer techniques  
2. constant traffic load  
3. the switching machines  
4. the current front runner  
5. the network access nodes  
6. Switched Multimegabit Data Service (SMDS)  
7. Fiber Distributed Data Interface (FDDI)  
8. rack space  
9. an optical interconnect cable  
10. Erbium doped fiber amplifier technology  
11. unrepeatered line systems  
12. transiting optical signals  
13. high speed optical switching  
14. the average software download action  
15. more sophisticated element control software  
16. wavelength conversion  


Exercise 4

Find the English equivalents for the following Russian technical word-combinations:


1. коммерческое использование ATM коммутаторов  
2. инновационные идеи  
3. излишний, чрезмерный, ненужный  
4. аварийное состояние  
5. коммутируемые услуги многомегабитовых данных  
6. распределяемые данные  
7. неуменьшающийся, неослабеваемый  
8. перекрестные искажения  
9. нехватка, недостаток  
10. реле цикла  


Exercise 5

Answer the following questions:

1.Does SDH currently have any real competitors?

2.What is ATM standard aimed at?

3.What are several other Fast Packet techniques aimed at?

4.What is the best way to view Frame Relay?

5.Why can a Frame Relay network deliver much higher throughputs than an X.25 network?

6.What is Switched Multimegabit Data Service aimed at?

7.What are some problems, relating to the maximum delay that an FDDI ring can withstand?

8.What will equipments implementing the SDH standards be strongly influenced by?

9.What does faster, smaller and cheaper traffic handling lead to?

10.What are the advantages to optical interconnection?

11.What devices are used to increase the maximum distance between transmitter and receiver?

12.Does the extinction of line system repeaters mean that the SDH RSOH in the STMs will eventually become redundant for terrestrial systems?

13.Is there any pressure to extend SDH at present?


Exercise 6

Make a short report on perspectives of memory and processing power (part 42.8.5)



Exercise 6

a) Translate into Russian in writing part 42.8.1,42.8.2 and 42.8.4.

b) Translate into Russian in writing part 42.8.3.



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