TIPOLOGÍA VIDA ÚTIL TOTAL

At the other end of list of considerations regarding AONs is a host of issues regarding any architecture itself (and the implementation of it, refer part I chapter 2 & 3). Optical network architectures may be classified as either of two general types

• Broadcast and select networks

• Selectively routed networks

These are diagrammatically illustrated in Fig. 4.1 & 4.2, respectively. The broadcast and select type of network relies upon the huge capacity available within optical networks and trades it for relative simplicity in the network control by broadcasting all the transmitted information to all the destinations so that actively manipulating the transmission spectrum through e.g. switching (which in optics tends to be non-trial) is avoided. Selection of the desired information is only performed at the receiving station by simply ‘tuning-in’ to only a portion o f the available information, e.g. in the time or the wavelength domain.

This type of optical network architecture is used mainly in networks topologically limited to shorter transmission distances, where the implementation is very cost sensitive, e.g. in the access network, see chapter 5, and/or where the service requirement is such that network functionality is not so critical such as e.g. IBM’s Rainbow network [5].

&1.ÀN

Xl-X{^

Fig. 4.1 - Broadcast and select optical network

In the cases where the volume of communicated information is such that even the enormous capacity o f optical fibre is not enough (this is typically emphasised when combined with restrictions imposed by topology and implementation) it is necessary to manipulate and select, i.e. switch, all or proportions o f the traffic. If this is done within the optical domain this may be referred to as the other type of optical network, namely a selectively routed optical network. This type of network is diagrammatically represented in Fig. 4.2. The access nodes each transmit or receive the data destined for them directly in pre-defined allocations of e.g. the wavelength and/or time spectrum. The individual

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proportions of the optical transmission space are manipulated directly in the optical domain by optical cross-connects (or add/drop multiplexers). This type of optical network architecture is typically used in extended networks or where the required functionality warrants the increased technological complexity and cost, see chapter 6.

Access node Optical cross c o n n e d

Fig. 4.2 - Selectively routed optical network

4.3.1 Modularity and scalability

In the author’s opinion, when considering any (all) optical network, two issues, which often seem to be either overlooked or diminished, are of key importance as they concern the fundamental assumptions upon which any telecommunication network is based. Namely, modularity and scalability. By a network’s modularity we mean to which extent it is singularly extendible In other words, can we extend the network, i.e. add nodes, one node at a time?

Several of the optical network architectures initially proposed, such as the wavelength routed shuffle-net, Manhattan Street Network (MSN) or hyper cube were

107 centred around the available wave guide optical technology. These networks are not intrinsically^ modular.

This is not necessary a disadvantage but the accompanying limitations and restrictions must be acknowledged and recognised. For example, it seems nonsensical to suggest and discuss, for instance, a national UK network based around e.g. the hyper cube topology as anything other than a logical exercise. Although the ‘requirement’ for 64 nodes would seem to map quite closely onto e.g. the UK core network the fact that they would have to be topologically arranged so that they conform to the hyper-cube structure over distances up to 1000km (or more) would seem to be too restrictive, certainly within the context o f a UK core network. That is o f course not to say that within the right frame­ work, such as e.g. a distributed multi-processor environment, this type of network might potentially be very powerful.

Associated with the question o f modularity is also the issue o f scalability. To what extent can the network be extended. In our example o f a hyper-cube network, can we keep on adding nodes? Yes, but only at the cost of additional wavelengths assuming that larger guided wave devices could also be constructed (and only in factors o f 2, see above). Hence, the hyper cube is not scalable (or modular).

Ultimately the importance o f these issues depends on whether the proposed network is scalable enough for the envisaged application now or in case of future expansion.

Scalability o f network control & management

A ‘subordinate’ and often even more neglected problem is the scalability of the control and management o f the investigated network. For instance, if we consider a single STM-16 bit stream (2.4GBit/s) made up from multiplexed ISDN channels it would consist of approximately 30000 circuit switched ‘calls’, e.g. telephone calls. If the average holding time for these is 2 minutes this corresponds to a call arrival rate o f about 300 (calls) per second. Given that telecoms switches are based around a 8kHz cycle rate this implies that a switch’s connection pattern should be re-programmed about 2.5 million (= 300x8000) times per second, where each new call (within the context o f a UK network) would be selected from one in 6.25x10*"^ (= 25 million terminations^) different possibilities. If the STM-16 channel was composed o f ATM cells a switch would need to responsively change its connection pattern about 5.5 million times per second although be it from a hugely

'The author is aware that work has been published which discusses how these types of networks may be extended, e.g. [6], but they are not inherently modular (or scalable).

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smaller ‘menu’. In either case, these extremely simplified ‘back of the envelope’ calculations^ imply control data flow and processing overheads that are non-trivial.

For example, if we are to consider communication between N nodes we find that there will be NxN possible connections and the minimum number o f control bits (in a binary switch matrix’) required to describe the connection is NlogzN. If the effective capacity"^ of the control channel is Cctri and we assume that the network should spend at least the same proportion of time transmitting, Tmsg, Eqn. (4.1), as controlling (the transmission of) information, Tctri, Eqn. (4.2), i.e. Tmsg < Tctri, we find that the maximum capacity of the network between each node pair, Cmax, is given by Eqn. (4.3), where M is the message length in bits.

(41)

(4.2)

^ c t r l

M C

A^log^A^ ^ ^

We plot the maximum communication capacity between any node pair, C.max, if the traffic carried is units of 2 minutes worth of circuit switched ISDN traffic (C.max.SW) and individually switched ATM cells (C.max. ATM) versus the number o f nodes, N, for a network with an effective control channel capacity, Cctri, of IGBit/s in Fig. 4.3 below.

For example, in the case of a network o f proportions equivalent to the UK, i.e. N = 2 5 x l0 \ the effective maximum communication capacity between any node pair would be in the order o f 12MBit/s or 600Bit/s for SW or ATM traffic respectively, assuming an effective control channel capacity of 1 GBit/s.

^For example, in the case of our ISDN traffic example, the author is aware that the number of connection patterns in practise is much less than 6.25x10’"’ due to the ‘call stripping’ and resource aggregation [1] which place in any real network.

^By a binary switch matrix we mean a switch fabric composed of (or represented by) 2x2 switches. Each of the 2x2 (pass-through or cross-over) switches require a binary control signal, i.e. one control bit.

"’Ey effective capacity we mean the actual end to end capacity including all transmission & processing delays etc.

109 l E + 1 6 - S lE + 1 4 - E 9 , lE + 1 2 - '"r lE + 1 0 - ~ r o C.max.SW §■ IE+08- Ü 100000 C.max.ATIV '5 0 ' --- ^ 10000 f T3 □ 100 -

z

Fig. 4.3 - Maximum node pair capacity, (Bit/s) versus the number of nodes (N) for circuit switched traffic (C.max.SW) and individual ATM packets (C.max.ATM)

Although the above ‘analysis’ is rather stylised it does seem to give some clear indications regarding any potential application of photonics in switching Namely, that transparent optical networks due to their relative non-responsive control would seem to be better suited for circuit switched or trunk-type traffic rather than distributed processing networks based around packet switched traffic; particularly when communicating between a large number of entities, i.e. nodes.

It would further more seem to suggest that optical networks would lend themselves towards aggregating or bundling of the information transmitted into larger units which then in turn may be switched so that the number of processed units is reduced. This would though mean that the utilisation of the available information space would not be maximised. However, we might not find this to be critical, particularly in view of the huge information space potentially available in optical fibre. In other words, the design of transparent AONs might be best served by “wasting” the huge information space available in such a way that the processing load is reduced. One possibility, which has been used in several AONs, e.g. LAMBDANET and others [2-4], might be to partition the available information space and designate each portion, e.g. wavelength channel, to a certain (transmitting) node. For a node to receive information from another node it is then only necessary to transmit the information in question on the allocated wavelength and the receiving node to tune to that wavelength [2-4]. Thus the information space has become

address space See chapter 5 & 6 for further development of this concept with application to the access and national network, respectively.

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