traffic control architectures for integrated broadband...

INTERNATIONAL JOURNAL OF DIGITAL AND ANALOG COMMUNICATION SYSTEMS, VOL. 3,167-176 (1990)

TRAFFIC CONTROL ARCHITECTURES FOR INTEGRATED BROADBAND NETWORKS

GERALD R. ASH AND STEVEN D. SCHWARTZ AT& T Bell Laboratories, Crawfords Corner Road, Holmdel, New Jersey 07733-1988, U.S.A.

SUMMARY Future broadband ISDN networks will provide a multiplicity of services on integrated transport network Switching nodes, interconnected by a flexible transmission network, provide connections for voice, data and broadband services. These connections are distinguished by estimated resource requirements, traffic characteristics and design performance objectives. This paper examines alternative traffic network architectures for integrated broadband networks, and provides integrated network routeing methods, bandwidth allocation strategies and traffidrouteing control plans for these networks. These architectures extend dynamic routeing control concepts to integrated broadband networks, and suggest perhaps radically different traffic architectures to which broadband networks might evolve. Strategies are examined for dynamic traffic routeing and dynamic trunk capacity routeing, which can adapt to load variations in customer requirements or to network resource failure conditions. Bandwidth allocation procedures are investigated which manage network bandwidth according to a virtual trunk concept, in which dynamic reservation controls are placed on the number of connections for each service category. We analyse the alternative traffic network architectures, for example broadband networks. The examples show that the architectures provide, to varying degrees, the advantages of increased network efficiency, improved customer service and increased network flexibility. Fully shared ring networks which integrate dynamic traffic and trunk capacity routeing yield many of these advantages and also provide greatly simplified network operation along with maximum flexibility to apportion network resources, especially when implemented with asynchronous transfer mode (ATM) technology. Such a traffic architecture provides a possible direction for future integrated broadband networks.

KEY WORDS Broadband ISDN Routeing Bandwidth allocation

1. INTRODUCTION

This paper describes and analyses alternative traffic network architectures for integrated broadband networks. We investigate these architectures with respect to various traffic network routeing and control techniques for these networks. We consider dynamic traffic routeing and dynamic trunk capacity routeing for broadband networks, which includes simplified routeing structures allowing full sharing of network resources. We investigate dynamic traffic control techniques which introduce a virtual trunk concept for implementing dynamic link and network bandwidth allocation, as well as alternative dynamic bandwidth reservation strategies. We study alternative traffic network architectures, which include dynamic traffic routeing networks with static trunk capacity, dynamic traffic routeing networks with rearrangeable trunk capacity, and static traffic routeing networks with dynamic trunk capacity. A traffic network with dynamic trunk capacity routeing offers advantages of simplicity of design and robustness to load variations and network failures. These routeing and control techniques apply across a spectrum of network services including voice, data and video and, therefore, provide a framework for network evolution to broadband ISDN.

1047-9627/90/020167- 10$05 .OO 0 1990 by John Wiley & Sons, Ltd.

Control of telecommunication traffic networks has undergone a steady evolution over the last decade. This evolution has been partly fuelled by greater sophistication in network and computer technology needed to implement traffic networks, but also by an increased awareness of certain attributes needed for effective network implementation. An important element of traffic network architecture has to do with the relationship of the facility network and the traffic network. An illustration of a facility network is shown in Figure 1, and Figure 2 illustrates the mapping of a subset of the trunks in the traffic network onto the facility network of Figure 1. It is clear from Figures 1 and 2 that in a highly interconnected traffic network, many node pairs will have a direct trunk connection where no direct physical path exists in the facility network. In this case a direct traffic trunk is obtained by cross-connecting through a switching location. This is distinct from the traffic situation known as alternative routeing in which a call is actually switched at an intermediate location. This distinction between cross-connecting and switching is a bit subtle, but is fundamental to traffic network call routeing, and facility network trunk routeing. Refer- ring to Figure 2 we may illustrate one of the logical inconsistencies we encounter when we design the

Received 17 January 1990 Revised 23 April 1990

168 G. R. ASH AND S. D. SCHWARTZ

Figure 1. Facility network model

TRAFFIC NETWORK VIEW FACILITY NETWORK VIEW

W-V i I\ i

Figure 2. Traffic network and facility network

traffic network essentially separating from the facility network. On our alternative route from node B to node D through A, in fact the physical path is up and back from B to A (a phenomenon known as ‘backhauling’) and then across from B to D. The sharing of capacity by various traffic loads in this way actually increases the efficiency of the network because the backhauled capacity to and from B and A is only used when no direct A to B or A to D traffic wanted to use it. It is certainly conceivable, however, that capacity could be put to more efficient use, as will be studied in the paper.

We first describe the traffic network architectures which are studied in the paper. Next we describe alternatives for dynamic traffic routeing, trunk facility routeing and traffic control strategies. Finally we illustrate the application of the network routeing and control strategies to integrated broadband networks; in particular we discuss broadband ISDN network designs.

2. TRAFFIC NETWORK ARCHITECTURES

The dominant trend in traffic network architecture is an evolution towards the greatest possible flexibility in resource allocation, which includes transmission, switching and operation system resource allocation. In this paper we discuss future directions for traffic network architectures with respect to a range of alternative directions, as follows:

1. Density of trunk group interconnection in the traffic network: we mean by this the frequency

2.

3.

with which node pairs have ‘permanently’ assigned logically connected direct trunk groups between them. We place permanently in quotes to indicate that this is a relative concept. Should a network provider provision direct trunk groups between nodes on the facility network through the use of manual cross- connects and hold these links up for weeks or months at a time, then we consider these links permanent. On the other hand, dynamic trunk group rearrangement or fully dynamic trunk group reallocation could change the assignment of facilities and would not result in a permanent trunk group interconnection. Complexity of the alternative routeing: a network which uses many ways to get from point a to point b can be regarded as having complex alternative routeing . One might imagine a scheme with simpler alternative routeing patterns and high aggregation of traffic to a smaller number of paths, perhaps only one path, as in a ring network. One way to achieve this alternative of simpler routeing patterns is to have completely automatic assignment of bandwidth from the fibre facility pipe. Another aspect of alternative routeing we investigate is the effect of the ability to rearrange trunks in the facility network between different traffic node pairs. In current networks, the assignment of trunks is static and can be changed only slowly and at fairly high cost. Circuit- or packet-orientated transport network: different traffic architectures lend themselves to different modes of operation. Broad- band ISDN is headed for a simple fixed length packet or ‘cell’ mode. We wish to identify worthwhile evolutions appropriate to each of the possibilities.

Integrated broadband networks might incorporate a dense mesh traffic architecture or a sparsely connected fully shared ring architecture. The mesh architecture has dynamic traffic routeing but static trunk capacity. That is, logical switching and transmission elements interconnect switches with logical trunk group connections forming a static yet dense logical mesh, as in Figure 2. The rearrangeable mesh architecture, Figure 3, is like the static mesh architecture except that the logical trunk capacities can be rapidly rearranged, that is they are not static or permanent. Note that the rearrangeable network actually is a hybrid of the mesh and ring architectures; it has aspects of both. In the fully shared ring architecture, Figure 4, we use static traffic routeing and dynamic trunk capacity routeing, in which ‘virtual trunk group’ capacity can be changed essentially in real time, or dynamically, on a sparse logical ring network. That is, the logical ring network has low connectivity in comparison to the mesh architecture, and directly overlays the transmission facilities network. As will be discussed below,

TRAFFIC CONTROL ARCHITECI'URES 169

TRAFFIC NETWORK VIEW 1

2

FACILITY NETWORK VIEW 1

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Figure 3. Rearrangeable mesh network

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Figure 4. Fully shared ring network

minimal alternative routeing is used in the fully shared ring architecture, but flexible bandwidth allocation is employed, and calls may follow a second path on network element failure. These integrated traffic architectures could use either packet mode, circuit mode, or hybrid circuit/packet mode connection formats for the services supported.

Summarizing, three traffic network architectures are examined:

1. Static mesh network (dynamic traffic routeing, static trunk capacity routeing)

2. Rearrangeable mesh network (dynamic traffic routeing, rearrangeable trunk capacity routeing)

3. Fully shared ring network (static traffic routeing, dynamic trunk capacity routeing)

We describe details of the operation and design of each below.

2.1. Static mesh network

The static mesh traffic architecture allows one- and two-link dynamic traffic routeing between nodes. The architecture provides the flexibility of dynamic routeing in responding quickly, without capacity augments, to unforecasted traffic load. Within the mesh network, separate routeing is provided for each service type supported by the

network (e.g. switched voice, switched B-channel (64 kb/s), switched HO-channel (384 kb/s), switched H1-channel (1536 kb/s)). Bandwidth is shared among services on traffic links and throughout the network through the use of dynamic bandwidth allocation techniques, described in Section 3.

2.2. Rearrangeable mesh network

In the rearrangeable mesh network we allow trunks between the various switches to rearrange rapidly, such as by hour of the day. Rearrangeable facility capability enables rearranging the trunking network on demand. This capability appears most desirable for use in relatively slow rearrangement of capacity, such as for busy-hour traffic, weekend traffic, peak day traffic, weekly redesign of trunking, or for emergency restoration of capacity under switch or facility failure. At various times the demands for facilities by the various node pairs and services that ride on the same optical fibres will differ. In this network, if a given demand between a certain node pair for trunks decreases and a second goes up, we allow the trunks to be reassigned to the second node pair. The ability to rearrange trunk capacity automatically rather than having to provision each trunk manually can result in cost savings. Large chunks of bandwidth can be provi- sioned in the form of fibre pipes, and then the facilities can be apportioned at will through the use of the rearrangement mechanism. This ability will be discussed in greater detail when network efficiencies are examined. This simplified network :ends itself to simplified switch, traffic and facility planning. We allow dynamic traffic routeing in this rearrangeable mesh network, as described in Section 3. This network may be viewed as a special case of the static mesh network.

2.3. Fully shared ring network

In the fully shared ring network the traffic network is exactly the same as the facility network. In the simplest version of this network we allow no traffic alternative routeing. We specify that the path taken between any two nodes in the network will be the shortest path route. This network is radically different from the structure of current networks. As was mentioned previously, current switched traffic networks are dense mesh networks, whereas the fully shared ring network is sparse. For example, in the facility network model discussed in Section 4, the fully shared ring network has only 168 direct connections out of a possible 2145 in our 66-node model. An immediate implication of this is that traffic paths in this network will require many more traffic links than in the mesh networks. This does not mean they will be longer-as a matter of fact they will be the same length or shorter because they follow the minimum distance facility route. It does mean that we will pass through many more actual


‘switching’ nodes. The average is between four and five segments, and there are some paths which pass through 12 segments. Implementation of this architecture requires rapid reallocation of facility bandwidth, or in effect dynamic trunk capacity routeing, among the node pairs and services which share a facility link. This rapid reallocation of bandwidth in fully shared ring networks is especially amenable to asynchronous transfer mode (ATM) technology, which is being introduced with broadband ISDN. Methods to accomplish this are described in Section 3.

3. NETWORK ROUTEING AND BANDWIDTH ALLOCATION

In this section, we focus on network routeing strategies for integrated broadband networks. Traffic routeing evolution envisions that future networks will have the property of high adaptivity and integration of all networks and services. Currently, we often implement separate networks for different services. The proliferation of networks has brought inefficient use of network resources and administrat- ive complexity, creating a need to integrate voice and data networks and services. In Reference 1 we discuss an integrated network routeing strategy, which includes the following steps for call establish- ment:

1.

2.

3.

Determine service type and destination: at the originating switch, the service type is identified and the destination information contained in the call request message is translated and the terminating switch identified. Select patWallocate bandwidth: the destination switch information and service type requested are used by the originating switch to specify the recommended paths between the originating switch and terminating switch. Note that a path may contain one or more links (connections between two switches) in tandem. Link and network bandwidth allocation rules are applied

capacity by using call-by-call decisions. The components of load important to dynamic path selection are not systematic or easily predictable-they are random from one day to the next and must ultimately be identified in real time. In addition to reacting to daily load variations, dynamic path selection also enhances network survivability under failure and overload conditions, in which the improved adaptivity provides robust routeing capabilities. We illustrate dynamic path selection methods with examples of dynamic traffic routeing and dynamic trunk capacity routeing.

3.1. Dynamic trafjic routeing

Dynamic non-hierarchical traffic routeing, which has been implemented and is described in References 2 and 3, allows at most two links in a path and introduces path sequences that depend on the time of the day and day of the week. Dynamic non- hierarchical traffic routeing uses a hybrid time- varying and simple adaptive routeing system to respond to network load variations. Time-varying routeing allows pre-specified routeing patterns to change as frequently as every hour to respond to expected changes in traffic patterns. The strategy is supplemented with simple adaptive routeing, which searches for idle capacity on a call-by-call basis, if needed. The simple adaptive routeing method appends to each sequence of engineered two-link paths, specified by the dynamic non-hierarchical routeing network design algorithm3 for the expected network load, additional two-link (real-time) paths to be used only when idle capacity is available. State-dependent dynamic traffic routeing leads to additional economic and service benefits relative to the dynamic non-hierarchical traffic routeing strategy and may be developed based on the trunk status map concept presented in Reference 4. In state- dependent dynamic routeing, the path sequences are determined in near real-time based on the real- time status of idle capacity in the network.

_ - in selecting paths. Establish connection: the call set-up function 3.2. Dynamic trunk capacity routeing uses multi-link destination intelligence signalling to send information in the call set-up message over the common channel signalling network from the Originating switch to each via (transit) switch and to the terminating switch.

With dynamic trunk capacity routeing, the physical and logical bandwidth are shifted rapidly among node pairs and services, through the use of dynamic cross-connect devices. Figure 2 illustrates the basic difference between the facility network and the traffic network. Figure 2 indicates that a direct

Integrated networks serve packet-switched services as well as circuit-switched services. Such ‘packet’ services could include X.25, frame relay and/or asynchronous transfer mode (ATM) broadband services. The discussion of routeing strategies in this section applies equally well to packet-switched services and circuit-switched services. The integrated routeing strategy uses dynamic path selection, implemented by dynamic traffic and trunk capacity routeing, which seeks out and uses idle network

traffic trunk is obtained by cross-connecting through a ‘switching’ location, which is further illustrated in the network block of Figure 5. This is distinct from traffic alternative routeing in which a call is actually switched at an intermediate switching location. Thus, the traffic network is a logically dense network overlaid on a sparse physical one. Dynamic trunk capacity routeing is needed for the rearrangeable mesh networks and for the fully shared ring network. The latter concept allows the traffic network archi-

TRAFFIC CONTROL ARCHITECTURES 171

NETWORK CONTROL TRAFFIC uB

I DATA

BANDWIDTH

ALLOCATION

DYNAMIC CROSS-CONNECl QRADDIDROP

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Figure 5. Traffic control model

tecture to overlap the facility network architecture. Both concepts assume that dynamic cross-connect or adddrop devices are traversed at each transport network cross-connect node on a given traffic path, as illustrated in Figure 5 . This is particularly promising when such a device has low cost. Two alternatives for the transport network cross-connect nodes are as follows:

1. Dynamic cross-connects: circuit-orientated dynamic cross-connects reconfigure the trunking network capacity at any instant under control of a network controller, as illustrated in Figure 5.

2. High-speed packet adddrop: high-speed packet add/drop devices provide simple routeing of call connections, and in the future may be implemented with photonic packet- switching technology.

High-speed packet network capability to share network capacity affords the most flexible use of the available network capacity. Such capabilities are emerging with the asynchronous transport mode (ATM) switching being incorporated into broadband ISDN standard^.^ This allows the possibility of a simpler and far faster dynamic trunk switching capability at the fibre line rate.

One possibility for such an implementation would be fibre add/drop devices such as that pictured in Figure 6. This device works most naturally in a packet format in which no operations on the packets need be performed other than the placing and reading of headers. This device would only need to decide what traffic is destined for a particular switching centre, and what traffic needed to be routed to the next node. This device also has the ability to write the simple routeing information necessary for this network on the header of cells entering the network. One might call this method of switching and routeing ‘label multiplexing’, and

ADD

1 EAST EAST

1 >a 0 ~ >

WEST WEST 1 c I c

I + DROP

Figure 6. Fibre add/drop device

we note the similarity of this process to the ATM standard for broadband ISDN. ATM switching calls for fixed-length packets, called cells, which are routed by switching labels placed in the header of the cell. This function is precisely what the a d d drop device does. The simplicity of this device also seems to lend itself to greater incorporation of photonic switching. One can well imagine an optical device (possibly under electronic control) which would accomplish the same task. Research is currently under way with ‘mirror-window’ devices which may some day be able to perform this routeing function.

This add/drop device would then pass all traffic it removed from the network to a traffic switch, as illustrated in Figure 5, in order to have the traffic broken down into its component pieces. The integration of this higher adddrop rate with the lower rate of the traffic switch processor follows current notions of design of integrated switching vehicles. The operation of the adddrop device might be viewed as that of a crosdconnect that can reconfigure at an extremely rapid rate. This blurring of the distinction between a cross-connect and a


switch is also a common theme of forward-looking transmission and switching research and development.

3.3. Bandwidth allocation

In a network where individual fibres have gigabits or terabits per second of capacity, bandwidth allocation is crucial to the smooth functioning of the network. In the bandwidth allocation strategy service demands are converted to elements of bandwidth or virtual trunks (VTs). Figure 5 illustrates a possible integrated network controller apportioning bandwidth to switch processors in large parcels of virtual trunks, and then having the switch processors allocate smaller portions to each call, and in doing so perform all necessary routeing functions such as writing appropriate headers on cells. This allocation of bandwidth creates what are normally known as direct trunk groups, and, owing to this new dynamic creation, we now refer to these groups as ‘virtual high usage groups’. The bandwidth contained on the physical links from which the network control algorithm allocates bandwidth to the virtual high usage groups is referred to as the ‘virtual backbone’ groups. In cases of overload of calls for a given node pair, or in the case of an unexpectedly high demand for bandwidth from a single user, then the switch processor would set up calls on the virtual backbone groups which connect the node pairs. If there is available bandwidth, then at the completion of the call, the bandwidth is retained in the virtual high usage group to which it is assigned. In a similar manner, in the event that bandwidth is under-used in a virtual high usage group, then excess bandwidth is released to the virtual backbone groups which constitute the physical path, and so is available to all network users. The integrated network controller reassigns network resources on a dynamic basis, through analysis of traffic usage data collected from the individual switch processors.

Figure 5 illustrates the bandwidth allocation model. We have suggested several service types which are distinguished by their traffic characteristics, bandwidth requirements and design performance objectives. These service types include (a) narrowband 64 kbls B-channel circuit-mode services, (b) wideband 384 kb/s HO-channel and 1536 kb/s H1-channel circuit-mode services, (c) broadband HZchannel and H3-channel services, (d) virtual- circuit packet-mode connections for constant-bit-rate and variable-bit-rate services and (e) connectionless packet-mode services. There are two levels of bandwidth allocation that are accounted for in our bandwidth allocation method:

1. Network bandwidth allocation: when virtual trunk connections are requested, dynamic traffic routeing and dynamic trunk capacity routeing procedures are used to determine on

which network path there exists sufficient bandwidth for the service; if no such path exists the connection is blocked.

2. Link bandwidth allocation: a minimum guaranteed number of virtual trunk connections is allowed for each service type on each link; a link bandwidth allocation procedure is used to ensure that this minimum allotted bandwidth is provided for each service type. This procedure uses dynamic trunk reservation strategies employed in present-day networks.

4. EXAMPLE APPLICATIONS TO INTEGRATED BROADBAND NETWORKS

We now discuss design results which illustrate applications to broadband ISDN networks. Using new design methods, model networks were designed, and we now examine in greater detail the advantages and disadvantages of the various architectures. By network efficiency we will mean here the cost to provide networks of the various architectures at the same level of performance. In addition we will suggest relative savings for the various architectures which might be expected from operations. This measure of efficiency is directly related to the efficiency with which a network architecture can carry traffic. A network that costs less to build for a given performance level will be able to carry more traffic when designed with the same cost. We examine the question of ‘robustness’ in greater detail. In addition to being less expensive to construct, we find that some of the architectures studied also show advantages in carrying load which is not forecast. We assume a 66-node network, which is a large-scale model, and traffic load models reflecting current projections of service mix.

We now briefly discuss the cost assumptions used in the study. For items in which it is possible that large changes in technology could provide large changes in costs we attempted to bound the two extremes. Thus at the high end for the cost of a DSO termination on a traffic switch we assume a price for 1995 of $206. With advances in VLSI technology these terminations might be at the DS3 level or higher. A lower bound for the termination of the DSO is then about $20. Costs for the rearrangeable network capabilities above and beyond what currently exists are abstracted from predictions for future cross-connect products. The greatest unknown in all the costs is the cost for the adddrop termination. The machinery which actually does the adding and dropping is in fact very inexpensive. It is less than a dollar per DSO. Almost the total cost for this device would come from the control necessary to run the fabric. Our logic in choosing the $20 to $5 range is that this device should cost no more than the lower bound for the switch termination and could cost not significantly less than the least expensive cross-connect. We find that the conclusions are not sensitive to small


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variations in these assumptions (if say an add/drop costs $30 per DSO).

SW TERM = $20 CCTERY = $5

ADDlDROP= $5 -

4.1. Network designs

We use a design methodology, for the mesh networks which adapts methods for integrated dynamic routeing networks. ‘ 9 For the fully shared ring network, since there is no traffic alternative routeing, the design procedure is relatively simple. The traffic demands of the various node pairs are aggregated to the facility links, and then each facility link is sized to carry the total traffic demand from all node pairs which use the facility link for voice, data and broadband traffic. The one subtlety of the design procedure is what blocking level to use for sizing the facility links. The difficulty is that many node pairs send traffic over the same facility link, and each of these node pairs will have a different number of links in its path. This means that for each traffic parcel, a different level of blocking on a given facility link will be needed to ensure say a

1 per cent level of blocking end to end. With many kinds of traffic present on the link we will be guaranteed an acceptable grade of service only if we size to the more restrictive condition. In our design, we identified the path through each facility link which required the largest number of links, n, and sized the link in consideration to (lln) per cent. We show that the fully shared ring network engineered in this simple manner still achieves significant efficiencies.

Figures 7-9 present results for all traffic network architectures with design results for voice and data services. The only variation in costs between the Figures is in the costs of the switch terminations and the terminations of the add/drop device. We use the term ‘add/drop’ as a shorthand for termination on the fully shared ring network bandwidth allocation vehicle. It is conceivable that other hardware could be used to run this network. In Figure 7, the switch termination and the addldrop termination each cost $20. The fully shared ring network is quite efficient in its use of facilities because of the aggregation of

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Figure 8. Cost comparisons of three traffic network architectures

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Figure 9. Cost comparisons of three traffic network architectures


traffic loads to the facility links. We get maximum benefit from the well-known non-linearity of the Erlang blocking formula. The difficulty with the fully shared ring network design in this instance is the large amount of money we have to spend on switchindbandwidth allocation. In this network we traverse an average of approximately four via nodes on a point-to-point path, and so we have to pay a great deal more for switching than in any of the other networks, in which we have to pass at most one via node. The conclusion to draw from these results is that if the switching needed for a fully shared ring network could not be made less expensive than the switching for a mesh network, then the fully shared ring network will be prohibitively expensive.

The static mesh network has the lowest cost, but not by as much as one might expect. The extra costs incurred in the building of the rearrangeable mesh network are almost completely offset by the increased traffic efficiencies. This points to the possible use of new, more sophisticated cross- connects for uses other than restoration. The capability of moving bandwidth among different node pairs can result in a more efficient network design; equivalently, for the same cost it can result in a more robust design. Efficiencies in network operations may well justify this type of network design.

Figure 8 shows results for the case in which the add/drop device is a factor of four less expensive than the switch termination. Savings from using the fully shared ring network in this case are fairly significant. The relative relations between the different mesh networks are constant for all of the Figures-we change none of the costs or conditions for these networks in the different examples. For the fully shared ring network it is a powerful statement that we can spend less money for this architecture and get a network with greater available bandwidth. In the next section this point will be made clear. Finally, Figure 9 shows the results for a perhaps extreme case in which the fully shared ring network terminations are a factor of 10 less expensive than switch terminations of the current technology. This Figure points to a possible significant benefit associated with fully shared ring networks if switching costs drop accordingly.

Figure 10 shows the comparison between a static mesh network designed to carry the load of Figures 7-9 along with a broadband load equal to 10 per cent of the voice load. We assume a cost of $600 for a DS3 termination for the broadband traffic for both the static mesh network and the fully shared ring network and assume $0.21 per DSO mile for all traffic. Switch terminations at the DSO level are assumed to cost $20 and low bandwidth adddrop terminations $5. In this case because the cost of transport of such high bandwidth services starts to become large, the significant savings in facility miles provided by the fully shared ring network are

FULLY 8uRED RHlQ STATIC MESH

Figure 10. Cost comparison of broadband network designs

economically significant. Figure 10 shows a fully shared ring network that is 25 per cent more efficient than the static mesh network. This computation also points to one of the great difficulties of planning for the future traffic network. If the high bandwidth services do materialize they could totally dominate all traffic carried by today’s network. At a level of broadband traffic equal to 10 per cent of voice traffic, the broadband portion of the network would have 70 times the bandwidth requirement of the low bandwidth traffic. Small uncertainties in the number of broadband calls therefore make for large uncertainties in the traffic design. Again this fact points to the need for the most flexible design possible, so that bandwidth can be reassigned as needed.

4.2. Network robustness

In the past, the most important goal of traffic network design was the production of the lowest cost design which would carry the forecast traffic at a given grade of service. There has been some attention given to the fact that forecasting is an art, not a science. The so-called forecast risk analysis algorithm6 is an example of a stochastic demand model. It is now recognized that a more appropriate approach would be to design networks which are in themselves more stable to variations in expected load. As we have mentioned previously, this will become increasingly important as the network begins to carry services which have no history from which to extrapolate required capacity. It is also the case that since these new services will probably have much greater bandwidth demands than simple voice calls, the ability to transfer bandwidth flexibly between many node pairs will be of even greater value. We can over-provide for some small percent- age of voice calls without paying a great penalty, but a network-wide over-provisioning of 45 Mb/s trunks could become prohibitive. The other important question to be addressed in network design is the ability to withstand failures of various components of the network. As the transmission and switching equipment has increased capacity there has been a tendency to concentrate load on a smaller number of facilities. Although cost effective, it leaves the network more vulnerable to large failures.


Figure 11 shows the ability of the traffic architectures to withstand forecast variations. The bars show the number of node pairs with blocking between the indicated levels. The total number of node pairs in the 66-node model network is 2145. These calculations are performed on network designs for voice and data traffic. The loads in the networks are all perturbed by 10 per cent about a zero mean. In other words some node pairs have the demands higher and some lower than has been forecast. This Figure shows the flexibility with which network bandwidth can be reconfigured to provide bandwidth where it is needed.

The first conclusion is that the fully shared ring network is the most robust of the traffic architectures we studied. Almost all of the node pairs provide performance better than the goal of 1 per cent blocking. In essence, the perturbation is not felt at all. The reason for this is that the node pair loads are perturbed about a zero mean, and so when these loads are aggregated to the links in the fully shared ring network, the law of large numbers (i.e. there are many node pairs riding on a given link in the fully shared ring network) implies that on average there will be little or no perturbation in the load on the link. It is in some sense a bit surprising that a network that allows no alternative routeing is so able to accommodate uncertainties in traffic. The ability to use an alternative route is often cited for just this advantage. The reader should realize that alternative routeing is only in the logical sense. The traffic still has to go over the same facility network. The fully shared ring network, as described above, provides almost call-by-call reconfiguration of network capacity. In that sense this network provides the greatest flexibility in bandwidth allocation.

Although alternative routeing is certainly useful, simplified schemes clearly provide many of the same advantages. The ability to reassign bandwidth dynamically is also highly useful. A broad conclusion to glean from this study is that integrated broadband networks should have expanded abilities to reassign facility bandwidth, either through the packet-orientated fully shared ring network or through the circuit-orientated rearrangeable meshes, and with that a significant decrease in the ability to use an alternative route will not degrade service to any great extent, and could possibly save operations costs.

We have also calculated the effects of sharing of capacity on the robustness of a broadband network. Here the network is sized according to the principles of forecast risk analysis, and bandwidth allocation is used with dynamic trunk reservation. We assume a uniform overload of 30 per cent on all voice traffic, although broadband traffic arrives at the expected rate. Again we show the number of node pairs experiencing blocking of voice calls in the range listed at the bottom of the chart. The results are shown in Figure 12. It is clear that integration

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Figure 11 . Network robustness of three traffic network architectures

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Figure 12. Network robustness of broadband network designs

of services in a fully shared ring network has great advantages for network performance.

4.3. Operations savings

Operations are perhaps the most expensive component of a telecommunications business. Large numbers of people are necessary to provision and route trunks (both public and private) over a typical facility network. Networks which have simplified trunk and traffic routeing will clearly save on administration expense. The greatest savings are probably to be found in the area of trunk provisioning. We spend a great deal of money putting trunks into the network and removing them when the demand shrinks or shifts. In the fully shared ring network, the provisioning of trunks then takes place at a logical level without the need for manual intervention. In a real sense the add/drop device is a cross-connect device that provisions a trunk dynamically when it is needed. Also the provisioning of fixed capacity, that is the virtual backbone trunks, could be done at a much higher level (DS3, or higher), with rearrangements at these higher bandwidth allocations becoming less frequently than in the current method of operation. We also point out that it is possible for the rearrangeable mesh networks to provide this same automated provisioning. The remapping of trunks that allowed savings on facility costs is exactly the provisioning auto- mation that is desired.


5 . CONCLUSION

We have presented and analysed alternative future traffic network architectures for integrated broadband networks, which include bandwidth allocation strategies, and traffichouteing control plans. These architectures extend dynamic routeing concepts to integrated broadband networks, and suggest perhaps radically different traffic architectures to which broadband networks might evolve. The architecture alternatives provide to varying degrees the advantages of increased network efficiency, improved customer service and increased network flexibility. We found that integrated broadband networks should have expanded abilities to reassign facility bandwidth, either through packet-orientated fully shared ring networks or circuit-orientated rearrangeable mesh networks. A significant decrease in the ability to use alternative routes will not degrade service to any great extent and could possibly save operations costs.

Fully shared ring networks yield many advantages and provide greatly simplified network operation along with maximum flexibility to apportion network resources, especially when implemented with asynchronous transfer mode (ATM) technology. In particular we found that a fully shared ring network is (a) the most robust of the traffic architectures studied, (b) provides the greatest flexibility in bandwidth allocation and (c) is the simplest network to design and operate. Such a traffic architecture provides a possible direction for future integrated broadband networks.

REFERENCES 1. G . R. Ash, B. M. Blake and S. D. Schwartz, ‘Integrated

network routing and design’, Proceedings of the 12th Inter- national Teletrafic Congress, Torino, Italy, June 1988.

2. G . R. Ash, A. H. Kafker and K. R. Krishnan, ‘Intercity dynamic routing architecture and feasibility’, Proceedings of the 10th International Teletrafic Congress, Montreal, Canada, June 1983.

3. G . R. Ash, R. H. Cardwell and R. P. Murray, ‘Design and optimization of networks with dynamic routing’, Bell Syst. Tech. J . 60, (8) 1787-1820 (1981).

4. G. R. Ash, ‘Use of a trunk status map for real-time DNHR, Proceedings of the 11th International Teletrafic Congress, Kyoto, Japan, September 1985.

5. S. E. Minzer, ‘B’roadband ISDN and asynchronous transfer mode (ATM)’, IEEE Communications Magazine, 27, (9), (1989).

6. D. F. Lynch and J . P. Moreland, ‘Economic trunk group sizing for stochastic traffic demands’, Proceedings of the 12th International Teletrafic Congress, Torino, Italy, June 1988.

Authors’ biographies:

Gerald R. Ash was born in Paterson, New Jersey on 1 August 1942. He received the B.S. degree from Rut- gers University in 1964, and the M.S. and Ph.D. degrees from the California Institute of Technology in 1965 and 1969, all in electrical engineering. Following his gradu- ation he spent two years in the United States Army Signal Corps, where he attained the rank of Cap- tain. In the Army he supervised the introduction of electronic sensor

technology to field units throughout Vietnam, and was involved in the development of error-correcting code technology at the U.S. Army Satellite Communications Agency, Fort Monmouth. He is Supervisor of Traffic Network Design at AT&T Bell Laboratories in Holmdel, New Jersey. He joined AT&T Bell Laboratories in 1972, and in 1976 became Supervisor of the Routing Studies Group: a group responsible for studies and analyses which led to the developmenb of dynamic non-hierarchical routeing (DNHR). In 1981 he took on his present responsibility for AT&T network evolution which includes traffic engineering for integrated services digital networks, real-time network control, and self-healing network design. He has been closely involved with the implementation of DNHR in the AT&T long-distance network, and in 1984 he was named an AT&T Bell Laboratories Fellow for his contributions in this area. He is author of over 25 articles and technical papers, and holds three patents. Dr. Ash won a New Jersey State Science Scholarship in a state-wide competition, was awarded the Bronze Star Medal for his Vietnam Service, and is the recipient of the 1989 Alexander Graham Bell Medal awarded by the IEEE. He is a member of Eta Kappa Nu, Tau Beta Pi and Sigma Xi.

Steven D. Schwartz received the Ph.D. degree from the University of California (Berkeley) in Theoretical Chemical Physics. The author then spent two years as post-doctoral research scientist at Columbia University in New York. In 1986 he joined the staff of AT&T Bell Laboratories where he has remained. He is also Assistant Professor of Chemical Physics at New York University.

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