Response Capability against Unexpected Traffic Changes

The ability of the network solution to respond to changes of traffic characteristics in terms of:

i) amounts of offered traffic ii) traffic pattern trends

is an important consideration calling for metrication.

Also, an indication of the scale of the changes (if necessary) in architecture of the network required to respond to these types of traffic changes, could be another metric. In order to develop some metrics describing this capability two scenarios deserve to be examined. The first scenario assumes that traffic changes occur only from growth, be it predicted or unpredicted. This implies that the traffic characteristics change due to amounts of traffic additional to that provisioned for the initial deployment of the network. The second scenario suggests that even originally provisioned final year traffic, on which the design was based, can change radically (complete churn) from fundamental changes in the future role of the network and in the type of services it will finally carry. The later questions the traffic carrying capabilities of a network solution in the absence of any advance information on the characteristics and amounts of traffic to be carried.

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5.3.1.1 Traffic Changes from Growth

In this case, the response capability can be demonstrated by the following four metrics:

i) The spare capacity {S). The idle capacity of the network which can be calculated simply as follows:

It can be defined as the amount of idle capacity that can be used for any extra traffic not provisioned in the planning case, without affecting the traffic already provisioned. ii) The average spare capacity {s). If a is the installed capacity not reserved for protection and Si is the spare capacity of the multiplex section i, and n is the total number of multiplex sections in the network solution, then

s = ^

S i

iii) Distribution of spare capacity (D ). This indicates how the spare capacity is distributed in the network. If again a is the installed capacity not reserved for protection and si is the spare capacity of the multiplex section i, and n is the total number of multiplex sections in the network solution, then

S i _

5

a

xlOO

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If the distribution of the spare capacity is calculated on a per subnetwork basis it should be noted that it does not have the same impact for SNCP subnetworks as for MS-SPRing subnetworks since in an MS-SPRing subnetwork routing flexibility can ease the disadvantages of a high distribution of spare capacity [132].

iv) Extra traffic carrying capabilities. This indicates how much more traffic could be carried by the network in addition to the provisioned traffic. Since this is very much dependant on the characteristics of this extra traffic a worst and a best case of the amount of extra traffic that could be carried can be defined, E T w and E T h

respectively. These can be used as crude metrics to quantify the limits in the amounts of extra traffic that could be carried by a proposed network solution.

When producing network solutions with SNCP-rings only, the spare capacity of each ring subnetwork equals the spare capacity of any multiplex section of the ring [78, 139]. In this case, E T w equals the spare capacity of the ring with the smallest amount

of spare capacity. This follows the assumption of the worst scenario that all growth traffic will originate/terminate within this subnetwork. E T h equals the sum of spare

capacities of each ring subnetwork by assuming the best case scenario of intra-ring growth traffic only.

When MS-SPRings form a candidate network solution, E T w and E T h are more

difficult to calculate since the extra traffic that can be carried depends on its traffic pattern trends [139]. E T w is calculated by assuming a future pure hub traffic pattern

trend of the growth traffic in the MS-SPRings of the solution. The hub node in each ring is located between the MS-SPRing's two multiplex sections for which the sum of spare capacities is the lowest. E T w equals the sum of the spare capacities of these two

multiplex sections. E T h is simply calculated by adding the sum of the spare capacities

of the multiplex sections of each MS-SPRing. This is after assuming a pure node to adjacent node traffic pattern trend in the growth traffic.

When the network solution is a mesh architecture, which employs 1-f-l protection,

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E T w we assume that the working or the protection trail of all growth traffic traverses

the multiplex section with the smallest amount of spare capacity. Therefore, the theoretical lower limit for E T w equals the smallest amount of spare capacity found on

a multiplex section of the mesh. A theoretical upper limit for E T h is calculated by

assuming only single hop growth traffic trails which will ideally require only 2-hop protection trails. Thus, this limit equals one third of the available spare capacity in the architecture.

In network architectures formed by combinations of SNCP-ring, MS-SPRing and mesh architectures the appropriate combinations can help the planner calculate E T w

and E T h .

5.3.1.2 Traffic Changes from Growth and Churn

In the case where all possible traffic changes can occur the only two metrics that can be used are the minimum and maximum total traffic that the proposed network solution can carry. These can be calculated using similar assumptions to the ones used in calculating the metrics for the extra traffic carrying capabilities. Let S C S i be the

capacity equivalent to the ring system level of each of the t SNCP-rings in the network solution and S C M i be the capacity equivalent to the ring system level of each

of the m MS-SPRing with m nodes in the network solution. Then the minimum total traffic equals the amount of traffic that can be carried by the SNCP-ring with the lowest ring system level and the maximum total traffic is:

Maximum total traffic = ^ S C S i + ^ nSCM,

i=\ i=\ 2

Again if the network solution comprises of a mesh architecture, it is very difficult to define the minimum and maximum total traffic that can be carried by the architecture.

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Using similar assumptions to the ones used in calculating the metrics for the extra traffic carrying capabilities we can calculate that the traffic that can be carried by the multiplex section with the least installed capacity produces a theoretical lower limit to the minimum total traffic that can be carried by the architecture. A theoretical upper limit to the maximum total traffic that can be carried by the architecture equals the installed capacity divided by three.

The above metrics will in any case produce only rough estimates of the mesh architecture's ability to respond to future traffic changes since they correspond to extreme situations. Of all the metrics proposed in these section the most important are the spare capacity, it's average value and it's distribution.