2.5 SRP/RPR AND DPT |
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2.5.1INTELLIGENT PROTECTION SWITCHING
One of the features of SRP is the ability to protect against single or double fibre breaks or the total loss of a router. For two such examples, see Figures 2.3 and 2.4. In Figure 2.3 the link between R1 and R8 is broken in both directions (although, in most senses, the same processes would apply if only one of the fibres was broken). As R1 and R8 notice that they are no longer receiving traffic, each will internally wrap the failed transmit circuit back onto the receive circuit. This will create a new topology, and all nodes will be informed that there is a ring wrap. IPS failure protection and restoration are achieved in less than 50 milliseconds, which is equivalent to the restoration speeds required by SONET and SDH systems.
In Figure 2.4, router R1 has failed. In this case, R2 and R8 will identify the failure. Each will then wrap the transmit port normally transmitting towards R1 back to the receive port from R1. Again, the topology will change and all nodes will be informed of the fact that there is a ring wrap on the network.
A well-designed DPT or RPR network can provide exceptionally efficient connectivity over local and wide area networks. In theory, it is possible to build DPT/RPR rings spanning entire continents and including up to 255 nodes. However, this is unlikely to be the most efficient use of this media. Constraining the diameter and, more importantly,
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X |
R1 |
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R8 |
R2 |
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R7 |
R3 |
R6 |
R4 |
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R5 |
Figure 2.3 Both fibres cut at a single point, ring wrapped at adjacent nodes
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TRANSPORT MEDIA |
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R1 |
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X |
R8 |
R2 |
R7 |
R3 |
R6 |
R4 |
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R5 |
Figure 2.4 Node lost, ring wrapped at adjacent nodes
the number of nodes on the ring will improve the efficiency of the network. In reality, no more than 64 nodes are permitted on a single ring.
2.6 (FRACTIONAL) E1/T1/E3/T3
These lower speed interfaces are commonly used between the provider edge and customer edge. Despite the significant inroads being made by Ethernet to the edge, these interfaces are still dominant because of the lack of widespread availability of Ethernet-based services beyond the urban centres.
The major problem associated with these interfaces is their large number. Traditionally, these are the interfaces over which all customers have been connected to their service providers’ networks. When each customer is presented over a single X.21, RJ48, or coax pair, there is a limit to the number of customers who can be serviced in a specific amount of router real estate. In order to overcome this constraint, it is now common for telcos to aggregate many of these circuits over a single SONET or SDH presentation (e.g. 61 E1s presented over a single STM-1, 12 DS-3s presented over a single OC12). This ‘channelization’ is actually fundamental to the way SONET and SDH work so it requires no additional functionality within those protocols. The only extensions required are to the service offerings of the telcos and to the interfaces on routers.