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3.2 Survivability Principles from the Ground Up

As in many robust systems, "defence in depth" is also part of communication network survivbility. We will now look at various basic techniques to combat failures and their effects, starting right at the physical layer. Table 3-2 follows the approach of [T1A193] and identifies four levels at which various survivability measures can be employed. Each layer has a generic type of demand unit that it provides to the next higher level. As in any layering abstraction, the basic idea is that each layer exists to provide a certain service to its next higher layer, which need know nothing about how the lower layer implements the service it provides. Here it is capacity units of various types that each layer provides to the next to bear aggregations of signals or traffic formed in the next higher layer. It is important to note that although a layered view is taken it is not implied that one or more methods from each layer must necessarily be chosen and all applied on top of each other. For instance, if rings are implemented at the system layer, then there may be no survivability measures (other than against intra-system circuit-pack level of failures) implemented at the logical layer, and vice-versa. Additionally, certain service layer networks may elect to operate directly over the physical layer, providing their own survivability through adaptive routing. In contrast, however, certain physical layer measures must always be in place for any of the higher layers to effect survivability. In this framework it is usually the system and logical layers, taken together, that we refer to when we speak of "transport networking" in general.

Table 3-2. Layered view of networks for survivability purposes

Layer

Elements

Service and Functions

Demand Units Generated

Capacity Units Provided

Generic Survivability Techniques

Service

IP routers, LSRs telephone switches,

ATM switches,

smart channel banks

Circuit-switched

telephony and data,

Internet, B-ISDN

private networks,

multi-media

OC-3, OC-12, STS-1s, DS-1s, DS-3s GbE, etc.

n/a

Adaptive routing, demand splitting, application re-attempt

Logical

OXC

DCS

ATM VP X-connects

Services grooming,

logical transport configuration, bandwidth allocation and management

OC-48, OC-192, wavelength channels,

wavebands

OC-3, OC-12, STS-1s, DS-1s, DS-3s GbE, etc.

Mesh protection or restoration

DCS-based rings

p-cycles

System

SONET OC-n TM, LTE, ADMs, OADMs WDM transmission systems

Point-to-point bit-transmission at 10 to 40 Gbs/s

Point-to-point fiber or wavelengths

fibers, cables

OC-48

OC-192 wavelength channels, wavebands

1:N APS 1+1 DP APS,

rings

Physical

Rights-of-way, conduits, pole-lines, huts, cables, ducts

Physical medium of transmission connectivity

n/a

Fibers, cables

Physical encasement,

physical diversity

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