Orientation to Transport Networks and Technology

Grover.book Page 15 Sunday, July 20, 2003 3:46 PM C HAPTER 1 Orientation to Transport Networks and Technology Many readers will have an initial picture of how the telecommunication network operates that is based on telephony circuit switching. This involves the switching of individual voice circuits through a network of trunk groups. Historically the trunk groups were cables comprising many twisted wire pairs and switching involved metallic connections between wire pairs in these trunk cables. Later, in the first step toward virtualiza- tion, point-to-point carrier transmission systems created more than one logical channel per twisted pair, and switching evolved to making connections between these logical channels. But in this context the transmission systems only provided “pair gain” between the switching nodes. In a second important step, cross-connect panels or cross-connect machines began to provide interconnection directly between carrier systems so that the apparent network seen by the circuit-switching machines could be rearranged at will. The same evolution has been repeated with wavelength division multiplexed (WDM) transmission, from point-to-point “pair gain” with coarse WDM, then to optical transport networking using dense WDM and optical cross-con- nects. In both cases the flexible interconnection of multiplexed carrier signals allows creation of virtual logical connectivity and capacity patterns for client service networks. This is the funda- mental idea of transport networking regardless of the technology involved. Through transport networking, an essentially fixed set of multi-channel point-to-point transmission systems are managed to create virtual network environments for all other services. Today, for example, the logical model of voice circuit switching still holds but the entire telephone trunk network is virtual. There are no actual twisted pair cables between voice switches. Nor are there cables or fibers dedicated between IP routers or cables that “belong” to a banking network or private company network. All these networks operate logically as if they did have their own dedicated transmission systems, but they are each just one of several virtual “service layer” networks supported by one underlying actual network; the transport network. 15 Grover.book Page 16 Sunday, July 20, 2003 3:46 PM 16 Chapter 1 • Orientation to Transport Networks and Technology Another widespread concept is packet switching. A compelling but oversimplified notion is that each packet takes an independent route through the network directly over the physical- layer cables. In reality, aggregations of data packets with common destinations or intermediate hubs en route are formed near the edges of the network and then follow preestablished logical conduits without further packet-by-packet inspection until they are near or at their destination. These logical conduits appear to the packet-switching nodes to be dedicated point-to-point transmission systems, but in fact they are cross-connected paths through the transport network, pro- viding the logical network for the packet-switching service. Both of these familiar types of network (e.g., circuit-switched telephony and packet data switching) have in recent times become essentially virtual. They are only examples of special- ized service layer networks operating over a common transport network. It is natural for us also to perceive and interact with other services such as the Internet, the banking network (ATM machines), credit verification, travel booking networks, and so on, as if they were separate physical networks. But they are all just logical abstractions created within one physical network by logical configuration of the carrier signals borne on fiber optic strands. Individual telephone calls, packet streams, leased lines, ATM trunks, Internet connections, etc., do not make their own way natively over the fiber systems. Rather, aggregations of all traffic types from site to site are formed by multiplexing these payloads up to a set of standard-rate digital carrier signals of rates such as DS3, STS1, STS3c and so on, to be discussed. Traffic of all sources, sometimes even from competing service providers, is combined (by statistical multiplexing or time division multiplexing) into composite digital signal streams for assignment to a given wavelength path or fiber route through the physical network. The transport network operates below these user-perceived service networks, and above the fixed physical network, to “serve up” logical connectivity requirements to such client networks from the underlying physical base of transmission resources. This client-server relationship can sometimes be observed across several layers. For instance, an optical transport network may implement lightpaths to create the logical topology for a SONET OC-192 network. That SONET network may itself provide routing and cross-connection for a mixture of logical paths and a variety of connection rates and types such as Gigabit Ethernet, ATM, IP, and DS3, or DS1 leased lines. An ATM client network of the SONET layer may itself then provide transport for various IP flows between Internet routers, and so on. Thus transport can also be thought of as an inter layer relationship, not always a single identifiable layer in its own right. An important tech- nological drive is, however, to reduce this historical accumulation of layers to a single layer- pair: that of “IP over optics.” This is an important development to which we will return. “IP over optics” both simplifies and enhances the application of the network design methods that follow. The mandate of this chapter is to establish familiarity with the basic concepts of transport networking, the technologies used, and some of the important issues that distinguish transport networking from other far more dominant networking concepts. This establishes context for the design problems that follow and equips a student to understand and explain to others—such as at a thesis defense—how it is, for example, that we don’t see individual packet routing consider- Grover.book Page 17 Sunday, July 20, 2003 3:46 PM 17 ations in a transport layer study, or how it can be that some nodes of the actual network do not appear in the transport network graph. (“Does it mean you have decided not to provide service to those cities?”) These questions, both of which were earnestly posed by committee members to the authors students during their dissertations, are typical of the misunderstandings about, or simple unawareness about, the concept of transport networking. Thus, there is a real need for students and practitioners to understand transport as a networking paradigm of its own, different from telephony switching and packet switching as well as leased-line private network design, and to be able to give clarifying answers to questions such as above. However, space permits only a basic introduction to transport networking; the minimum needed to support later develop- ments in the book. To delve deeper, the book by K.Sato [Sato96] is recommended as a supple- ment. It gives a more extensive treatment of key ideas and technologies for transport networking without considering network design or survivability. 1.0.1 Aggregation of Service Layer Traffic into Transport Demands Ultimately, whether through IP over optics or through a stack of DS-3, ATM and SONET layers, the net effect is that a set of user services is mapped onto a set of physical high-capacity transmission, multiplexing and signal switching facilities that provide transmission paths to support the logical connectivity and capacity requirements of all service layer flows. The aggregation of flows between each pair of nodes on the transport network defines what is called the demand on the transport network (or the respective transport layer). The term demand has a spe- cialized meaning, distinct from the more general term traffic [Wu92]. A demand unit is a quan- tum of transmission and routing capacity used to serve any aggregations of traffic flow from the service layers. Whereas traffic may refer to measures of voice, data, or video flow intensities (Erlangs, packets per second, Mb/s, frames/sec, etc.), demands on a transport network are speci- fied in terms of the number of managed units of transmission capacity that the aggregation of traffic requires. Demand may thus take on standard transmission units such as lightpaths, OC- 192s, OC-48s, DS3s or DS1s. An optical backbone network may typically be managed at the OC-48 (~2.5 Gb/s of aggregated data) and the whole lightpath (i.e., contiguous optical carrier signal frequency assignments) level. Each lightpath could be formatted to carry an OC-192 that may be structured as 4 OC-48s or to carry a 10GigE aggregation of Ethernet packet frames, or many other application-specific payload formats for which mappings are defined. These are just examples of the general concept of aggregations of payload being matched onto standard units of demand for routing and capacity management in the transport layer. Figure 1-1 illustrates the basic concept of multiple traffic sources being aggregated based on their common destination (or a route to a common intermediate destination) thereby generat- ing a demand requirement on the transport network. In the example, the bulk equivalent of 76 STS-1s would in practice be likely to generate two OC-48 demands. It is important to note that, as the word “transport” suggests in general language, the individual packets, cells, phone calls, leased lines, and so on are no longer recognized or individu- ally processed at the nodes en route. In effect they have been grouped together in a standard Grover.book Page 18 Sunday, July 20, 2003 3:46 PM 18 Chapter 1 • Orientation to Transport Networks and Technology SITE i traffic sources to: SITE j Telephony: (18) 500 DS1s M U Internet: (30) 5 STS3c L T Bulk I equivalent= 76 ATM: (15) P 5 STS3c STS-1s L Video: (8) E 8 DS3s X Private networks: 100 DS1 (5) Frame-relay services: 36 DS1 SERVICES TRANSPORT di,j = 76 Figure 1-1 Traffic sources are aggregated from service layers into transport demand units “container” for shipment toward their destination.

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