In carrier networks to date, the major division has been between circuit switching and transmission (transmission divides into SONET/SDH—Synchronous Optical Network/Synchronous Digital Hierarchy and optical networking using DWDM—Dense Wave-Division Multiplexing) (Stern, Bala, and Ellinas 1999). Traditionally, both switching and transmission have been voice oriented (Figure 1).
Figure 1: The current-generation network architecture.
The switching/transmission divide is not just technological, but also a structural feature of organizations and even engineering careers. There are still many telecoms engineers around who will proudly state they are in switching or transmission, and each will have a less-than-detailed view of what the other discipline is all about. Data people, formerly X.25, Frame Relay, and ATM, and latterly IP, were historically the new, and rather exotic next-door neighbors.
The traditional problem of switching is essentially one of connection: how to identify end-points (by assigning phone numbers), how to request a connection between end-points (by dialing and signaling) and how to physically set-up and tear-down the required voice connection (using telephone switches). Once upon a time this was done by analogue technologies, but that is going back too far. From the 1980s, the state of the art was digital switching, and telecom voice switches became expensive versions of computers.
Once people were digitally connected, more advanced services could be introduced such as free-phone numbers, premium rate numbers, call blocking, call redirect, and so forth. Initially this was done by increasing the complexity of the call-control software in the digital telephones switches. Unfortunately, such code was proprietary to the switch vendors: the carriers paid handsomely to buy it, and were then locked-in for their pains. The solution was for the carriers to get together and design a standardized architecture for value-added voice services called the Intelligent Network (IN). In North America the preferred term was Advanced Intelligent Network (AIN). The IN architecture called for relatively dumb switches (service switching points—SSPs) invoking service-specific applications running on high-specification computers called service control points (SCPs) during the progression of the call. Since the very same vendors sold SSPs and SCPs as sold the original switches, prices did not go down and the IN was only a partial success at best.
Transmission solves a different problem—that of simultaneously carrying the bit-streams corresponding to many different voice calls, data sessions, or signaling messages over long distances on scarce resources, such as copper wire, coaxial cable, radio links, fiber optic strands, or precisely-tuned laser wavelengths. A transmission engineer would start with a collection of nodes—towns and cities where telecoms equipment was going to be placed—and an estimated traffic matrix showing the maximum number of calls to be carried between any two nodes. The next step was to design a hierarchy of collector links that aggregated traffic from smaller nodes to larger hub nodes. These hubs would then be connected by high-capacity backbone links. This sort of hub-and-spoke architecture is common in physical transportation systems as well: roads, rail, and air travel.
Voice traffic never traveled end-to-end across the transmission network, because it had to be routed at intermediate voice switches. The telephone handset connected to a local exchange switch (or a similar device called a concentrator) at a carrier Point-of-Presence (PoP) located within a few miles of the telephone. The local switch or concentrator then connected to transmission devices to send the call to a much bigger switch at the nearest hub. From there, the call bounced via transmission links from switch to switch until it reached the called telephone at the far end.
Switch engineers called the transmission network “wet string,” based on the child’s first telephone—two tin cans connected by wet string (wetting decreases sound attenuation). Transmission engineers, on the other hand, considered voice switches as just one user of their transmission network, and in recent years a less interesting user than the high-speed data clients. These are to voice switches as a fire hose is to a dripping tap. For transmission engineers, it’s all about speed and they boast that they don’t get out of bed for less than STM-4 (Synchronous Transfer Module level 4, running at 622 Mbps).
Just a note on terminology. The word “signal” is used in two very different ways. In session services such as voice and multimedia calls, signaling is used to set up and tear-down the call as previously noted. Here we are talking about a signaling protocol. However, in transmission, signals are just the physical form of a symbol on the medium. So, for example, we talk about analogue signals, where we mean a voltage waveform on the copper wire copying sound waves from the speaker’s mouth. We talk about digital signals when we mean bits emitted from a circuit, suitably encoded onto a communications link (cf., digital signal processing). The two uses of the word “signal” are normally disambiguated by context.
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