IP/MPLS Transport and Routing Layer

This is the classic Internet model. In an all-IP world, hosts, or end systems (computers, servers, or anything that can run an IP stack) communicate over any convenient wired or wireless access transmission link (wet string) to edge routers. These edge routers look at packet headers and then forward them on the correct “next hop,” to the next router in the chain, or to the final destination host. Routers started as ordinary computers running routing software (this still works!) in the earliest days of the Internet, and then became special purpose machines with a custom architecture. Initially focused on enterprise applications, a new generation of ultralarge and ultrareliable machines came into service in the Internet boom of 1999–2001. The current state of the art is the Terabit router, the Tera prefix (10^12) indicating aggregate router throughput of thousands of billions of bits per second.
Add a note hereOnly routers at the edge of modern Service Provider networks actually see IP packets. The Provider Edge routers encapsulate a packet into MPLS by attaching a label to the front of the packet that indicates its final destination (unlike IP addresses, labels are only locally unique and may be altered at each intermediate label-switching router, thereby supporting scalability). Interior or core routers forward the labeled packets—based on their label information—along label-switched paths. The threading of label-switched paths through network routers is under the explicit control of the operator, and this control is used for a number of purposes:

§  Add a note hereLoad-balancing between alternative routes,
§  Add a note hereThe creation of virtual private networks (VPNs) for enterprises,
§  Add a note hereSegregation of traffic between different quality of service classes,
§  Add a note hereNetwork survivability via failover to backup label-switched paths.
Add a note hereThere used to be many concerns about the robustness and service quality of IP networks in general, and the Internet in particular. But as the Internet has become more central to business, significant care and attention, as well as capital resources have been invested by telecom carriers. The Internet is no longer a byword for flakiness and delay. Many carriers privately believe that the Internet is currently “too good,” and as the inexorable rise of Internet traffic fills up the currently rather empty pipes, expect to see a harder-nosed commercial attitude emerging.
Add a note hereMany carriers will focus their NGNs on connectivity services based directly on IP such as Internet access and MPLS-based VPNs. Services such as leased lines, frame relay, and ATM will either be discontinued, or will be supported only on legacy platforms that will eventually be phased out—this may take a while for leased lines services based on SDH, for example. However, some carriers want to phase out and decommission legacy networks early, to get the OPEX advantages of a simpler network infrastructure, but still leave these legacy services in place to avoid disruption to customers.
Add a note hereSurprisingly, there is a way to do this. It involves emulating leased line, Frame Relay, and ATM services over the new MPLS network, using service adaption at Multi-Service Access Nodes (MSANs) or Provider Edge routers at the edge of the network. There is obviously a considerable cost in MSAN or edge router device complexity, service management overhead and in dealing with the immaturity of the MPLS standards for doing this (using MPLS pseudo-wires). The advantage seems to be in decoupling platform evolution to the NGN from portfolio evolution to “new wave” products and services.

The Current-Generation Network

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.
Add a note here
Add a note hereThe 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.

Add a note hereCircuit Switching
Add a note hereThe 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.
Add a note hereOnce 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.

Add a note hereTransmission
Add a note hereTransmission 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.
Add a note hereVoice 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.
Add a note hereSwitch 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).
Add a note hereJust 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.