6.4 An mpls Connection

MPLS introduces a connection-oriented structure into the otherwise connectionless IP network. An MPLS-ready IP router does not forward IP packets based on the destination address in the header. Rather, it forwards them based on a label that is very similar in functionality to the VPI/VCI value carried in the header of an ATM cell.

Let us consider an MPLS-enabled IP network that runs over Ethernet. In this case, a special MPLS header, sandwiched between the IP header and the LLC header, is used. The MPLS header contains a label that is a short, fixed-length connection identifier. The MPLS-ready IP router, known as a label switched router (LSR), maintains a table of labels. When an IP packet arrives at the LSR, the label carried in the MPLS header is cross-referenced to the table of labels to find the next hop. The IP packet is then switched to the destination output port of the LSR that connects to the next hop LSR. The table contains labels for only the existing connections, and therefore it is not as large as the forwarding routing table in an IP router.

The procedure is similar to ATM. In order for a user to transmit over an MPLS-enabled IP network, it has to first request the establishment of a connection. This is done using a signaling protocol, such CR-LDP or RSVP-TE. The connection is known in MPLS as a label switched path (LSP). As in the case of ATM, an LSR is aware of all of the connections that pass through its switch fabric; therefore, it can decide whether to accept a new connection or not based on the amount of traffic that will be transmitted and the requested QoS. The LSR allocates a portion of its bandwidth to a new connection, and it stops accepting new connections when it either runs out of bandwidth or reaches a certain percentage of utilization.

6.5 A Wavelength Routing Optical Network Connection

Optical networks are based on the wavelength division multiplexing (WDM) technology, which combines multiple wavelengths onto the same optical fiber. A wavelength is a frequency on which a data stream can be modulated. Each wavelength, therefore, is a separate transmission channel. Transmission over a WDM fiber requires W-independent transmitters. Each transmitter is a light source (e.g. a laser), and is independently modulated with a data stream. The output of each transmitter is an optical signal on a unique wavelength: λi , i = 1, 2, . . . , W . The optical signals from the W transmitters are combined into a single optical signal at a wavelength multiplexer and transmitted out onto a single optical fiber. At the receiving end, the combined optical signal is demultiplexed into the W individual signals, and each one is then directed to the appropriate receiver, where it is terminated and converted to the electric domain.

A wavelength routing optical network consists of optical cross-connects (OXCs) interconnected with WDM fibers. An OXC is an N × N optical switch, with N input fibers and N output fibers. Each fiber carries W wavelengths. The OXC can switch optically; that is, all of the incoming wavelengths of its input fibers are switched to the outgoing wavelengths of its output fibers without having to convert the optical signal to an electrical signal. For instance, the OXC can switch the optical signal on incoming wavelength λi of input fiber k to the outgoing wavelength λi of output fiber m.

A wavelength routing network is a circuit-switching network. That is, in order for a user to transmit data to a destination user, a connection has to be first set up. This connection is a circuit-switching connection, established by using a wavelength on each hop along the connection’s path. For example, let us consider that two IP routers (router A and router B) are connected via a three-node wavelength routing network (see Figure 1.5(a)). The link from router A to OXC 1, OXC 1 to OXC 2, OXC 2 to OXC 3, and OXC 3 to router B is assumed to be a single fiber with W wavelengths, referred to as λ1, λ2, . . . , λW . Data is transmitted only in one direction: from router A to router B. Another set of fibers (not shown in Figure 1.5(a)) has to be used in order to transmit data in the opposite direction (i.e. from router B to router A).

Figure 6.5 A lightpath

Assume that IP router A wants to transmit data to IP router B. Using a signaling protocol, A requests the establishment of a connection to B. The connection between routers A and B is established by allocating the same wavelength, say wavelength λ1, on all of the links along the path from A to B (i.e., links A to OXC 1, OXC 1 to OXC 2, OXC 2 to OXC 3, and OXC 3 to B). Also, each OXC is instructed to switch λ1 through its switch fabric transparently. As a result, an optical path is formed from router A to B, over which data is transmitted optically. This optical path is called a lightpath, and it connects routers A and B in a unidirectional way from A to B. In order for B to communicate with A, a separate lightpath has to be established in the opposite way over a different set of fibers which are set up to transmit in the opposite direction.