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1.2.3 A Telephone Connection
The telephone network is probably the oldest connection-oriented network. A telephone switch, known as the central office, serves many thousands of subscribers. Each subscriber is directly connected to a central office via a dedicated twisted pair line, known as a local loop. Central offices are interconnected via time-division multiplexing (TDM) links, such as SONET/SDH links and PDH links (i.e., T1, E1, T3, and E3).
Figure 1.4 shows two telephones interconnected via two central offices. For presentation purposes, let us assume that the two central offices are connected via a T1 line. Transmission on a T1 line is organized into frames, with each frame containing 24 time slots. Each time slot is 8 bits long and carries a single voice call. The frame repeats every 128 ^sec, meaning that a particular time slot occurs once every 128 ^sec (i.e. 8000 times per second). Since it carries 8 bits at a time, the total bit rate of a time slot as it continuously repeats frame after frame is 64 Kbps.
Transmission on a T1 line is unidirectional; that is, data is routed from central office 1 to central office 2. For a bidirectional transmission between the two central offices, two separate T1 lines - each transmitting in the opposite direction - are needed.
In order for subscriber A to talk to subscriber B, a connection has to be first established. This connection is set up by the telephone network when A picks up the receiver and dials the number for the called party. A signaling protocol is used to set up a connection that runs through the central offices that are along the path from subscriber A to subscriber B. The connection involves:
(1) a dedicated line from subscriber A to central office 1;
(2) a time slot (e.g. time slot i) on the T1 line from central office 1 to central office 2; and
(3) a dedicated subscriber line from central office 2 to subscriber B.
Subscriber A office 1 office 2 Subscriber B
Figure 1.4 A simple telephone network.
EXAMPLES OF CONNECTIONS
In the opposite direction, it involves:
(1) a dedicated line from subscriber B to central office 2;
(2) time slot i on the T1 line from central office 2 to central office 1; and
(3) a dedicated subscriber line from central office 1 to subscriber A.
These resources are allocated to the phone call between subscriber A and subscriber B until one of them hangs up. A telephone connection is known as a circuit; thus, the telephone network is a circuit-switching network.
1.2.4 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: ki,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 x 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 Xi of input fiber k to the outgoing wavelength Xi 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
Router A °XC 1 ------------------1)
(a) A three-node wavelength routing network
Router A OXC 1
(b) A lightpath between Routers A and B
Figure 1.5 A lightpath.
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 X17X2,..., XW. 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).