er 7 3. To route a trunk, prune the unsuitable links from the topology. 4. Pick the shortest path on the remaining topology. 5. Use RSVP extensions to route the trunk and perform admission control. 6. MPLS handles the forwarding. 7.3.2 Optical Elements In general, the point-to-point links between optical switches consist of bun- dles of optical fibers. An optical switch can choose to switch any possible subset of the optical bundle as a single unit-all the data traffic in such a unit on the incoming interface is switched to a corresponding type and size of unit on the outgoing interface. The switching variants used in optical net- works are described in the following section. A key feature of all of them is that the optical switch switches large flows of data as a unit and does SO based on quantities (wavelengths, time slots, and SO on) inherent in the net- work medium rather than by examining data headers at the individual packet or application level. Switching Entire Fibers A basic entity to switch is an entire fiber. All of the data that arrives on a single fiber is switched to be transmitted out of another fiber. Lambda Switching Within a single fiber, the available bandwidth can be divided up by frequency into wavelengths (also known as lambdas). An optical switch could switch all of the data in wavelength A on the incoming fiber to wavelength B on the outgoing fiber. A restriction of some optical switches (such as MEMS) is that they are incapable of wavelength conver- sion, in which case B must be the same as A. Note that this is still differ- ent from switching the entire fiber because two different wavelengths on a single incoming fiber could be switched to retain their wavelength values, but exit the optical switch on two different outgoing fibers. Waveband Switching Waveband switching is a generalization of lambda switching. If a fiber's bandwidth is divided by frequency, wave- lengths may be grouped together and switched as a block. This has poten- tial benefits in reducing the number of LSPs in place,