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The space switch fabric can switch a packet to any of the N output optical buffers. The output of a tunable wavelength converter is fed to a decoupler which distributes the same signal to N different output fibers, one per output buffer. The signal on each of
OPTICAL BURST SWITCHING
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Figure 10.3 An architecture with a space switch fabric.
these output fibers goes through another decoupler which distributes it to d + 1 different output fibers, and each output fiber is connected through an optical gate to one of the FDLs of the destination output buffer.
The packet buffer consists of couplers and output buffers, which are implemented in FDLs. Specifically, an output buffer consists of d + 1 FDLs, numbered from 0 to d. FDL i delays a packet for a fixed delay equal to i slots. FDL 0 provides zero delay, and a packet arriving at this FDL is simply transmitted immediately out of the output port. Each FDL can delay packets on each of the W wavelengths. For instance, at the beginning of a slot FDL 1 can accept W optical packets - one per wavelength - and delay them for one slot. FDL 2 can accept W optical packets at the beginning of each time slot and delay them for two slots. That is, at slot t, it can accept W packets (again, one per wavelength) and delay them for two slots, in which case, these packets will exit at the beginning of slot t + 2. However, at the beginning of slot t + 1, it can also accept another batch of W packets. Thus, a maximum of 2 W packets can be in transit within FDL 2. The same goes for FDL 3 through d.
The information regarding which wavelength a tunable wavelength converter should convert the wavelength of an incoming packet and the decision as to which FDL of the destination output buffer the packet will be switched to is provided by the control unit, which has knowledge of the state of the entire switch.
OPTICAL BURST SWITCHING (OBS)
10.2 OPTICAL BURST SWITCHING (OBS)
OBS was designed to efficiently support the transmission of bursty traffic over an optical network. OBS was based on the ATM block transfer (ABT), an ITU-T standard for burst switching in ATM networks (see Section 4.6.3). OBS is still been developed and it has not as yet been standardized.
An OBS network consists of OBS nodes interconnected with WDM fiber in a mesh topology. An OBS node is an OXC (see Section 8.3.5). It consists of amplifiers, multiplexers/demultiplexers, a switch fabric, and an electronic control unit (see Figure 10.4). The OBS node can switch an optical signal on wavelength of an input fiber to the same wavelength of an output fiber. If it is equipped with converters, it can switch the optical signal of the incoming wavelength to another free wavelength of the same output fiber, should wavelength of the output fiber be in use. (Assume that full conversion applies, and that each converter can convert an optical signal to any other wavelength). Unlike wavelength routing networks, where a connection can remain active for a long time, the switch fabric of an OBS node demands an extremely short configuration time.
The OBS network is accessed by OBS end devices, which are IP routers, ATM switches, or frame relay switches, equipped with an OBS interface. (Devices which produce analogue signals, such as radar, can also be attached to the OBS network.) Each OBS end device is connected to an ingress OBS node.
An OBS end device collects traffic from various electrical networks, such as ATM, IP and frame relay, and it then transmits it to destination OBS end devices optically through the OBS network. The collected data is sorted based on a destination OBS end device address and is assembled into larger size units, called bursts. As shown in Figure 10.5, in order to transmit a burst, the end device first transmits a control packet, and after a
Figure 10.4 Reference OBS node.
Figure 10.5 A control packet is transmitted prior to transmitting a burst.
OPTICAL BURST SWITCHING
delay, known as the offset, it transmits its burst. The control packet contains information such as the burst length and the burst destination address. It is basically a request to set up a connection (i.e., a lightpath, end-to-end). After the transmission is completed, the connection is torn down.
As in wavelength routing networks, two adjacent OBS nodes can be linked by one or more optical fibers, each carrying W wavelengths. Therefore, up to W bursts per fiber can be simultaneously transmitted out. An end device might also have the ability to transmit W bursts to its ingress OBS node simultaneously, or it might have only one wavelength available on which to transmit one burst at a time.