Optical Switching Techniques

In optical networks, there are three main optical switching techniques: Optical Circuit Switching (OCS), Optical Packet Switching (OPS) and Optical Burst Switching (OBS). OCS and OPS were first developed based on conventional circuit switching and packet switching. Though OCS and OPS have their own particular applications, both techniques have significant drawbacks and limitations. Recently, OBS proposed to combine the advantages of OCS and OPS to overcome their limitations. In this section, the benefits and limitations of these three main optical switching techniques are discussed.

2.3.1 Optical Circuit Switching

OCS was the first optical switching technique used in optical networks. In an OCS based optical network, a dedicated wavelength on each link is used to establish physical connections between pairs of source and destination nodes through switching nodes [20]. Therefore, the end-to-end paths from source nodes to destination nodes are connected sequences of physical links between nodes. At each switching node the incoming data is switched to the appropriate outgoing link. Three phases are involved in an OCS process: circuit establishment, data transfer and

circuit disconnection. The circuit establishment must be finished before data transmission begins [38]. The circuit establishment phase is a two-way reservation overhead for setting up a lightpath. First a source issues a requirement for setting up a circuit link, and then waits for an acknowledgment from the corresponding destination. [20].

OCS is suitable for large data transmissions that need long connection hold times and therefore are widely used in core networks. A key challenge in the practical implementation of OCS based optical networks is the development of efficient algorithms and protocols for establishing lightpaths [39]. To date many Routing and Wavelength Assignment (RWA) algorithms, that select the routes and assign wavelengths to all optical circuits efficiently, have been proposed and developed [40, 41]. Furthermore, dynamic signalling protocols have also been proposed to manage connections, distribute control messages and network state information in SONET and WDM networks. These signalling and control protocols include the Generalised Multiprotocol Label Switching (GMPLS) control plane, which uses the same mechanisms as Multiprotocol Label Switching (MPLS) to manage a circuit [42] and also the Automatic Switched Optical Network (ASON). The goal of these approaches is to specify a common control plane for providing QoS and equipment interoperability across domains and carriers [43].

OCS techniques have some drawbacks. A major issue is that each wavelength must be dedicated between sources and destinations (if no wavelength conversion is used), and cannot allow grooming or statistical multiplexing, which results in low channel efficiency. Another issue is that for bursty traffic, the setup time of a lightpaths can be longer than the burst duration. This results in low bandwidth utilisation. Therefore the development of alternative switching techniques which are suitable for bursty traffic is essential.

2.3.2 Optical Packet Switching

OPS techniques are developed based on mature packet switching concepts, which are widely used in computer networks. It should be noted however that unlike the concepts, some of the optical technologies needed to realise OPS are still lacking.

For example to date there are no viable optical memories [44] Taking full advantage of the available resources and occupying the wavelength only when data has to be sent, a significant statistical multiplexing gain can be provided by OPS as the bandwidth can be shared by multiple data flows. In OPS based optical networks, traffic data is divided and assembled in the payload with a preceding header that contains the destination information. The intermediate nodes function as packet routers, based on the header information, to decide where packets should be forwarded.

However, as mentioned earlier, there are some main reasons that make the implementation of all-optical OPS difficult. The two main obstacles are the need for all-optical packet header processing and the lack of optical Random Access Memory (RAM) [45] Therefore, hybrid OPS approaches, employing OEO conversion have been proposed, such as the relatively complex approach where the packet header is processed in the electrical domain [46]. To attach the header to a packet, different techniques have been proposed; such as transmitting headers and payloads on separate wavelengths, sub-carrier multiplexing (SCM) [47, 48] and serial transmission of headers and payloads on the same wavelength [49]. As the header is separated from the payload and its bit rate is lower than the payloads, electrical processing is feasible. Alternative approaches for the lack of optical RAM have also been proposed based on Fibre Delay Lines (FDLs) [50] and deflection routing [51]. Furthermore, other relevant technologies are needed for OPS and are not fully developed such as burst mode optical packet synchronisation, regeneration, wavelength conversion and packet oriented optical switching fabric [52].

2.3.3 Optical Burst Switching

OBS is proposed to overcome the limitations of OCS and OPS. In OBS, the switching granularity is at the burst level, rather than the wavelength level in OCS (holding time of minutes to months) and the packet level (ms level) in OPS. OBS provides statistical multiplexing and only the wavelength of the control packet (sent ahead of the burst to align switches and reserve wavelengths and other resources) needs OEO conversion at intermediate nodes [53]. Therefore, compared to OPS,

OBS has better transparency. Fig 2-2 gives a simple OBS based optical network architecture.

Fig 2 - 2: An OBS based optical network architecture [20]

In the OBS networks proposed in the literature (there are no commercial OPS or OBS networks), there is an OBS interface at the user end, and the OBS node comprises an optical switching fabric, switch control unit and routing processors [54]. There are some key functions in OBS, such as burst assembly, signalling, contention resolution etc. In the following parts, the details of these key functions in OBS will be discussed.

A burst assembly algorithm is used for aggregating traffic into fixed or variable size data bursts. The performance of the OBS network is affected by the choice of this algorithm. There are many different burst assembly algorithms that have been investigated. Typically, based on the time threshold T and the burst length threshold

B, these algorithms can be classified into four categories, time-based assembly

algorithms, burst length-based assembly algorithms, mixed time/burst length-based assembly algorithms and dynamic assembly algorithms [55]. The first and second categories algorithms use a fixed T or a fixed B respectively as the criterion to send out a burst. The third category algorithms use both T and B as the criterion. The last category of algorithms uses dynamic thresholds. Compared to the first three categories of algorithms, dynamic assembly algorithms are more flexible and therefore improve the performance.

Fig 2 - 3: Distributed OBS signalling with one-way reservation [53]

In the OBS network, signalling is used to build a connection for an assembled optical burst. Distributed signalling with a one-way reservation is the most widely used signalling in proposed OBS network architectures [55]. Fig 2-3 gives the time- space diagram for the distributed signalling with one-way reservation scheme. In the one-way reservation scheme a Burst Control Packet (BCP) containing information about the burst, is used for reserving the required transmission and switching resources and is sent on a separate channel. After a specified time delay (offset), the data burst is sent out without waiting for an acknowledgement (ACK) of the successful establishment of the connection between the source and the destination [53]. The offset offers sufficient time to allow the BCP to be processed at intermediate nodes, plus time to reserve the required resources and configure the switching fabric at each intermediate node.

Contention resolution in OBS has been investigated using different approaches based on time, wavelength, space domains etc. In addition to solutions such as the use of Fibre Delay Lines (FDLs), deflection routing and wavelength conversion, burst segmentation is another way to solve the contention issue. Compared to dropping bursts when there are no sufficient wavelengths and switching resources, burst segmentation which can allow the parts of a burst which overlap with other contending bursts to be dropped, improves packet loss probability in OBS networks [53]. In the process of burst segmentation, a burst is divided into basic transport units called segments. There are two possible burst segment dropping policies; tail dropping and head dropping [56]. Compared to head dropping, prioritised burst segmentation using tail dropping has better successful in-order delivery capabilities.