IP over WDM

The Internet Protocol (IP) came as a natural progressive choice to carry the variety of service forms (voice, data, and media content such as video) by encapsulating them into the packets format provided by the IP protocol. This is the result of the

popularity of the Internet and the rapid advances in broadband technologies. WDM as the direct provider of capacity is the logical choice to carry the IP payload and hence the selection of the current layered combination as IP over WDM [23] [24]. This combination was a result of multiple protocol stack changes as technologies were being displaced for a better alternatives.

In the early 1990’s IP was carried over ATM, a protocol that proved itself in terms of integrating multiple services and providing QoS. This is fed to another layer that multiplexes many ATM low rate flows into a higher rate flow using the TDM based SONET/SDH protocol, which is finally handed to the WDM layer where the latter is carried over fibre. This stack required four major devices, IP routers, ATM switches, SONET Add drop multiplexer and WDM equipment. It lacked efficiency, there was overlap in functionality, and the protocols overhead was large. SONET and IP both perform aggregation, each layer has its own restoration functionality, and all layers perform routing, while the MPλS performs QoS. Having multiple layers with shared functionality decreased the reliability of the system.

In the late 1990’s, MPλS (the optical equivalent to MPLS) [25], was introduced to the stack replacing the ATM. It was first carried over Point-to-Point Protocol (PPP) and was encapsulated in SONET frames. But later in 2000 the PPP was replaced by Ethernet. IP over WDM emerged as the winning combination [26]. IP routers are now capable of running at 10Gbps, 40Gbps and 100Gbps and the IP layer is used for traffic aggregation, protection and route calculation [27]. WDM layer is used for providing the routes, flow protection and network restoration. The protection and route assignment are coordinated between these layers [19], [28]. Figure 2.2 shows the components used in an IP over WDM network (3 nodes shown for simplicity). We describe them here briefly:

Figure 2.2: The architecture of IP over WDM networks

IP routers are the parts responsible for routing in the IP layers and represent the client side of the optical terminal. They receive the aggregated traffic from edge routers and interface the optical side of the network.

Optical Cross Connects (OXC) are essential elements in the WDM network as they provide switching of optical signals between input and output ports. They can pro- vide light path instantiation and can close down paths dynamically. Additionally they can support switching light paths, management and protection functionality. They may also include wavelength conversion capabilities, multiplexing and groom- ing capabilities internally [29].

OXC have migrated from being opaque to transparent with the third option of be- ing translucent [30] [31]. In the opaque OXC the switching fabric is either entirely electrical or a mixture of optical fabric with optical-electrical-optical conversion. In transparent OXC the signal is switched between the input and output ports without the need for an optical-electrical-conversion. The opaque OXC supports regenera-

tion, wavelength conversion and bit-level monitoring at the cost of the electrical conversion delay and power consumption. In translucent OXCs, the switching stage consists of both the electronic module and optical module offering both options for switching signals. While the optical switching module is preferred, the electronic module comes in handy when the optical switching module is occupied or the func- tionality of regeneration is needed, therefore providing a compromise between full transparency and the choice of regeneration.

Transponders are used when the format of the optical signal needs to be adapted to another format. The client side of the transponder receives the signal from the IP router, which is generally carried on an optical fibre operating at the 1310nm wavelength. The transponder converts the client side signal into an electronic signal which is back converted to the required WDM wavelength. The reverse is done on the receiving side of the network. Although this functionality is carried out using Optical-Electrical-Optical conversion, it is expected to change to all optical with the maturing of the all optical technology. Transponders can also have network management functionality and monitoring by adding an overhead and they have the ability to add Forward error correction.

Optical amplifiers amplify optical signals directly without converting them to elec- trical signals. Erbium doped fibre amplifiers (EDFAs) have revolutionised optical communications in WDM networks, because they provide high gains in the 1.55µm window in the order of 25dB and are independent of the signal shape and bit rate. They can be used as in-line amplifiers where the links are attenuation limited. Here they are placed at regular link intervals (80-120 km) to compensate for the 0.2 dB/km attenuation of the fibre. They are used also as post amplifiers to increase the power of the transmitter, or as preamplifiers to improve receiver sensitivity. ED- FAs can also be used as booster amplifiers when couplers are used to compensate

for the losses [32], [33].

2.2.3.1 Multiplexers/Demultiplexers

Multiplexers are used to combine and separate the multiple wavelengths carried in the fibre. Currently the standards for multiplexing are the coarse and dense WDM multiplexing (CWDM and DWDM) differing by the channel separation; 20nm for the CWDM and 0.8 nm for the DWDM [20], [21].