Terabit Switching and Routing
In the present network infrastructure, world's communication service providers are laying fiber at very rapid rates. And most of the fiber connections are now being terminated using DWDM. The combination of fiber and DWDM has made raw bandwidth available in abundance. 64-channel OC-192 capacity fibers are not uncommon these days and OC-768 speeds will be available soon. Terabit routing technologies are required to convert massive amounts of raw bandwidth into usable bandwidth.
Present day network infrastructure is shown in Fig 1. Currently, Add/Drop multiplexers are used for spreading a high-speed optical interface across multiple lower-capacity interfaces of traditional routers. But carriers require high-speed router interfaces that can directly connect to the high-speed DWDM equipment to ensure optical inter operability.
This will also remove the overhead associated with the extra technologies to enable more economical and efficient wide area communications. As the number of channels transmitted on a single fiber increases with DWDM, routers must also scale port densities to handle all those channels. With increase in the speed of interfaces as well as the port density, next thing which routers need to improve on is the internal switching capacity. 64-channel OC-192 will require over a terabit of switching capacity.
Considering an example, a current state-of-the-art gigabit router with 40 Gbps switch capacity can support only a 4-channel OC-48 DWDM connection. Four of these will be required to support a 16-channel OC-48 DWDM connection. And 16 of these are required to support 16-channel OC-192 DWDM connection with a layer of 16 4::1 SONET Add/Drop Multiplexers in between. In comparison to that a single router with terabit switching capacity can support 16-channel OC-192 DWDM connection. With this introduction, we now proceed to understand what is required to build full routers with terabit capacities.
This section gives a general introduction about the architecture of routers and the functions of its various components. This is very important for understanding about the bottlenecks in achieving high speed routing and how are these handled in the design of gigabit and even terabit capacity routers available today in the market.
The Forwarding Decision: Routing table search is done for each arriving datagram and based on the destination address, output port is determined. Also, a next-hop MAC address is appended to the front of the datagram, the time-to-live(TTL) field of the IP datagram header is decremented, and a new header checksum is calculated.
Forwarding through the backplane: Backplane refers to the physical path between the input port and the output port. Once the forwarding decision is made, the datagram is queued before it can be transferred to the output port across the backplane. If there are not enough space in the queues, then it might even be dropped.
Output Link Scheduling: Once a datagram reaches the output port, it is again queued before it can be transmitted on the output link. In most traditional routers, a single FIFO queue is maintained. But most advanced routers maintain separate queues for different flows, or priority classes and then carefully schedule the departure time of each datagram in order to meet various delay and throughput guarantees
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