Unbuffered Crossbars Research Articles

PBC: A Partially Buffered Crossbar Packet Switch

The crossbar fabric is widely used as the interconnect of high-performance packet switches due to its low cost and scalability. There are two main variants of the crossbar fabric: unbuffered and internally buffered. On one hand, unbuffered crossbar fabric switches exhibit the advantage of using no internal buffers. However, they require a complex scheduler to solve input and output ports contention. Internally, buffered crossbar fabric switches, on the other hand, overcome the scheduling complexity by means of distributed schedulers. However, they require expensive internal buffers-one per crosspoint. In this paper, we propose a novel architecture, namely, the partially buffered crossbar (PBC) switching architecture, where a small number of separate internal buffers are maintained per output. Our goal is to design a PBC switch having the performance of buffered crossbars and a cost comparable to that of unbuffered crossbars. We propose a class of round robin scheduling algorithms for the PBC architecture. Simulations results show that using as few as eight buffers per fabric column and irrespective of the number N of input ports of the switch, we can achieve similar performance to buffered crossbars that use N buffers per fabric output.

Efficient Variable Length Block Switching Mechanism

Most popular and widely used packet switch architecture is the crossbar. Its attractive characteristics are simplicity, non-blocking and support for simultaneous multiple packet transmission across the switch. The special version of crossbar switch is Combined Input Crossbar Queue (CICQ) switch. It overcomes the limitations of un-buffered crossbar by employing buffers at each crosspoint in addition to buffering at each input port. Adoption of Crosspoint Buffer (CB) simplifies the scheduling complexity and adapts the distributed nature of scheduling. As a result, matching operation is not needed. Moreover, it supports variable length packets transmission without segmentation. Native switching of variable length packet transmission results in unfairness. To overcome this unfairness, Fixed Length Block Transfer mechanism has been proposed. It has the following drawbacks: (a) Fragmented packets are reassembled at the Crosspoint Buffer (CB). Hence, minimum buffer requirement at each crosspoint is twice the maximum size of the block. When number of ports are more, existence of such a switch is infeasible, due to the restricted memory available in switch core. (b) Reassembly circuit at each crosspoint adds the cost of the switch. (c) Packet is eligible to transfer from CB to output only when the entire packet arrives at the CB, which increases the latency of the fragmented packet in the switch. To overcome these drawbacks, this paper presents Variable Length Block Transfer mechanism. It does not require internal speedup, segmentation and reassembly circuits. Using simulation it is shown that proposed mechanism is superior to Fixed Length Block Transfer mechanism in terms of delay and throughput.

Departure Batch-size Distribution of Unbuffered Crossbar Packet Switches

In this letter, we present an exact closed-form solution for the probability distribution of the packet batch-size at the output ports of unbuffered crossbar based packet switches. The general case of a switch with M inputs and N outputs is considered under balanced traffic and using an optimum arbitration scheme. The solution can be applied to a wide range of queuing systems, even beyond the field of packet switching.