CS 4594 Broadband Switching Elements and Fabrics

Some Examples of Switch Technology Crossbar Knockout Multistage Interconnect Network (Min) Delta Networks

Crossbar N inputs and M outputs Lines from inputs and perpendicular to outputs, and where they cross there is a switch to connect or not. Problem: Does not scale well in out

Knockout Switches Introduced in 1987 by Yeh and others at ATT Bell Laboratories Essentially output queuing N inlets and N outlets operate at the same speed Transfer medium consists of N broadcast busses, one for each inlet N bus interface units, one for each outlet. Each takes input from all the broadcast busses. Each delivers output to its own outlet.

Top-level Diagram of Knockout Switch Bus Interface Unit Bus Interface Unit Bus Interface Unit N Inputs N Outputs 1 2 N 1 N

Knockout Bus Interface Unit Cell Filters Concentrator Shifter Cell Buffer N inputs 1 output

Knockout Concentrator (8:4) Inputs CCCC CC C D D D D D CC CC C C D D D D C C C C C D D D D D C C C C C = Contention Switch D = Delay Lost Cells

Multistage Interconnect Networks (MIN) Has large number of inlets and outlets Often has identical switching building blocks, forming a fabric Fabric has multiple stages

Diagram of a small MIN

Self Routing in a MIN  up 1  down

Delta Networks Constructed of identical k by k switching elements Regular interconnection patterns – suitable for large scale chip integration Self-routing, requiring log_k N digits Log-k N stages, each with N/k switching elements

Diagram of Bigger Delta Network (Banyan)

Contention in Delta Networks Cells can be simultaneously switched through the network However, cells may have to contend for same switching element

Diagram of Contention in Delta Network

Some Ways to Reduce Contention Provide buffers in every basic switching element Increase the internal link speed relative to the external speed Use backward feedback Use multiple networks in parallel Provide multiple links internally between switching elements Shuffle cells first so that they don’t collide later (Batcher-Banyon)

Some Extra Challenges Multicasting

Routing Types in a MIN Connection-based versus cell-by-cell. Cell-based (routing tags) versus network- based (routing tables).

Tags With tags, cells can be augmented to include extra header information when they enter a switch. For example, with the Siemens ATM chip set, each 53 octet cell is augmented to a 64-octet cell as it enters the switch. This extra information speeds up the routing.

Connection-based The path is determined once for the duration of a the connection. All cells of the connection follow the same path. The guarantees proper sequencing of cells. Connections may be refused. Paths can be determined by –Combination of Random selections to distribute the load Careful selection to arrive at the correct destination –Centrally or step-by-step

Cell-by-Cell Routing The routing decision is done cell by cell. Cells belonging to the same sequence or stream can be in different parts of the fabric. Cells can arrive out of sequence (depending on other design factors) With tags, each cell finds its own way. With tables, destinations have global meaning. Tables are consulted at various steps.