What Is The CLOS Architecture?

- Nov 18, 2020-

The Clos architecture was born in 1952 and was created by a man named Charles

Clos people proposed it, so it is not a new concept. This architecture mainly describes the structure of a multi-level circuit switching network. The biggest advantage of Clos is the improvement of the Crossbar structure. Through the Clos architecture, a non-blocking network can be provided. 1. Let’s take a look at the Crossbar architecture first,


It can be understood as an 8-port switch, 4-port input and 4-port output. The intersection of the Input line and the Output line (CrossPoint) is a switch. Then, if we want to establish a 1-2 port session, we must turn on the switch of input1 line and output2 line. Similarly, session(2,4)(3,1)(4,3);


Another feature of Crossbar is that when performing MxN switching (ie M inputs, N outputs), there is a blocking exchange. For example, the (3, 2) path occupies (4, 1), so ( 4,3) It is blocked and cannot be forwarded at the same time.

To sum up, the advantages of a. Crossbar are:

   1. Simple structure;

   2. Simple control;

   3. NXN exchange is strictly non-blocking; b. The shortcomings of Crossbar are: 1. The deployment scale is limited, and the number of CrossPoints is N*N; 2. There is only one path from Input to Output, without redundancy;

  3. This simple structure is not conducive to making large-scale integrated circuits (VLSI). To make large capacity, it is necessary to stack enough Crossbar;

  4. The input and output are not flexible and can only be used as an NxN switch. If it is an MxN switch, it becomes a blocked switch.

The last picture of the Crossbar exchange structure around 1960, let’s feel it


  Next, let's take a look at the CLOS architecture:


This architecture has obvious advantages;

1. In large-scale input and output, the number of Crosspoints is small. Assuming that in NxN mode, see the figure below for the specific algorithm


Therefore, we calculate a total of N=20 inputs, n=10 inputs on the sub-switch on stage1, and k=3 sub-switches on stage2, we need a total of 136 crosspoints, while using Clossbar requires N* N is 400 crosspoints. 2. Each Session has a redundant link; 3. Any input can find lines that are not used at the same time, so it is called a non-blocking architecture (of course, it is also divided into situations, and Clos is not completely non-blocking in some cases). The advantage of CLOS is that it saves costs and increases efficiency.