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The Switch Fabric Concept


Throughout the era of electromechanical exchange advancement, engineers dedicated countless hours innovating switching techniques. The concept of the “fabric” switch gained popularity.  As more and bigger switches were needed in large exchanges, it was discovered that interconnecting small switches could approximate the functionality of a single large switch.


Imagine ten interconnected switches, each potentially different. Next, draw a box around the collection. The boxed aggregate would define a fabric switch with I/O ports. Here, the internal connections are invisible to the fabric user and only the I/O ports are seen.  


The internal switches may be of any type in theory but crossbar switches are best for flexibility, at least for telephone exchanges. Here is a basic switch fabric diagram using crossbar switches.  










10x10 switch fabric example (Peter Dordal, Loyola University)


This example demonstrates a fabric composed of 6 switches made to look like one larger 10x10 switch. For this example, it’s guaranteed that any 2 inputs can independently be connected to any 2 outputs. In this case, 5 is connected to 2 and 6 is connected to 1. So the fabric is non-blocking for any 2 inputs. 

However, any 2 additional inputs may or not connect to any 2 other outputs. For example, inputs 1-10 can connect to 6-10 but not to 1-5 due to blocking by the first two connections. Fully non-blocking switch fabrics are more costly than partially-blocking fabrics. So, exchange designs that support the necessary call traffic and can use partially-blocking fabrics are more cost effective than designs that requires only non-blocking fabrics. See Endnote on optimum fabric design methods.  


Unlike the Step-by-Step system that uses progressive dial control, this type of switch cannot be directly controlled by dial pulses in real-time. There is no simple relation between the dialed digits and what switches should be closed inside the fabric to make an in-to-out connection. Importantly, the fabric can be scaled to meet any switching size (NxM) by adding more internal switches. In a metro area exchange, this type of flexibility is essential. The #1 and #5 Crossbar exchanges use multiple switch fabrics per exchange. The crossbar switch was a significant advancement in switch technology when it was developed.  See the crossbar section with a video explaining switch operations.   

Exchange fabrics

A switching fabric refers to a network topology where nodes are interconnected through a cluster of switches. So, the switching topologies used in panel, rotary, SxS and especially crossbar telephone exchanges can be classified as fabrics. For a given exchange type, the same basic switch is replicated multiple times, forming the fabric that facilitates end-to-end phone calls.

Why is this important?  Knowing about fabrics adds insight for how all exchange implementations share common design patterns when viewed at a high-level. The Exchange Anatomy section shows how all the major components fit together to build exchanges. So, fabrics play a key role in all medium to large exchange designs.  



A famous 1953 paper by Bell Labs' engineer Charles Clos [Clos] proved how to guarantee that a switch fabric made of crossbars can be non-blocking. Clos concludes his paper by saying, "In present day commercial telephone systems the use of non-blocking switching networks is rare.....By first designing a non-blocking system with a reasonable number of switching stages and then omitting certain of the paths, the designer can arrive at a network with a given level of blocking and be very close to a minimum of cross points."


Clos provided methods to design optimum blocking and non-blocking fabrics. He is a hero among switch designers. 


Clos: A Study of Non-Blocking Switching Networks, Charles Cos, Bell System Technical Journal, March 1953. 

10x10 switch fabric example  cross point crossbar telephone exchagne
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