Networks: Circuit Switching Vs. Packets Switching


A network is set of artifacts (or simply procedures) that allows network terminals to share information each other. So, a tin can phone can be considered a kind of network without doubts.
In the Hyperuranium network model ( usually Hyperuranium citizens are software people and CEOs ), a network is a complete graph. It means that every network end is connect to all other ends. Please, have a look at following picture:

This isn’t my grandma doily. This is a complete graph.

A so designed network is perfect. When the network end that we call Joe wants to talk with network end that we call Mary, he simply picks up the phone, presses the Mary button and talks. No resources competition is expected in a doily (sorry complete graph) network model.
On another hand, if Mary breaks up with Joe, after the last sad phone call, the edge that connects  Mary and Joe became useless. It still needs to be maintained but no traffic is going to travel over it at least until Mary decides to give Joe a second chance (if she does).
In the real world, we can’t waste money over Mary and Joe’s affair maintaining useless network connections. A good compromise between traffic congestion and communication availability must be identified.
An interesting high level network concept is the Circuit Switched Network. In this model, we don’t have a full connection between network ends. If Mary wants to call James (yes, she decided to find an alternative), she calls the operator and asks to be connected to James’s terminal. The operator asks a line to an operator in James city, both operators link up phone plugs and a physical connection is established between  Mary’s and James phones. When they end their call, the operators are free to disconnect the lines that can be used for other communications.
This is a good model. When Mary and James aren’t connected, operators and lines can be used for other communications. For this reason, the circuit switching model doesn’t need a complete graph to connect network ends. Unfortunately, there are two problems: If Mary and James base their relationship on long, meaningful and romantic silences, the circuit established remains unused but cannot be used for other talkative users. It stays allocated until the connection between Mary and James lasts. This phenomena can induce the second problem: the network congestion. If all circuits are assigned, no other resources can’t be assigned to other users.
A fully racked set of pre Cisco era switches with a couple of walking routers managing network traffic.
Please, note that old network elements were prettier than actual ones.
In circuit switching model, the network has solid logic and the peripheral terminals can be dumb objects. If we refer to the old telephone network, a couple of microphones and speakers were “physically connected” each other with long, long cables (and repeaters). By the physical point of view, not so different from a tin can phone. The logic of communication is fully placed in the network (the operators) and no smart terminals are required. 

On the other side, we can imagine to have a smarter terminal and a kind of less smart network. In this case, we want that network ends take a part of the work trying to split their data stream in small pieces (packets) embedded in an envelope containing information about packet’s source, packet’s destination, some wake up signals, and an error control code. In this way, the data network becomes like a road network where packets travel from source to destination filling all available resources.  Sender and receiver are smart guys on a packet switched network. They assemble/disassemble packets, they put/check some error correction code and, sometimes, they put/decode a sequence number to be sure that the information will be fully read in the right sequence. The network, on its side, does it best (it works at best effort). Traffic lights and circulation rules will still be necessary, but the circuit established between ends is only “virtual”. It means that resources will be fully used multiplexing data over physical connections.

The intelligence on the network’s ends and the rules of the routing managed by network elements all create a fascinating set of technologies based on information exchange models, routing policies and network services definitions. This will be discussed in future posts.