The communication between the computers needs an infrastructure. With the increasing need for capacity, speed, quality, mobility, and safety that we experience today, a lot of quick development is required within these fields. The form of telecommunication we have used ever since the start is called circuit switching. This method let’s two phones who have established a dedicated line through the network to communicate. The circuit/line that is established can offer full capacity for as long as it’s active and is quite practical for you and the partner you’re talking to. However, for the data networks, this is highly inefficient. This form of communication usually comes with intense “waves” of information and then waits for the processing and an eventual response.
To better utilize the network they invented techniques to multiplex more conversations together at the same time over one single line. Frequency Division Multiplexing (FDM) consists of both analog and digital signals. The method is created so that you can have different channels that are sending information on their own frequency while having as many conversations/channels as you want (as long as the bandwidth can stand it). The great thing about FDM is that it’s just the same as switching TV Channels or radio channels with broadcasting. As the revolution of digitalizing of the networks started, something called Time Division Multiplexing (TDM) came as another solution. TDM is a method that lets each connection/user send information in turns in bits. Instead of everyone talking at the same time, each user has their own set of time to send information before the next user can and so on- going on in turns. From that time, digitalizing networks became standard for the structure of the networks. This was back in 1988 when Integrated Services Digital Network become standard as well as the lines in the networks.
We will later discuss how the different networks consist of different layers on top of each other. But for this introduction, it’s important to understand the telecommunication network’s infrastructure so we further can understand how the networks today are physically set up. Under the telecommunication network, where all the telephone centrals are connected, we have something called a transport network. This consists of physical lines that can be either copper or fiber. These physical lines are multiplexed for more than one connection to get as much capacity in the network as possible.
Traditionally we would multiplex channels on a 64 kb/s to higher capacity lines. These basic channels would be multiplexed together in a group of 32 on a line of 2 Mb/s. These high capacity lines would then be multiplexed 4 and 4 together, which then would be multiplexed to higher capacity lines. This extremely hard way of sending information is called Plesiochronous Digital Hierarchy (PDH), and in the 1990s got replaced by a more flexible technology called Synchronous Digital Hierarchy (PDH) which was made for optical networks.
A visual of how lines were multiplexed together, creating groups on groups, eventually resulting in “supergroups”.
PDH, also known as SONET, is a more modern solution that uses laser or diodes to deliver information on optical lines. The technology operates on a basic capacity of 155 Mb/s. This system offers an automatic redirection of traffic that is sent in the wrong places, as well as its own operation and monitoring system. Over this modern transport network, you could set up different circuit switching communication links for different purposes, such as telephone centrals, data connections between routers on the networks or connections that was leased by an ISP to different customers. However, circuit switching wasn’t the best option for all networks. Read more about this in the next article as we talk about data communication and packets.
picture 1: self-made
picture 2: https://images.slideplayer.com/24/7050643/slides/slide_1.jpg
picture 3: https://media.apnarm.net.au/media/images/2017/01/27/b88550082z1_20170127101055_000gnqfv0e02-0-dlfdxknhhrnc86d2nn2_ct834x465.jpg
picture 4: https://www.rfwireless-world.com/images/PDH-level-hierarchy-in-Europe.jpg
Next in the series: https://x9security.com/networking-3-data-communication/