During my 4 years of college, I’ve been fed with the idea that IP based transmission is overall better than TDM based transmission. For the most part, it is known that IP transmission will survive link failures, and more efficient in terms of resource usage. But now that I’ve been to the actual telco field, there’s something else that you should know about TDM and IP based transmission.
The first one is TDM, often referred as E1 (or T1 in several other countries). The transmission media is your usual copper cable. In case of E1 connection, each copper wires may carry data with speed up to 2 MB (Tx/Rx). However, the real problem is when you look at it from the RNC’s perspective..
The picture above is a digital distribution frame (DDF). It is used to connect the copper wires coming from the sites (either NodeBs or BTSes) with copper wires coming from BSC/RNC. If you’re an electrical engineering student, you might be familiar with the breadboard, where you need to plug in copper wires into interconnected holes, and make sure that the wires connect with each other correctly, forming a working electrical circuit. Well, this one is quite similar, except that now there’s more than a hundred pair of copper wires, and you need to ensure that each one of them is connected to the correct sites or you’ll start pulling your hair in frustration.
In case you need to add transmission capacity to the sites using TDM transmission, you’ll need to add another pair of E1 cables. Transmision checking is also difficult, because you need to create a physical loopback using the wires, and coordinate with people on the sites to perform the test. The problem gets even more complicated when somebody made a mistake with the wiring, forcing you to swim through a bunch of colored wires.
IP, on the other hand..
Well, that’s that. A LAN switch. Because the links from the sites are using IP based transmission, we can aggregate the links using twisted pair ethernet cables and plug them into the LAN switch. There isn’t really much confusion regarding the physical connection, because the switching can be done using the LSW’s software. Oh and also, you can check the connection simply by using a ping command. Troubleshooting can be done without much involvement of the physical connection.
There you have it. For those dealing with IP transmission, good for you, huh?