Analog Services
1- Analog Services uses FDM (Frequency Division Multiplexing) which is an analog technique. We use it because every modulated analog signal having different frequency because they are composite signals. So it is not possible that every time bandwidth can be used completely by the signals. By this the signals can overlap or a lot of capacity on the specific link can be wasted. FDM helps to multiplex the different channels which have the signals lesser than the bandwidth of channel.
2- Analog services uses telephone services in form of analog switched services. In this type of analog services a subscriber connect to the central office through telephone using a dial up modem. The twisted pair cable plays role as transmission medium and the connection is called local loop. It joins us to PSTN (Public switched telephone network). Signals are in analog form and bandwidth range is usually from 0HZ to 4000HZ. Voice passes through series of switched and it remains connected while the communication between two subscribers is in process. When communication ends, the switches then serve the other subscribers to make a connection.
3- Analog services uses telephone services in form of leased services as well. In this type a dedicated connection/line is available between two parties. There is no need to dial to start a communication. Just pick up the phone and communicate.
4- Analog hierarchy is used to improve the performance of analog services. In which different small Bandwidth signals multiplexed in a specific line and create more Bandwidth carriage. One bigger line carries much more bandwidth.
5- In analog services, modems are used to modulate the analog signals. Receiver will receive the signals in analog form too. Noise is also in analog form, antenna receive analog. But analog get voice and noise in same format. Analog services are not good in this condition because noise free communication is somehow impossible here.
Digital Services
1- 1- Digital Services uses TDM which is a digital technique. In this case we deal with time instead of frequency. Our link is shared sequentially. There are synchronous and asynchronous implementations of TDM.
In synchronous every transmitting device is allocated with exactly the same time slot as allocated to other devices. If any device has to send something so that time slot is dedicated for that then the next device’s turn. Inputs from each device grouped in form of frames and ready to send.
And in asynchronous there is available support for more devices than synchronous. Multiplexer scans and looks for the slot which need to be filled yet. It can send the frames partially filled as well instead of empty areas in frames like synchronous. But here we are not comparing synchronous and asynchronous because both are strengths of digital services.
So Digital services are better in all the aspects than analog services.
2- Digital version of Analog Switched Services
Digital Services uses digital version of analog switched services in which subscribers don’t need modems to modulate the signals. Its already digital, it needs DSU (Digital Service Unit) which changes rate of the data to 56kbps then encodes it and send it. Local loop carries digital form of signals. DSU dials by itself automatically. Digital line is good because of speed and noise free characteristics. DSU is much expensive but with better speed and quality. It is the example of bandwidth on demand, which you have to pay for that time when you are connected, when you are using bandwidth only.
3- Digital Data Services
Digital Services uses DSS which is digital version of analog leased line. In this we don’t need to dial obviously because of dedicated line. By this we also reduced the cost of DSU. So its cheaper than DSU used in digital version of analog switched services. It provides maximum rate of 64kbps through dedicated connection between subscribers.
4- Digital Signal Services
Digital Hierarchy using TDM gives us voice channels ranges 24, 96, 672, 4032 and data rate from 1.5mbps up to 274.176mbps. This is much better and from analog hierarchy.
5- In digital services, signals are in digital format. So when receiver will receive the signals so the noise can be separate easily from the original data. It helps to avoid the noise. It is better from analog services.
Last updated: March 19, 2014
