Page 86 - From GMS to LTE

72 From GSM to LTE-Advanced Pro and 5G

Exclusive channel for a connection
MSC
BSC BSC
MSC

Figure 2.1 Exclusive connections of a circuit‐switched system.

As the circuit‐switched channel has a constant bandwidth, the sender does not have
●
to worry about a permanent or temporary bottleneck in the communication path.
This is especially important for a voice call. As the datarate is constant, any bottleneck
in the communication path would lead to a disruption of the voice call.
Furthermore, circuit‐switched connections have a constant delay time. This is the
●
time between sending a bit and receiving it at the other end. The greater the distance
between the sender and receiver, the longer the delay time. This makes a circuit‐
switched connection ideal for voice applications as they are extremely sensitive to a
variable delay time. If a constant delay time cannot be guaranteed, a buffer at the
receiving end is necessary. This adds additional unwanted delay, especially for applications
like voice calls.

While circuit‐switched data transmission is ideally suited to voice transmissions,
there are a number of significant disadvantages for data transmission with variable
bandwidth usage. Web browsing is a typical application with variable or ‘bursty’ band-
width usage. For sending a request to a web server and receiving the web page, as much
bandwidth as possible is desired to allow the web page to be received as quickly as
possible. As the bandwidth of a circuit‐switched channel is constant, there is no possi-
bility of increasing the data transmission speed while the page is being downloaded.
After the page has been received, no data is exchanged while the subscriber reads the
page. The bandwidth requirement during this time is zero and the resources are simply
unused and are thus wasted.

2.2 Packet‐Switched Data Transmission over GPRS

For bursty data applications, it would be far better to request for resources to send and
receive data and for the resources to be released again after the transmission, as shown
in Figure 2.2. This can be done by collecting the data in packets before it is sent over
the network. This method of sending data is called ‘packet switching’. As there is no
longer a logical end‐to‐end connection, every packet has to contain a header. The
header, for example, contains information about the sender (source address) and the
receiver (destination address) of the packet. This information is used in the network to
route the packets through the different network elements. In the Internet, for example,
the source and destination addresses are the Internet Protocol (IP) addresses of the
sender and receiver.