Page 88 - From GMS to LTE

74 From GSM to LTE-Advanced Pro and 5G

Transfer of web pages,
billing based on volume

t

While the user views the web pages
there is no data transfer and thus no
cost is incurred
Figure 2.3 Billing based on volume.

Since the subscriber does not pay for the time when no data is transferred, the call
●
does not have to be disconnected to save costs. This is called ‘always on’ and enables
applications like e‐mail programs to poll for incoming e‐mails in certain intervals or
allows messaging clients to wait for incoming messages.
When the subscriber is moving, by train for example, the network coverage frequently
●
becomes very bad or is even lost completely for some time. When this happens, cir-
cuit‐switched connections are disconnected and have to be reestablished manually
once network coverage is available again. GPRS connections, on the other hand, are
not dropped as the logical GPRS connection is independent of the physical connec-
tion to the network. After coverage is regained the interrupted data transfer simply
resumes.
GPRS was initially designed to support different types of packet‐switching technologies.
The great success of the Internet, which uses the IP exclusively for packet switching, has
led to IP being the only supported protocol today. Therefore, the terms ‘user data trans-
fer’, ‘user data transmission’ or ‘packet switching’ used in this chapter always refer to
‘transferring IP packets’.

2.3 The GPRS Air Interface

2.3.1 GPRS vs. GSM Timeslot Usage on the Air Interface
Circuit‐Switched TCH vs. Packet‐Switched PDTCH
As discussed in Chapter 1, GSM uses timeslots on the air interface to transfer data
between subscribers and the network. During a circuit‐switched call, a subscriber is
assigned exactly one traffic channel (TCH), which is mapped to a single timeslot. This
timeslot remains allocated for the duration of the call and cannot be used for other
subscribers even if there is no data transfer for some time.
In GPRS, the smallest unit that can be assigned is a block that consists of four
bursts of a packet data traffic channel (PDTCH). A PDTCH is similar to a TCH in
that it also uses one physical timeslot. If the subscriber has more data to transfer, the
network can assign more blocks on the same PDTCH right away. The network can