Page 96 - From GMS to LTE

82 From GSM to LTE-Advanced Pro and 5G

frequency to avoid using a constantly jammed channel. Although the approach is good
for voice services that can hide badly damaged blocks from the user up to a certain
extent, it poses a retransmission risk for packet data if one of the frequencies used in the
hopping sequence performs very badly. Thus, limiting the size of MCS‐7, MCS‐8 and
MCS‐9 blocks to two bursts helps to cope better with such a situation.

2.3.5 Mobile Device Classes
The GPRS standard defines three different classes of mobile devices, namely class A,
class B and class C. Class C mobiles can only be attached to GPRS or GSM at one time.
As this is quite inconvenient for a user, class C mobiles are suited only for embedded
applications that need only either GSM or GPRS to transfer data.
Today, all mobile devices available on the market are class B devices. They can be
attached to both GPRS and GSM at the same time, but early GPRS specifications had
one important limitation: GSM and GPRS could not be used at the same time. In most
networks this meant and still means today that during an ongoing voice call it is not
possible to transfer data via GPRS. Similarly, during data transmission no voice call is
possible. For outgoing calls this is not a problem. If a GPRS data transmission is ongo-
ing, it will be interrupted when the user starts a telephone call and is automatically
resumed once the call is terminated. There is no need to reconnect to GPRS as only the
data transfer is interrupted. The logical GPRS connection remains in place during the
voice call.
During data transmission, the mobile device is unable to listen to the GSM pag-
ing channel. This means that without further mechanisms on the network side, the
mobile device is not able to detect incoming voice calls or short messaging service
(SMS) messages. When applications generate only bursty data traffic, the probabil-
ity of missing a Paging message is reduced. Once the current data transfer is com-
pleted, the PDTCHs are released and the mobile device is again able to listen to the
paging channel (PCH). As the Paging message is usually repeated after a couple of
seconds, the probability of overhearing a Paging message and thus missing a call
depends on the ratio between active data transmission time and idle time. As this
is clearly not ideal, a number of enhancements have been specified to allow the
circuit‐switched and packet‐switched parts of the network to exchange informa-
tion about incoming calls or SMS messages. This is described in more detail in
Section 2.3.6 and ensures that no Paging message is lost during an ongoing data
transfer. In practice, it can be observed today that most GSM/GPRS networks use
one of these mechanisms.
The GPRS standard has also foreseen class A mobile devices that can be active in both
GSM and GPRS at the same time. This means that a GPRS data transfer and a GSM
voice call can be active at the same time. Today, there are no such devices on the market
as the practical implementation would require two sets of independent transceivers in
the mobile device. As this has been deemed impractical, a further enhancement was put
into the GPRS standard that is referred to as ‘dual transfer mode’, or DTM for short.
DTM synchronizes the circuit‐ and packet‐switched parts of the GSM/GPRS network
and thus allows GPRS data transfers during an ongoing GSM voice call with a single
transceiver in the mobile device. Even though many mobile devices support DTM today,
there is no widespread use of it on the network side.