Page 89 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 75

Timeslot X TS 5 TS 6 TS 7

PDTCH bursts
for user 1

PDTCH bursts
for user 2

PDTCH bursts
for user 1

PDTCH bursts
for user 3

Figure 2.4 Simplified visualization of PDTCH assignment and timeslot aggregation.

also assign the following block(s) to other subscribers or for logical GPRS signaling
channels. Figure 2.4 shows how the blocks of a PDTCH are assigned to different
subscribers.
Instead of using a 26‐ or 51‐multiframe structure as in GSM (see Section 1.7.3), GPRS
uses a 52‐multiframe structure for its timeslots. Frames 24 and 51 are not used for
transferring data. Instead, they are used to allow the mobile device to perform signal
strength measurements on neighboring cells. Frames 12 and 38 are used for timing
advance calculations as described in more detail later on. All other frames in the 52‐
multiframe are collected into blocks of four frames (one burst per frame), which is the
smallest unit for sending or receiving data.

Timeslot Aggregation
To increase the transmission speed, a subscriber is no longer bound to a single TCH as
in circuit‐switched GSM. If more than one timeslot is available when a subscriber wants
to transmit or receive data, the network can allocate several timeslots (multislot) to a
single subscriber.

Multislot Classes
Depending on the multislot class of the mobile device, three, four or even five
timeslots can be aggregated for a subscriber at the same time. Thus, the transmis-
sion speed for every subscriber is increased, provided that not all of them want to
transmit data at the same time. Table 2.1 shows typical multislot classes. Today,
most mobile devices on the market support multislot class 10, 12 or 32. As can be