Page 48 - From GMS to LTE

34 From GSM to LTE-Advanced Pro and 5G

frequency. Up to 64 different frequencies can be used per base station for frequency
hopping. To inform the mobile of the use of frequency hopping, the Immediate
Assignment message used during the establishment of a signaling link contains all the
information about the frequencies that are used and the hopping pattern that is applied
to the connection.
For carriers that transport the SCH, FCCH and BCCH channels, frequency hopping
must not be used. This restriction is necessary because it would be very difficult for
mobile devices to find neighboring cells.
In practice, network operators use static frequencies as well as frequency hopping in
their networks.
The interface which connects the base station to the network and which is used to
carry the information for all logical channels is called the Abis interface. An E‐1 con-
nection is usually used for the Abis interface, and owing to its 64 kbit/s timeslot
architecture the logical channels are transmitted in a way that differs from their trans-
mission on the air interface. All common channels as well as the information sent and
received on the SDCCH and SACCH channels are sent over one or more common 64
kbit/s E‐1 timeslots. This is possible because these channels are only used for signaling
data that are not time critical. On the Abis interface, these signaling messages are sent
by using the Link Access Protocol (LAPD). This protocol was initially designed for the
ISDN D‐channel of fixed‐line networks and has been reused for GSM with only minor
modifications.
For TCHs that use a bandwidth of 13 kbit/s on the Abis interface, only one‐quarter of
an E‐1 timeslot is used. This means that all eight timeslots of an air interface frame can
be carried on only two timeslots of the E‐1 interface. A base station composed of three
sectors, which use two carriers each, thus requires 12 timeslots on the Abis interface
plus an additional timeslot for the LAPD signaling. The remaining timeslots of the E‐1
connection can be used for the communication between the network and other base
stations as shown in Figure 1.28. For this purpose, several cells are usually daisy chained
via a single E‐1 connection (see Figure 1.28).

Abis E-1 frame with 32 timeslots (64 kbit/s)
…

LAPD
BCCH, timeslot ¼ E-1 timeslot per traffic channel (TCH)
PCH, on the air interface
AGCH,
SDCCH,
etc.

0 1 2 3 4 5 6 7

One carrier with eight timeslots on the air interface (Um)
Figure 1.28 Mapping of E‐1 timeslots to air interface timeslots.