The line resources on the Abis-interface usually are not used efficiently. The reason is that a BTS, typically, has only a few TRXs, which implies small traffic volume capability. Consequently, the line between the BTS and the BSC is used only to a fraction of its capacity. Figure 6.1(a), the star configuration, shows the case of a BTS with four TRXs, in which only 47% of the 2 Mbps
bit
TS 7 6 5 4 3 2 1 0
0 FAS / NFAS
1 Air 0 Air 1 Air 2 Air 3 BTS 1 / TRX 1 2 Air 4 Air 5 Air 6 Air 7
3 Air 0 Air 1 Air 2 Air 3 BTS 3 / TRX 1 4 Air 4 Air 5 Air 6 Air 7
5 Air 0 Air 1 Air 2 Air 3 BTS 1 / TRX 2 6 Air 4 Air 5 Air 6 Air 7
7 Air 0 Air 1 Air 2 Air 3 BTS 3 / TRX 2 8 Air 4 Air 5 Air 6 Air 7
9 Air 0 Air 1 Air 2 Air 3 BTS 2 / TRX 1 10 Air 4 Air 5 Air 6 Air 7
11 Air 0 Air 1 Air 2 Air 3 BTS 4 / TRX 1 12 Air 4 Air 5 Air 6 Air 7
13 Air 0 Air 1 Air 2 Air 3 BTS 2 / TRX 2 14 Air 4 Air 5 Air 6 Air 7
15 Air 0 Air 1 Air 2 Air 3 BTS 4 / TRX 2 16 Air 4 Air 5 Air 6 Air 7
17 not used
18 partial O&M data 19 BTS 4 / TRX 2 signaling 20 BTS 4 / TRX 1 signaling 21 BTS 3/ TRX 2 signaling 22 BTS 3/ TRX 1 signaling 23 BTS 2/ TRX 2 signaling 24 BTS 2/ TRX 1 signaling 25 BTS 1/ TRX 2 signaling 26 BTS 1/ TRX 1 signaling 27 BTS 4 / O&M signaling 28 BTS 3 / O&M signaling 29 BTS 2 / O&M signaling 30 BTS 1 / O&M signaling 31 Transmission control
information bit
TS 7 6 5 4 3 2 1 0
0 FAS / NFAS
1 Air 0 Air 1 Air 2 Air 3 TRX 1 2 Air 4 Air 5 Air 6 Air 7 3 Air 0 Air 1 Air 2 Air 3 TRX 5 4 Air 4 Air 5 Air 6 Air 7 5 Air 0 Air 1 Air 2 Air 3 TRX 2 6 Air 4 Air 5 Air 6 Air 7 7 Air 0 Air 1 Air 2 Air 3 TRX 6 8 Air 4 Air 5 Air 6 Air 7 9 Air 0 Air 1 Air 2 Air 3 TRX 3 10 Air 4 Air 5 Air 6 Air 7 11 Air 0 Air 1 Air 2 Air 3 TRX 7 12 Air 4 Air 5 Air 6 Air 7 13 Air 0 Air 1 Air 2 Air 3 TRX 4 14 Air 4 Air 5 Air 6 Air 7 15 Air 0 Air 1 Air 2 Air 3 TRX 8 16 Air 4 Air 5 Air 6 Air 7
17 not used
18 not used
19 partial O&M data
20 not used
21 O&M signaling 22 TRX 8 signaling 23 TRX 7 signaling 24 TRX 6 signaling 25 TRX 5 signaling
26 not used
27 TRX 4 signaling 28 TRX 3 signaling 29 TRX 2 signaling 30 TRX 1 signaling
31 not used
(a) (b)
Air 4
Figure 6.1 (a) Star configuration (fullrate) and (b) serial connection (four BTSs with two TRX each).
actually is needed. The shaded areas mark the unused channels. When the BTS has only one TRX, that value goes down to 16%. Such waste of resources has a historical background, and it would not change much if halfrate channels were used.
When GSM specified the BTS, it defined that a BTS may have up to 16 TRXs. Two 2-Mbps interfaces are required to connect such a BTS to the BSC, because a single 2-Mbps interface is able to support only up to 10 TRXs, including O&M signaling.
Proportionally fewer resources are required on the Abis-interface when a BTS with a smaller number of TRXs is installed. The remainder cannot easily be used.
Experience has shown that the optimum for a BTS is in the range of one to four TRXs. This compromise reflects several parameters:
• Capacity. How many traffic and signaling channels does a BTS need to provide, on average and during busy hours, to avoid an overload condition?
• Available frequency range. What is the minimum distance between BTSs beyond which a given TRX frequency may be reused?
Network operators worldwide have had bad experiences, particularly with the latter point.
When digital radio was introduced, the assumption was that the impact of the disturbances, same-channel interference or neighbor channel interfer- ence, would be relatively minor. Soon after the introduction of commercial service, that assumption was found to be wrong, when more and more interfer- ence problems between BTSs appeared and degraded the quality of service.
Problems with large, powerful cells were experienced, particularly in urban areas and city centers, where more and more minicells and microcells are being used.
The conclusion was to move in the direction of using more cells with fewer TRXs and smaller output power (<1W) rather than in the direction of fewer cells with more TRXs and high output power. That configuration requires a larger number of BTSs than the alternative to cover any given area.
Connecting the larger number of BTSs to the BSCs, in turn, requires a larger number of links (Abis-interfaces).
Because of that trend, together with the high costs for links between the BTS and the BSC and the low efficiency when using such links, another con- figuration was introduced, the serial connection of BTSs.
6.2.1 BTS Connection in a Serial Configuration
In a serial configuration, the BTSs are connected in a line or a ring topology.
Only one BTS, for the line topology, or two BTSs, for the ring topology, are physically connected to the BSC. Figures 6.2 and 6.3 illustrate those topolo- gies. For the network operator, the advantage of the serial approach over the star configuration is that it saves line costs. Furthermore, the serial connection allows for more efficient use of resources, as illustrated in Figure 6.1(b). This advantage becomes particularly obvious, when colocated or sectored BTSs are used (see Section 3.1.2.3). The disadvantage, however, is that a single link failure causes the loss of the connection to a large number of BTSs
BSC BTS
TRX
BTS
TRX
BTS
TRX
Figure 6.2 Serial connection of BTSs in a line topology. The disadvantage is that a single link failure results in total loss of connection to a number of BTSs.
BTS
TRX
BTS
TRX
BTS
TRX
BSC
Figure 6.3 Serial connection of BTSs in a ring topology. The advantage is that a single link failure never results in total loss of connection to any BTS.
(for serial configuration). For that reason, the use of a ring configuration provides some redundancy in which the signal can always go in one of two directions, so that in the event of a link failure, it is still possible to provide an alternative connection.
F6.2.2 Connection of BTSs in Star Configuration
The star configuration was the most popular when the first systems were deployed in 1991–1992. In a star configuration, every BTS has it own connec- tion, an Abis-interface to the BSC. Figure 6.4 illustrates a star configuration with three BTSs.