This document discusses selecting the appropriate capacity for a Base Station Controller (BSC) in a mobile telecommunications network. It provides the following guidelines: 1. Allow a 20% margin for additional TRXs and space for future upgrading. Minimize handovers between BSCs. 2. Calculate required capacity based on offered traffic plus a 10% margin, not installed capacity. 3. Use Erlang B calculations to determine the number of channels needed to support the traffic load at a 0.1% blocking rate. 4. Divide the number of required channels by the number supported per Ater link or interface to determine the number of links needed between the BSC and core network.

1. 1www.TempusTelcosys.com
Telecom Tutorials

2.  BSC
 Ater , A-if and TCSM
 SDCCH

3. BTS
BTS
BSC TCSM MS
C
Ate
r
A-if
BSC Model
BSC2i
248BCF/248BTS/512TRX
BSC3i
248BCF/248BTS/660TRX
How to select BSC capacity
- 20% TRXs margin
- space and future
upgrading
- minimize inter BSC HO

4. BTS
BTS
BSC TCSM MS
C
Ate
r
A-if
Use offer traffic to calculate. Not install capacity
Add 10% traffic margin
Offer 823Erl
(sum from
each BTS
under this
BSC)
823+10%
= 915Erl We calculate at
915 Erl

5. BTS
BTS
BSC TCSM MS
C
Ate
r
A-if
From Erl-B we calculate number
of channel need at 915Erl and
0.1% blocking
= 985 channel (64kbps circuits)
TCSM 1->4 : 1 Ater = 116 channels
TCSM 1->3 : 1 Ater = 90
1 A-if handle 30 channels
with TCSM can multiplex 4
A-if to 1 Ater
But if use TCSM 4->1 will
lost 1 TSL(4channels) to
signaling
Actually we must have O&M link so 1 Ater = 114 channels but only 1 or 2 link/BSC so we
can ignore it. Except very small BSC have only 1 or 2 Ater.

6. BTS
BTS
BSC TCSM MS
C
Ate
r
A-if
Ater = 985/116
= 8.49
= 9 Ater links
A-if =
985/29
= 33.9
= 34 A-if
links
This case use TCSM 1->4
Ater 116 = 4 A-if with 29
channels

7. BSC BSC BSC
TCSM TCSM
MSC
Ate
r
A-
if
TCSM racks need =
Ater within same MSC / 8

8. When SDCCH get high
congest TCH utility become
degrade
MS terminated but no
SDCCH for new call
setup
To solve congestion
and utility don’t
forget SDCCH!