bts: bts: Explain Ericsson's interface switch (IS)

The built in interface switch in ericsson RBS base stations no where
explained. From the example configuration files alone it is not possible
to understand how the IS configuration works. Let's add a chapter that
explains how the IS configuration works.

Related: OS#5198
Change-Id: Ib6ebd7fdfe9063c0d8cacf53ffd27f6099d9038a

2 files changed, 138 insertions, 1 deletions

diff --git a/doc/manuals/chapters/bts-examples.adoc b/doc/manuals/chapters/bts-examples.adoc
index 1f3652bbe..f840b39e9 100644
--- a/doc/manuals/chapters/bts-examples.adoc
+++ b/doc/manuals/chapters/bts-examples.adoc
@@ -263,7 +263,7 @@ network
 
 <4> OML always requires a TEI (Terminal Equipment Identifier) to set up. This number can be found in the manual of the BTS.
 
-<5> This BTS has an built in “Interface Switch” (IS) that offers flexible way to reconfigure the interconnection between the internal components of the BTS and the external E1 line. This depends on the exact BTS type and configuration.
+<5> This BTS has an built in “Interface Switch” (IS) that offers flexible way to reconfigure the interconnection between the internal components of the BTS and the external E1 line. This depends on the exact BTS type and configuration. See also <<cfg_ericsson_rbs_is>>
 
 <6> Similar to OML we assign TS1 to RSL as well.
 
diff --git a/doc/manuals/chapters/bts.adoc b/doc/manuals/chapters/bts.adoc
index d119acca1..e6179aa20 100644
--- a/doc/manuals/chapters/bts.adoc
+++ b/doc/manuals/chapters/bts.adoc
@@ -645,3 +645,140 @@ network
 <2> This number affects how long the MS waits before (re-)transmitting RACH
 requests.
 <3> How often to retransmit the RACH request.
+
+[[cfg_ericsson_rbs_is]]
+=== Configuring Ericsson RBS Interface Switch (IS)
+
+Ericsson RBS2000/RBS6000 base stations feature a so called "Interface Switch" (IS),
+which is a built-in switchboard that interconnects between internal components
+of the BTS. It also connects to the external E1 connections. This allows to
+adapt the BTS to specific E1 networking requirements that may differ from the
+usual timeslot configuration.
+
+The internals of an Ericsson RBS are quite complex. In the following we will
+only explain how to connect transceiver units (TRU) to an E1 interface pointing
+to the outside world.
+
+==== Understanding the is-connection-list VTY option
+
+The IS operates on 16kbps subslots (ICPs), which means that there are no fixed
+borders between E1 timeslots. Any number of consecutive subslots may be
+connected through. However, depending on the components that are connected it
+may still be a requirement to align on E1 timeslot borders.
+
+The configuration of the IS is done using the is-connection-list command. The
+first two numbers are the ICP numbers that specify the first subslot on both
+sides that shall be interconnected. The third number (contiguity index) specifies
+how many of the following subslots shall be connected.
+
+In the following example we connect 4 blocks with 12 subslot each. The numbers
+on the left are the ICP numbers of the E1 connection pointing to the outside.
+The numbers in the middle are the ICP numbers of the subslots occupied by the
+transceivers (one TRX per block). The third number is the contiguity index that
+spans over 12 subslots or 3 E1 timeslots.
+
+.Example: 4 TRX BTS (4 x 12 subslots)
+----
+network
+ bts 0
+  is-connection-list add 4 512 12
+  is-connection-list add 16 524 12
+  is-connection-list add 28 536 12
+  is-connection-list add 40 548 12
+----
+
+==== E1 port and TRU ICP numbers
+
+On the outside connection, the ICP counting begins at E1 timeslot 0 (port A)
+but since E1 TS 0 is reserved for framing and synchronization of the E1 line
+itself the first usable subslot is subslot 4 (beginning of E1 TS 1). Depending
+on the configuration the BTS may have multiple E1 ports. The counting scheme
+will repeat itself. This means the next usable ICP can be found at an offset
+of 128.
+
+.External connections of a BTS with two E1 ports
+[options="header",cols="50%,25%,25%"]
+|===
+|Function		|Subslot offset (ICP)	|ICP count
+|E1 port A		|4	 		|124
+|E1 port B              |132			|124
+|===
+
+Depending on the transceiver configuration, a RBS2000/RBS6000 base station
+usually features two sets of ICPs for each TRX. The reason for this is that with
+the introduction of EGPRS more bandwidth than a single 16kbps subslot could
+deliver was required. The solution to this was to add an entirely new set of IS
+ICPs where full 64kbps E1 timeslots instead of 16kbps subslots could be
+used to serve a single air interface timeslot. The two sets of ICPs must not be
+mixed. Only one set may be used at a time.
+
+.ICPs to use TRU with 16kbps subslots per TRAU
+[options="header",cols="50%,25%,25%"]
+|===
+|Function		|Subslot offset (ICP)	|ICP count
+|TRU-0, RSL/OML		|512	 		|4
+|TRU-0, TRAU TS0..TS7	|516			|8
+|TRU-1, RSL/OML		|524			|4
+|TRU-1, TRAU TS0..TS7	|528			|8
+|TRU-2, RSL/OML		|536			|4
+|TRU-2, TRAU TS0..TS7	|540			|8
+|TRU-3, RSL/OML		|548			|4
+|TRU-3, TRAU TS0..TS7	|552			|8
+|TRU-4, RSL/OML		|560			|4
+|TRU-4, TRAU TS0..TS7	|564			|8
+|TRU-5, RSL/OML		|572			|4
+|TRU-5, TRAU TS0..TS7	|576			|8
+|TRU-6, RSL/OML		|640			|4
+|TRU-6, TRAU TS0..TS7	|644			|8
+|TRU-7, RSL/OML		|652			|4
+|TRU-7, TRAU TS0..TS7	|656			|8
+|TRU-8, RSL/OML		|664			|4
+|TRU-8, TRAU TS0..TS7	|668			|8
+|TRU-9, RSL/OML		|676			|4
+|TRU-9, TRAU TS0..TS7	|680			|8
+|TRU-10, RSL/OML	|688			|4
+|TRU-10, TRAU TS0..TS7	|692			|8
+|TRU-11, RSL/OML	|700			|4
+|TRU-11, TRAU TS0..TS7	|704			|8
+|===
+
+NOTE: Each air interface timeslot is served by its individual TRAU, so it is
+possible to route each subslot (ICP) dedicated to TRAU individually. The
+connections on the other end may contain gaps and do not have to be
+consecutive.
+
+.ICPs to use TRU with 64kbps subslots per TRAU
+[options="header",cols="50%,25%,25%"]
+|===
+|Function		|Subslot offset	(ICP)	|ICP count
+|TRU-0, RSL/OML		|712	 		|4
+|TRU-0, TRAU TS0..TS7	|716			|32
+|TRU-1, RSL/OML		|748			|4
+|TRU-1, TRAU TS0..TS7	|752			|32
+|TRU-2, RSL/OML		|784			|4
+|TRU-2, TRAU TS0..TS7	|788			|32
+|TRU-3, RSL/OML		|820			|4
+|TRU-3, TRAU TS0..TS7	|824			|32
+|TRU-4, RSL/OML		|856			|4
+|TRU-4, TRAU TS0..TS7	|860			|32
+|TRU-5, RSL/OML		|928			|4
+|TRU-5, TRAU TS0..TS7	|932			|32
+|TRU-6, RSL/OML		|964			|4
+|TRU-6, TRAU TS0..TS7	|968			|32
+|TRU-7, RSL/OML		|1000			|4
+|TRU-7, TRAU TS0..TS7	|1004			|32
+|TRU-8, RSL/OML		|1036			|4
+|TRU-8, TRAU TS0..TS7	|1040			|32
+|TRU-9, RSL/OML		|1072			|4
+|TRU-9, TRAU TS0..TS7	|1076			|32
+|TRU-10, RSL/OML	|1108			|4
+|TRU-10, TRAU TS0..TS7	|1112			|32
+|TRU-11, RSL/OML	|1144			|4
+|TRU-11, TRAU TS0..TS7	|1148			|32
+|===
+
+NOTE: In case voice TRAU frames are transferred, only the first of the four
+16kbps subslots is used. When the timeslot is switched to GPRS/EGPRS, the
+full 64kbps bandwidth will be used. This also means that the set of four
+ICPs per TRAU must be connected consecutively. Also the connection
+to the outside must be aligned to E1 timeslot borders.
\ No newline at end of file