doc: running: Describe how to configure a co-located PCU

The manual does not yet mention the possibility to configure a BSC
co-located PCU. Lets add a short description to the chapter
Running OsmoBSC, Configure primary Links that enables users to get an
idea where exactly the BSC co-located PCU has its place in the RAN
infrastructure and which links it is connected with. Also give a short
example how to setup the unix domain socket path.

A more detailed description, especially about the timeslot configuration
will be added with a follow up patch for bts.adoc

Related: OS#5198
Change-Id: I3af3cd8ef7099bb94f4cb25513e9dfdc5fcc1b5a
---
M doc/manuals/chapters/bts.adoc
M doc/manuals/chapters/running.adoc
2 files changed, 65 insertions(+), 0 deletions(-)

diff --git a/doc/manuals/chapters/bts.adoc b/doc/manuals/chapters/bts.adoc
index e6179aa..9107aa1 100644
--- a/doc/manuals/chapters/bts.adoc
+++ b/doc/manuals/chapters/bts.adoc
@@ -181,6 +181,7 @@
 means of `si5 neighbor-list (add|del) arfcn <0-1023>` for SACCH (SI5).
 
 
+[[config_gprs_pcu_pars]]
 === Configuring GPRS PCU parameters of a BTS
 
 In the case of BTS models using Abis/IP (IPA), the GPRS PCU is located
diff --git a/doc/manuals/chapters/running.adoc b/doc/manuals/chapters/running.adoc
index 70f9d28..6d690ce 100644
--- a/doc/manuals/chapters/running.adoc
+++ b/doc/manuals/chapters/running.adoc
@@ -218,3 +218,47 @@
 ----
 
 More detailed configuration is described in <<smlc-config>>.
+
+
+==== Configure BSC co-located PCU
+
+While small IP based BTSs usually come with a built in PCU (BTS co-located
+PCU), this does not have to be the case with any BTS. Especially larger E1 BTS
+usually make use of a BSC co-located PCU.
+
+In the case of OsmoBSC this means that an instance of OsmoPCU is running next
+to OsmoBSC. Both processes share a unix domain socket to exchange signaling
+traffic and configuration parameters.
+
+.OsmoBSC with co-located OsmoPCU'
+[graphviz]
+----
+digraph G {
+        rankdir=LR;
+        BTS [label="BTS"];
+
+        subgraph cluster_ran {
+                label="RAN";
+                PCU [label="OsmoPCU"];
+                BSC [label="OsmoBSC"];
+                MGW [label="OsmoMGW"];
+	        { rank=same BSC MGW PCU }
+        }
+
+        BTS->PCU [label="GPRS/TRAU", style=dotted];
+        BTS->BSC [label="Abis"];
+        BTS->MGW [label="SPEECH/TRAU", style=dotted];
+        BSC->MGW [label="MGCP"];
+        BSC->PCU [label="PCU_SOCK"];
+}
+----
+
+Apart from the configuration of the PCU socket path the configuration is not
+much different from those where the PCU is integrated inside the BTS. See also
+see also <<config_gprs_pcu_pars>> for a detailed description.
+
+.Configure socket path to co-located PCU
+----
+network
+ pcu-socket /tmp/pcu_bts
+----

