== IPA Multiplex

The ETSI/3GPP specifications for A-bis transport (ETSI/3GPP TS 08.56)
specify the transmission of RSL and OML messages over a LAPD based
framing on top of 64kBit/s signalling times slots (B-channels) on E1
lines.

OsmoBTS does not implement this LAPD based transport, but instead
implements A-bis over IP in a flavor first observed by ip.access nanoBTS
products.  The OsmoBTS implementation is a clean-room re-implementation
based on the observation and dissection of protocol traces.

LAPD as used in E1 signalling channels provides in-order transmission
and reliable delivery.  This is why TCP was chosen as Layer 4 transport
protocol on top of IP.  TCP however, is a stream based transport
protocol, which doesn't preserve the boundaries of messages.

To work around this shortcoming, an additional framing layer called the
IPA multiplex was introduced between TCP and the RSL and OML messages.

.Protocol Stacking
[width="30%"]
|===
|RSL + OML (this document)
|IPA (this document)
|TCP (IETF RFC 793)
|IP (IETF RFC 791)
|Ethernet (IEEE 802.3)
|===

=== IPA Header

Each higher-layer PDU is encapsulated by a three-byte IPA header with
the following structure:

.IPA Header Structure
[options="header",cols="10%,15%,15%,60%"]
|===
|Offset (Octets)|Length|Name|Description
|0|2|Length|Length of the variable-length payload section in network
byte order (excluding the length of the IPA Header)
|2|1|Stream Identifier|Identifies the stream of the payload
|3|Variable|Payload|higher-layer PDU (i.e. RSL or OML message)
|===

=== IPA Stream Identifiers

The IPA Stream Identifier serves to differentiate different streams
within the multiplex.  In the context of A-bis, it can be seen as
analogous to the LAPD TEI on classic A-bis over E1.

The following IPA stream identifiers are being used in A-bis/IP:

.IPA Stream Identifiers
[options="header",width="70%",cols="20%,20%,60%"]
|===
|Value (Hex)|Name|Description
|0x00|RSL|A-bis RSL according to this document, TRX 0
|0x01|RSL|A-bis RSL according to this document, TRX 1
|0x0n|RSL|A-bis RSL according to this document, TRX n
|0xfe|CCM|IPA Connection Management
|0xff|OML|A-bis OML according to this document
|===


=== IPA Connection Management (CCM)

The IPA Connection Management is a sub-layer underneath the IPA
multiplex which is used to manage the connection itself.  It supports
functions like Identity Management and Keep-Alive.

==== Identity Management

When a BTS connects to the BSC, the BSC must identify the connected BTS
somehow.  In ETSI/3GPP A-bis, the E1 multiplex + signalling timeslot
number is used for this.  In IP, there is no similar usable identity.
Hence, the Unit ID is used for this purpose.

.Procedure for IPA peer identification is as follows
[options="header",cols="20%,80%"]
|===
|Direction|Operation
|BTS -> BSC|BTS connects the TCP connection to be used with IPA
|BTS <- BSC|BSC requests BTS identity with ID_GET
|BTS -> BSC|BTS responds BTS Unit ID with ID_RESP
|BTS <- BSC|BSC responds with ID_ACK, if the Unit ID is known to the BSC
|===

Following the above peer identification procedure, transfer of
higher-level messages such as OML or RSL commences.

==== IPA CCM Messages

IPA CCM supports the following messages

.IPA CCM Messages
[options="header"]
[cols="10%,15%,75%"]
|===
|Value|Name|Purpose
|0x00|PING|Request a PONG from the peer
|0x01|PONG|Response to a PING
|0x04|ID_GET|Request Identity from peer
|0x05|ID_RESP|Response to ID_GET
|0x06|ID_ACK|Identity Acknowledged
|===