STP SCCP configuration

Revision as of 16:00, 26 October 2017

A STP represents a Service Transfer Point in SS7 network. This means that it does not process messages, instead it transfers the messages to other SS7 nodes that need to do the processing.
Given this fact, a STP needs to be connected to multiple SS7 nodes, usually other STPs and possible one or more SEPs.

To configure a SCCP layer on a STP you need the following:

• MTP interconnections
• SCCP layer
• GTT
• Optional SCCP Management.

Example network overview

Below is the schema of the network we are trying to configure. We'll be configuring STP2.

In the figure we have:

• two STPs,
  • STP1 is a interconnect STP
  • STP2 represents our interconnect STP.
• three SEPs, SEP represents an SCCP endpoint in SS7 network,
  • SEP1 is our master SCCP message processor
  • SEP2 is the backup for SEP1.

The message flow scenario:

• STP1 sends all SCCP messages to STP2.
  
• STP2 will send the SCCP messages, for HLR and VLR, to SEP1. If communication with SEP1 is lost then the messages will be sent to SEP2.
  

Each SS7 node is connected with a M2PA link.

The following configuration is for STP2.

MTP interconnection

The MTP stack is composed from three layers:

• Layer 3: Routing layer

• Layer 2: Transport layer

• Layer 1: Signalling layer

For each SS7 interconnection we need to specify the three MTP layers.

Layer 1

For M2PA this is specified in sigtransport.conf

[sctp:STP2-STP1]
; Transport type. We will use SCTP
type=sctp

; Use the SCTP in message mode
stream=false

; Local address of M2PA link. IP 100.1.1.1 Port:2080
local=100.1.1.1:2080

; Remote address of M2PA link. IP 80.1.1.1 Port:2080
remote=80.1.1.1:2080

[sctp:STP2-SEP1]
type=sctp
local=100.1.1.1:2092
remote=100.1.1.5:2092
stream=false

[sctp:STP2-SEP2]
type=sctp
local=100.1.1.1:2093
remote=100.1.1.6:2093
stream=false

Layer 2

Layers 2 and 3 are specified in ysigchan.conf

[m2pa:STP2-STP1]
; Component type. In our case M2PA
type=ss7-m2pa

; Signalling layer reference in our case sctp:eSTP-lSTP1 which represents the section name defined in sigtransport.conf.
sig=sctp:STP2-STP1

[m2pa:STP2-SEP1]
type=ss7-m2pa
sig=sctp:STP2-SEP1

[m2pa:STP2-SEP2]
type=ss7-m2pa
sig=sctp:STP2-SEP2

Layer 3

Layers 2 and 3 are specified in ysigchan.conf

[ls:STP2-STP1]
; Component type. In our case MTP3
type=ss7-mtp3

; Point code type: ITU standard.
netind2pctype=ITU

; Network indicator: national
netindicator=national

; Local Point Code: 10
local=ITU:10

; Adjacent Point Code: 1
adjacent=ITU:1

; Layer 2 reference:  m2pa:STP2-STP1
link=m2pa:STP2-STP1

[ls:STP2-SEP1]
type=ss7-mtp3
netind2pctype=ITU
netindicator=national
local=ITU:10
adjacent=ITU:20
link=m2pa:STP2-SEP1

[ls:STP2-SEP1]
type=ss7-mtp3
netind2pctype=ITU
netindicator=national
local=ITU:10
adjacent=ITU:21
link=m2pa:STP2-SEP1

All Layer 3's are hold together by SS7Router. This class actually makes the SS7 MTP routing in YATE. It is created by default but in STP case it needs additional parameters to be configured, so will will add it in ysigchan.conf

[ss7router]
; Component type
type=ss7-router

; Act as a STP: yes
transfer=yes

SCCP layer

SCCP

SCCP is configured in ysigchan.conf file.

[sccp]
; Component type: SCCP
type=ss7-sccp

; Pointcode type: ITU
pointcodetype=ITU

; Network indicator
netindicator=national

; Local pointcode: 10
localpointcode=10

; Reference to SCCP management
management=sccp-mgm

GTT

[gtt]
; Component type: GTT
type=ss7-gtt

; SCCP component for which this GTT should rote messages: sccp; SCCP component section name
sccp=sccp

The GTT component does not make any heavy lifting, it just dispatches a sccp.route message. Anyone who knows how to route a SCCP message should process the sccp.route message and return a valid response. In this example we'll use a Javascript routing script that will be presented below.
The presence of the GTT component also indicates that the SCCP component created in this node is capable of SCCP routing.

SCCP Management

SCCP Management is used to avoid SCCP message routing to unavailable targets.
SCCP management can be made aware of the remote SCCPs nodes in SS7 network and monitor their states.
Each time it detects that the state of a remote SCCP has changed, it emits a sccp.update message to inform all concerned parties about the state change.
When a monitored remote target is unavailable SCCP management starts sending SST messages for that target to detect if the state has changed.

The SCCP management can also keep a list of remote SCCPs who are concerned about the state of local SCCP subsystems. In this way the SCCP management can inform the remote SCCPs when status of a local subsystem has changed.

[sccp-mgm]
; Type of component: SCCP ITU Management
; This section is referenced by SCCP component
type=ss7-sccp-itu-mgm

; State monitoring:

; Remote point code 20
remote=20
 
; Remote point code 21
remote=21
 
; Remote Subsystems 6 and 7 at point code 20
remote=20:6,7

; Remote Subsystems 6 and 7 at point code 21
remote=21:6,7

SCCP routing

This is a sample SCCP Javascript routing script.

// A SEP is defined by it's pc and the SSNs for the HLR and the VLR
// At the start we'll consider that both the mainSep and the backupSep are active (1) and both their ssn(vlr and hlr) are active
var mainSep;
mainSep["pc"] = 1;
mainSep["hlr"] = 1;
mainSep["vlr"] = 1;

var backupSep;
backupSep["pc"] = 1;
backupSep["hlr"] = 1;
backupSep["vlr"] = 1;

function onRoute(msg)
{
   // The routing is done based on the gt parameter from the sccp.route message

   var gt = msg["gt"];
   if (gt == "407520801") { // HLR
	if (mainSep["pc"] == 1 && mainSep["hlr"] == 1) {
           // if mainSep pc and hlr ssn are active then send to mainSep
	    msg["RemotePC"] = 20;
	    msg["gt.ssn"] = 6;
	    msg["route"] = "ssn";
	    return true;
	} else if (backupSep["pc"] == 1 && backupSep["hlr"] == 1) {
           // if backupSep pc and hlr ssn are active then send to backupSep
	    msg["RemotePC"] = 21;
	    msg["gt.ssn"] = 6;
	    msg["route"] = "ssn";
	    return true;
	}
	return false;
   } else if (gt == "407520802") { // VLR
	if (mainSep["pc"] == 1 && mainSep["vlr"] == 1) {
           // if mainSep pc and vlr ssn are active then send to mainSep
	    msg["RemotePC"] = 20;
	    msg["gt.ssn"] = 7;
	    msg["route"] = "ssn";
	    return true;
	} else if (backupSep["pc"] == 1 && backupSep["vlr"] == 1) {
           // if backupSep pc and vlr ssn are active then send to backupSep
	    msg["RemotePC"] = 21;
	    msg["gt.ssn"] = 7;
	    msg["route"] = "ssn";
	    return true;
	}
   } else {

       // If we were to have an interconnect STP for some GT then we would add it here
       // if (gt=="gt_of_external_system") {
       //      msg["RemotePC"] = 1;
       //      return true;
       // }

       // We could also consider that all other messages go to the interconnect STP
       // msg["RemotePC"] = 1;
       // return true;
  
       // Unknown gt
	return false;
   }
}

function onUpdate(msg)
{
   // when cheking for update information we follow the PC state and the SSN states for each PC 
   var pc = msg["pointcode"];
   if (pc != 20 || pc != 21) // Received sccp.update for unknown pointcode
	return false;
   var pcstate = msg["pc-state"];
   var ssn = msg["subsystem"];
   var ssnState = msg["subsystem-state"];

   // First check the PC state. If the PC in down then all SSNs associated to that PC are down 
   if (pcstate != "") {
	if (pcstate == "allowed") {
	    if (pc == 20)
		mainSep["pc"] = 1;
	    else
		backupSep["pc"] = 1;
	} else {
	    if (pc == 20)
		mainSep["pc"] = 0;
	    else
		backupSep["pc"] = 0;
	}
	
   }
   if (ssn == "" || ssnState == "")
	return true;

   // Check the SSN state and set the SSN state for the appropriate SEP
   if (ssn == 6) { // hlr
	if (ssnState == "allowed") {
	    if (pc == 20)
		mainSep["hlr"] = 1;
	    else
		backupSep["hlr"] = 1;
	} else {
	    if (pc == 20)
		mainSep["hlr"] = 0;
	    else
		backupSep["hlr"] = 0;
	}
	return true;
   }
   if (ssn != 7) // Not VLR return;
	return false;
   if (ssnState == "allowed") {
	if (pc == 20)
	    mainSep["vlr"] = 1;
	else
	    backupSep["vlr"] = 1;
   } else {
	if (pc == 20)
	    mainSep["vlr"] = 0;
	else
	    backupSep["vlr"] = 0;
   }
   return true;
}

// install message handlers for sccp.route and sccp.update
Message.install(onRoute,"sccp.route",120);
Message.install(onUpdate,"sccp.update",120);

See also:

• SCCP decoder
• SEP SCCP configuration
• SCCP address translatation
• Configure SCCP and GTT
• SCCP Introduction