SIGTRAN SS7 Over IP

 

Benefits of SS7 Over IP

1. Decreased cost of infrastructure

2. Improved ease of network management

3. Ability to rapidly deploy new services

SIGTRAN based Network Architecture

The SIGTRAN is transition from the traditional circuit-switched network to packet-switched network,

1. Media Gateway handles the media or bearer. It converts media from one format to other format. For eg. it can terminate TDM trunks from PSTN network and converts the audio to RTP format and deliver to IP network.

2. Media Gateway Control , contains call processing, manages resources of MGs that it controls.

3. Signaling gateway terminates circuit switched network signaling such as SS7 and converts to SS7 over IP. It transports IP based elements signaling.

4. As we see in IP based architecture, the Signaling and Media control plane are separated.

SIGTRAN Protocol Layers

SCTP ( Stream Controlled Transmission Protocol)

Why SCTP was developed ?

During initial phases when SIGTRAN was developed, the existing transport layer protocols TCP and UDP were not fit to support requirements of Telephony signaling transports. So, a new transport layer protocol SCTP was developed,'

1. UDP was not met basic requirement for reliable and in-oder transport.

2. TCP met basic requirements but had below limitations

a. Head Of Line blocking - TCP is strictly sequential means a single packet loss can cause subsequent packets also getting delayed.

b. For , Most of the implementation of TCP, Retransmission timer is often large and not tunable.

c. Build-in support for multihoming was lacking ( to meet reliability requirement and no single point of failure)

d. Lack of built in hearbeat mechanism, so takes long time to detect failure.

 

What is SCTP ?

1. SCTP is connection oriented protocol. 

2. Each end of the connection is known as SCTP Endpoint.

3. A end point defined by  SCTP transport address, which will have one or more IP addresses and port.

4. The SCTP end points exchange the state information during initialization procedure  tocreate SCTP association. What they exchange ?

Association is combination of 2 SCTP endpoints.

5. Once association is established, user data exchange take place.

What are feature of SCTP over TCP/UDP ?

6.  SCTP reduces impact of Head of Line blocking by using STREAMS concept. Stream is logical unidirectional channel within association. STREAMS allow to send separate sequence of ordered messages that are independent of each other. So one packet transmission failure in one stream does not impact the packet exchange in other streams.

An association can have multiple streams.

7. SCTP supports quick detection and recover from transmission failures.

a. Using Path Max Retrans threshold, maximum number of consecutive retransmission that are allowed in path.

b. Use of heartbeat mechanism

The configuration of parameters like maxPathRetrans, minRTO, maxRTO etc are available that can be used while defining SCTP scoekts, this option was not present in TCP.

8. SCTP provides Multihoming. It allows SCTP endpoints to support multiple transport addresses. 

Each transport address is equivalent to a different path for sending and receiving data through network. These paths are known as primary path and secondary path. If primary path goes down then secondary path is used by SCTP end points.

Retransmitting on secondary path decreases failure recovery time.

What is UA layers ?

1. The UA layer encapsulate the SCN (Signalling Communications network) for transport over IP network using SCTP. 

2. M3UA encapsulated all SCCP packets and transport over SCTP. Similarly M2UA, encapsulates MTP layer 3 packets and transport over SCTP.

Means M3UA is user adaptation layer so it is used so that existing SCCP logic can run  without any change as it runs in TDM case over MTP3. M3UA does required handling so that it can run over SCTP over IP.

SUA - SCCP User Adapation Layer

2. What is AS (Application server) ?

 It is a logical entity that serves specific routing key.

An example is virtual switch element that handles processing of unique range of PSTN trunks identified by SS7 SIO/OPC/DPC/CIC-Range.

Another example is virtual Database element which handles all HLR transaction for particular SS7 OPC/DPC/SCCP_SSN combinations.

There is 1:1 relationship between Routing key and a AS.

3. What is ASP (AS Process) ?

A process instance of application server is Application Server  process.

An ASP contains an SCTP endpoint and can be configured to process signaling traffic within more than one Application Server.

4. What is routing key ?

A routing key is uniquely identified by 32 bit value called routing context.

A routing key is used to distribute a message from SS7 network to AS.

A routing key can be combination of following SS7 parameters.

a. Network Indicator

b. Service Indicator

c. OPC

d. DPC

e. SCCP SSN

What is M3UA ? (MTP Level 3 UA)

1. M3UA provides transport of MTP level 3 user part (SCCP and ISUP) signalling over IP using SCTP.

2. M3UA provides seamless operation between the user part peers by fully supporting MTP Level 3 upper layer primitives.

3. The MTP level 3 user part are not aware that they running over M3UA not MTP level 3.

4. MTP service primitives contains

a. MTP Transfer request and indication

b. MTP Pause indication

c. MTP Resume indication

d. MTP Status indication

5. MTP Primitives purpose

a. MTP Transfer -> To transfer user data

b. MTP Pause -> Affected Point code is unavailable

c. MTP Resume -> Affected Point code is availble

d. MTP Status -> Congestion and User Part availability info on an affected Point code

6. M3UA layer on SGP maintains the state of configured ASPs and ASes.

M3UA on ASP maintains state of SGPs and SGs.

7. M3UA layer on SG supports message distribution of incoming messages from SS7 and IP based sources. This distribution is based on matching the incoming message against routing key. When a routing key is selected state of AS is checked. if active then message forwarded to ASPs of AS.

8. M3UA layer at SGP and ASP maintains state of SCTP association. It works on client server model.

During establishment of association , several inbound and outbound streams are negotiated between SCTP peers. M3UA laayer assigns data traffic to individual streams based on some parameter that ensures proper sequencing of msg like SLS.

9. M3UA registered port number is 2905. SCTP payload protocol identifier is 3.

Messages

Transfer Message

1. The user data is payload Data message type. This maps directly to the MTP Transfer primitives'.

2. It contains OPC,DPC. SIO,SLS and ISUP information. It can contain a routing context, Network apperance.

3. Routing context assocates message with preconfigured routing key or AS. Its mandatory if SCTP association supports more than one AS.

4. Network Appearance(NA_ provides SS7 network context for point codes in message, Useful when SG is connected to more than one ss7 network and traffic associated with different networks is sent to ASP over a single SCTP association. Network apperance might be necessary to indicate format of OPC and DPC.

Routing context and NA are optional.

SSNM Messages

1. SSNM messages map to other MTP primitives - MTP Pause, MYP Resume, MTP Status

Also, there is support for ASP to audit state of an SS7 destination.

2. Routing context and NA are optional.

3. Messages

a. DUNA - Destination unavailable - MTP Pause primitive

a1. Can be generated from SS7 network if an ASP send message to unavailable SS7 destination or when ASP audits SS7 destination.

a2. It contains affected point code parm, PC which are affected.

b. DAVA - MTP Resume

b1. one oe more ss7 destinations are reachable

c. DAUD - Destination Audit - No map to any primitive.

Sent by ASP to audits SS7 destination that are of interest

d. SCON - Signalling Congestion - Maps to MTP Status primitive

SGP sends it to all concerned ASPs when SG determines or is notified the congestion state of an SS7 destination is changed

e. DUPU - Destination User Part unavailable - Maps to MTP Status primitive

SGP sends  to concerned ASPs to indicate availability of a user part

f. DRST - Destination restricted

SGP sends this message to concerned ASPs to indicate, one or more SS& destination are restricted. also sent in response to DAUD message.

ASPSM and ASPTM Messages

1. These messages provides means of controlling state of ASP.

2. ASP cam have these states

a. ASP Down

b. ASP Active

c. ASP Inactive

3. Meaning ->

a. ASP Down - ASP is unavailable

b. ASP Active -  ASP available and can send receive traffic

c. ASP Inactive - ASP available but cant handle traffic

4. AS can have

a. AS-DOWN - all ASPs in AS are down

b. AS-Inactive - atleast one ASP in AS is ASP-Inactive state and no ASP in active satte

c. AS-Active - atleast one ASP in Active state

d. AS-Pending - It is entre when last active ASP transition to ASP Inactive or ASP Down It provides means for AS to recover without loosing message if another ASP quickly become active

5. ASPSM Messages

a. ASP UP - Transtion from ASP Down to ASP inactive

b. ASP UP Ack - Ackn in response to ASP UP message

c, ASP DOWN - transtion to ASP Down from any other state

d. ASP Down Ack -SG P can send this message asynchronously for eg. if SGP going out of service

e. Hearbeat Message - to query if peer still available

f. Hearbeat Ack Message - response to hearbeat msg

6. ASPTM Messages

a. ASP Active message - transition from ASP inactive to ASP active

b. ASP Active Ack message - Response to ASP Active msg

c. ASP Inactive message - transtion from ASP active to ASP inactive

e. ASP Inactive Ack - response to ASP Inactive msg

7. Management Message

a. Error- Notifying peer of an error even associated with received msg like invalid routing context, invalid NA, No configured AS for ASP

b. Notify- Notify can indicate ASP fialure, lack of resource for load share

8. Routing Key Management Messages

a. RK can be statically or dynamically provisioned.

b. Static means through CLI or NW mgmt team.

c. Dynamic - There are following msg for this -

c1. Registrtion request and response - Register a Routing key with SGP or peer IPSP. The response will indicate result and Routing context assigned to routing key.

c2. De-reigsteration request and response msg - Deregister a routing key with SGP or IPSP. Must contain the routing context received in registration response message, Response indicate success or failure.

MISC

1. SUA - SCCP User Adapation Layer

2. M2UA - MTP Level 2 User adaption

3. IUA - ISDN User adaption

4. TALI - Transport Adaption Layer Interface (TALI) - One of proprietary soltuion sim ilar like M3UA, only it uses TCP at transport layer