What is SS7 protocol?

SS7 protocol or signaling system 7,  is the legacy protocol used by the Traditional or legacy telephony system. SS7 standards are defined by the standard body ITU-T(International Telecommunication Union Telecommunication Standardization Sector ).  E1 or T1 cables are used in ss7 for physical connections. An E1 having 32-time slots and T1 have 24-time slots, there are others as well.  The protocol is layer architecture which follows the OSI model. The protocol layer below is the service provider and the layer above is a service user.  SS7 trunks are setup between telephone exchange to carry ss7 signaling and voice. An ss7 trunk has multiple circuits for voice. When a call originates from a switch/exchange an ISUP circuit is reserved from the trunk.

 

 

 SS7 Trunk
SS7 Trunk

Now, these days there are other IP based telephony (VOIP) that are also very popular. VoIP is discussed in a separate post with SIP Trunking.

Ss7 protocol stack or Ss7 protocol Layers:

SS7 protocol stack or signaling system 7, have the following layers.

SS7 Protocol Stack
SS7 Protocol Stack

Message Transfer Part 1 (MTP1)

This is the physical layer of signaling system 7, which transfers bits over a physical channel.


Message Transfer Part 2( MTP2)

This is the data link layer. It transfers the error-free frames between two directly connected nodes. It maintains receiving and sending windows.

Functionalities of MTP2 layer:
Error Control checks the error in a received frame based on 16 bits CRC. Re transmits lost message or error message.
Sequencing maintains the sequencing of messages between directly connected nodes. Forward and backward sequencing numbers are maintained for sequence and retransmissions.
Link Alignment does link alignment based on the request from MTP3.
Flow control, if the receiver is slow, sending mtp2 gives an indication to its user for congestion.
Link Status, link status is given to the user, the status may be, in service, out of service or congestion.

Message Transfer Part 3(MTP3),

This is the network layer which implements mtp3 protocol, this does end to end routing of SS7 messages. This is the network layer in the ss7 protocol stack. For routing mtp3 routing level is defined. The routing level is defined in the MTP3 header, which contains OPC, DPC, and SLS. Mtp3 specification is given in Q.704.  DPC is the point code of destination ss7 node, OPC is the originating point code and SLS is used for load distribution of MTP3 user messages over links.Mtp3 Routing Level

When a message is passed from MTP2 to MTP3, the routing level, SIO, and MTP3 user data is passed in the message.  MTP3 has two major functions, discrimination, and distribution. Discrimination function decides if the message is for the self node or for some other distance node. The discrimination function uses DPC.  If a message is for a self node, then it is passed to the distribution function.  The distribution function checks SI, which is part of SIO. The value of SI identifies the user of  MTP3. Eg id SI is 5, the message is delivered to the ISUP layer over MTP3, if the value of SI is 3 then the message is delivered to the SCCP layer over MTP3.

Mtp3 Layer Functions:
Remote Point code status management, mtp3 maintains the configuration and remote point code status. The status may be available or unavailable. The user of mtp3 gets pause or resume indications for a remote point code. While routing a message the status of remote point code is checked.
Link Alignment:
Once a link is configured at MTP2 and MTP3 level. MTP3 layer starts the link alignment by sending an alignment request to the MTP2 layer.  This starts link alignment at the mtp3 level. Once a link comes up on mtp2 level. Mtp3 receives an in-service indication from the MTP2 layer. MTP3 sends an SLTM message to the remote end. If SLTA received in response to SLTM then the link is marked up at mtp3 level. If the link is the first link in a link set then it’s an emergency link alignment procedure, else its normal link alignment procedure.
Transfer Prohibited (TFP): 
This is the MTP3 procedure by which a signaling point code can mark a route unavailable for a destination point code. This updates the routing table for a destination point code. When a signaling transfer point (STP), detects that a point code is not reachable. 
Load Sharing does the load sharing on over Linkset and links based on SLS received from the user.
Change Over, when a link fails the failed link’s traffic is distributed to the available links. 
Change Back, when a failed link comes back, the traffic is rerouted back to the link. 

Signaling Connection Control Part (SCCP) Layer

This is the transport layer and user of MTP3. Do the segmentation/reassembly of large messages if required. A user application is identified by an SSN (subsystem number at SCCP level). An SSN is a standard numeric value that identifies an application/node in the GSM network.

  • HLR – SSN 6
  • VLR-SSN 7
  • MSC – SSN 8
  • SGSN – SSN 149

The node on the SCCP layer is identified by a Global title. A global title is a sequence of digits, that are unique globally.  The phone number of a person is an example of a global title. SCCP layer does SSN management and global title translation.

SSN management:

SSN configured on the SCCP layer can be local or remote. Before sending a message to peer SSN, SCCP layers check the status of remote SSN, if is down message is dropped. To check the status of an SSN,  a peer transmits the SST and wait for SSA.  This procedure is optional, a remote SSN can be marked as available when peer point code comes up after successful link alignment.

Global Title Translation:

An SCCP address has called party and calling party address in the SCCP header, both addresses are global titles.  Upon receiving, SCCP checks if GT, matches the current node. If yes then the Called Party GT is translated to the point code of the current node. The message delivers to the local user identified by the called party SSN present in the incoming messages.

If a message is not of local use, the GT can be translated into another GT or PC. Or it can be forward to the next hope. If the current nodes know the point code which is serving the called party address then the gt is translated on point code else forwards to next hope.

SCCP message classes:
Class 0, connection less non sequenced message. 
Class 1, connection less sequenced delivery of messages.
Class 2, connection oriented without flow control.
Class 3, connection oriented with flow control. 
User Primitives of SCCP Layer:

A user primitive is the interface a layer provides to access its services, SCCP Provides request primitives, to request a service from layer and Indication primitives, to inform events received from the network.

Sccp Primitives (Connection Less) 
SCCP Connection Less Messages
SCCP Connection-Less Messages

Belongs to the class 0 and 1. No prior logical connection is setup. The messages are routed based on the called party address parameter. Class 0 messages are delivered not in sequence while class 1 messages delivered in sequence. ISUP and TCAP use connectionless SCCP protocol.

Data Request, this sends the user data in the SCCP protocol message. In UDT or XUDT SCCP messages, depends on the length of a user message. 
Data Indication, When a protocol data received from the network in UDT or XUDT.

Sccp Primitives (Connection-Oriented)
SCCP Connection Oriented Messages
SCCP Connection Oriented Messages

Belongs to message classes 2 and 3. A prior connection setup before sending user data.  BSSMAP and RANAP use connection-oriented SCCP protocol.

Connection Request (CR), this starts request towards the remote node for an SCCP connection.
Connection Confirmation (CC), remote SCCP user issues CC primitives to sccp for accepting connection.
Connection Release request, user issues this primitive when a connection has to close.
Connection Release Conformation, to confirm a release connection request.
Data Request issues a user to send data (e.g RANAP ) to the remote peer.
Data Indication, issues user for received data from the network.

ISDN User Part (ISUP) Layer

ISUP uses the services of MTP3. ISUP layer implements the protocol messages required for call control functionalities and voice circuit management.  When a call starts, ISUP signaling traverse many intermediate telephone exchanges. The voice carries on SS7 circuits, these are E1 or T1 lines. Each E1 has 32 logical channels.  A single channel is required for a call. Channel is identified by a circuit identification code (CIC).

ISUP basic messages:

Initial Address message (IAM), the first message which sends by the call originating SSP.

Address Complete Message (ACM), the first message originated by call terminating exchange for a received call.

Answers Message (ANM), when called side goes off-hook, terminating exchanges sends ANM to the calling exchange.

Release Message (REL), send by the exchange from which connected user (Called or Caller) goes on hook.

Release Complete message (RLC), Response of ISUP release message.

ISUP Call Flow :

ISUP Call Flow
ISUP Call Flow

TCAP Layer, This is the transaction layer. Its the user of the SCCP layer. This layer provides the functionalities to its user for maintaining end to end the dialogue with the peer node.  TCAP message carriers application context name for the application running over TCAP. A TCAP dialogue starts with a TCAP begin protocol message.  Begin message and originating transaction id and application context.

If remote peer accepts the dialogue then sends a TCAP end or a TCAP continue message, with dialogue response as accepted. In case of dialogue is not accepted, peer sends abort or reject.  Abort is of two types. User abort is generated by the user of the TCAP layer. Protocol Abort, generated by the transaction layer.

A TCAP message may have user components. A component is for operation, from the sending application that want to invoke on remote application.

 

MAP Layer, this is an application-level protocol provider. It implements the messages required for Roaming and Messaging for the GSM handset.


SS7 network elements or nodes:

SS7 Network
SS7 network

SS7 network has three types of node, each node has specific functionalities.

Signaling Switching Points (SSP)

This is the node that terminates the wireless side (BSC/BTS) and connects to the rest of the network over SS7. An SSP may be connected to other SSP directly or it may be connected via signaling gateway. MSC or mobile switching center is an example of SSP.  MSC provides roaming, messaging, billing and call control functionalities.

Signaling Transfer Point (STP)

This node is working as a  router in ss7 network. The main important role is to do Global Title Translation and selects the next node for the message. The next node may be SSP or another STP.

Signaling Control Point (SCP)

This node has a database and maintains a centralized service. For a service, these nodes are contacted. Many SSPs can contact a single SCP. In GSM network HLR or Home Location Register is an example of SCP. When a mobile phone switched on, it tries to attach first on MSC (SSP) and then MSC sends location update to the HLR.  The MSC may change but the HLR remains same. A mobile operator can map location information with time.


Sigtran or SS7 Signaling over IP:

In earlier days the internet was not such a strong, fast and reliable network. Now with the advancement of hardware and software technologies, the internet is much faster and reliable. Sigtran is the SS7 signaling over IP network.  With the invention of the SCTP protocol, Sigtran became possible.  In Sigtran the user applications are the same, the SS7 user adaptation layers have been developed. A Sigtran layer uses the services of SCTP protocol and provides service to an SS7 layer.


M3UA or MTP3 user adaptation layer:

This layer is the service provider for the MTP3 user in the SS7 network. The MTP3 users are ISUP and SCCP, mean M3UA carries the signaling for ISUP and SCCP. The primitives provided to its users by this Sigtran layer is the same as the mtp3 layer.

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