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tutorial_ss7_ip_interworking

tutorial_ss7_ip_interworking
tutorial_ss7_ip_interworking

Table of Contents

Interworking Switched Circuit and Voice-over-IP Networks (3)

Introduction (3)

Signaling in Switched Circuit and VoIP Networks (3)

Signaling in Switched Circuit Networks (3)

Signaling in VoIP Networks (4)

Sigtran Protocols (6)

Performance Considerations for SS7 over IP (6)

Security Requirements for SS7 over IP (6)

SCTP: Stream Control Transmission Protocol (7)

Transporting MTP over IP (8)

M2UA: MTP2 User Adaptation Layer (9)

M2PA: MTP2 User Peer-to-Peer Adaptation Layer (9)

M3UA: MTP Level 3 User Adaptation Layer (10)

Transporting SCCP over IP (11)

SIP, PINT, SPIRITS, ENUM, TRIP (11)

SIP Protocol Components (12)

SIP-T (12)

PINT and SPIRITS (13)

ENUM (13)

TRIP (13)

Bibliography (13)

Abbreviations (14)

For more Information (14)

Figure 1: SS7 Signaling End Points (SEPs) in a Switched Circuit Network

In SS7 networks, ISUP (Integrated Services Digital Network (ISDN) User Part) signaling messages are used to setup, manage and release trunk circuits that carry voice calls between central office switches. ISUP messages also carry caller ID information, such as the calling party's telephone number and name. ISUP is used for both ISDN and non-ISDN calls between central office switches.

TCAP (Transaction Capabilities Application Part) signaling messages support telephony

Figure 2: Example of a VoIP Network Configuration

Several security mechanisms are currently available for use in IP networks. For transmission of signaling information over the Internet, sigtran recommends the use of IPSEC (see the IETF RFC2401). IPSEC provides the following security services:

· Authentication: to ensure information is sent to/from a known and trusted partner

· Integrity: to ensure that the signaling information has not been modified in-transit

· Confidentiality: to ensure that the transported information is encrypted to avoid illegal use or violation of privacy laws

· Availability: to ensure communicating endpoints under attack remain in service for authorized use

The sigtran protocols do not define new security mechanisms as the currently available security protocols provide the necessary mechanisms for secure transmission of SS7 messages over IP networks.

SCTP: Stream Control Transmission Protocol

To reliably transport SS7 messages over IP networks, the Internet Engineering Task force sigtran working group devised the Stream Control Transmission Protocol (SCTP). SCTP allows the reliable transfer of signaling messages between signaling endpoints in an IP network.

To establish an association between SCTP endpoints, one endpoint provides the other endpoint with a list of its transport addresses (multiple IP addresses in combination with an SCTP port). These transport addresses identify the addresses that will send and receive SCTP packets.

IP signaling traffic is usually composed of many independent message sequences between many different signaling endpoints. SCTP allows signaling messages to be independently ordered within multiple streams (unidirectional logical channels established from one SCTP endpoint to another) to ensure in-sequence delivery between associated endpoints. By transferring independent message sequences in separate SCTP streams, it is less likely that the retransmission of a lost message will affect the timely delivery of other messages in unrelated sequences (called head-of-line blocking). Because TCP/IP does enforce head-of-line blocking, the sigtran Working Group recommends SCTP rather than TCP/IP for the transmission of signaling messages over IP networks.

There are three types of messages in SS7:

· Message Signal Units (MSUs)

· Link Status Signal Units (LSSUs)

· Fill-In Signal Units (FISUs)

MSUs originate at a higher level than MTP Level 2 and are destined for a peer at another node. LSSUs allow peer MTP Level 2 layers to exchange link status information. FISUs are sent when no other signal units are waiting to be sent across the synchronous link. This purpose is preserved by the heartbeat messages in SCTP. FISUs also carry acknowledgment of messages, a function also assumed by SCTP.

In summary, SCTP provides:

· acknowledged error-free non-duplicated transfer of signaling information

· in-sequence delivery of messages within multiple streams, with an option for order-of-arrival delivery of individual messages

· optional bundling of multiple messages into a single SCTP packet

· data fragmentation as required

· network-level fault tolerance through support of multi-homing at either or both ends of an association

· appropriate congestion avoidance behavior and resistance to flooding (denial-of-service) and masquerade attacks

To meet stringent SS7 signaling reliability and performance requirements for carrier-grade networks, VoIP network operators ensure that there is no single point of failure in the end-to-end network architecture between an SS7 node and a media gateway controller. To achieve carrier-grade reliability in IP networks, links in a linkset are typically distributed amongst multiple signaling gateways, media gateway controllers are distributed over multiple CPU hosts, and redundant IP network paths are provisioned to ensure survivability of SCTP associations between SCTP endpoints.

Transporting MTP over IP

For MTP messages transported over SS7 or IP networks, the following requirements are specified by the International Telecommunication Union:

· MTP Level 3 peer-to-peer procedures require a response time within 0.5 sec (500 milliseconds) to 1.2 seconds (1200 ms).

· no more than 1 in 10 million messages will be lost due to transport failure.

· no more than 1 in 10,000 million (US terminology: 1 in 10 billion) messages will be delivered out-of sequence (including duplicated messages) due to transport failure.

· no more than 1 in 10,000 million (US: 1 in 10 billion) messages will contain an error that is undetected by the transport protocol or 1 in 1,000 million (US: 1 in 1 billion) for ANSI

(American National Standard Institute) specifications.

· availability of any signaling route set (the complete set of allowed signaling paths from a given signaling point towards a specific destination) is 99.9998% or better (downtime of

approximately 10 minutes/year or less).

· the message length (payload accepted) is 272 bytes for narrowband SS7 and 4091 bytes for broadband SS7.

The M3UA layer at an IP endpoint may also indicate to the signaling gateway that M3UA at an IP endpoint is congested.

Transporting SCCP over IP

SUA (SCCP User Adaptation Layer) is a protocol defined by the IETF sigtran Working Group for transporting SS7 SCCP (Signaling Connection Control Part) user part signaling messages (e.g., TCAP and RANAP) over IP using the Stream Control Transmission Protocol (SCTP). SUA is used between a signaling gateway and an IP signaling endpoint and between IP signaling endpoints. SUA supports both SCCP unordered and in-sequence connectionless services and bi-directional connection-oriented services with or without flow control and detection of message loss and out-of-sequence errors (i.e., SCCP protocol classes 0 through 3).

For connectionless transport, SCCP and SUA interface at the signaling gateway. From the perspective of an SS7 signaling point, the SCCP user is located at the signaling gateway. SS7 messages are routed to the signaling gateway based on point code and SCCP subsystem number. The signaling gateway then routes SCCP messages to the remote IP endpoint. If redundant IP endpoints exist, the signaling gateway(s) can load share amongst active IP endpoints using a round-robin approach. Note that load sharing of TCAP messages occurs only for the first message in a TCAP dialogue; subsequent TCAP messages in the same dialogue are always sent to the IP endpoint selected for the first message, unless endpoints share state information and the signaling gateway is aware of the message allocation policy of the IP endpoints. The signaling gateway may also perform Global Title Translation (GTT) to determine the destination of an SCCP message. The signaling gateway routes on global title, i.e. digits present in the incoming message, such as called party number or mobile subscriber identification number.

For connection-oriented transport, SCCP and SUA interface at the signaling gateway to associate the two connection sections needed for connection-oriented data transfer between an SS7 signaling end point and an IP endpoint. Messages are routed by the signaling gateway to SS7 signaling points based on the destination point code (in the MTP-3 address field) and to IP endpoints based on IP address (in the SCTP header).

SUA can also be used to transport SCCP user information between IP endpoints directly rather than via the signaling gateway. The signaling gateway is needed only to enable interoperability with SS7 signaling in the switched circuit network.

If an IP resident application is connected to multiple signaling gateways, multiple routes may exist to a destination in the SS7 network. In this case, the IP endpoint must monitor the status of remote signaling gateways before initiating a message transfer.

SIP, PINT, SPIRITS, ENUM, TRIP

SIP (Session Initiation Protocol) is a signaling protocol for creating, modifying and terminating sessions, such as IP voice calls or multimedia conferences, with one or more participants in an IP network. SIP is currently undergoing standardization by the Internet Engineering Task Force SIP Working Group. While the sigtran protocols are currently the protocols of choice for interworking

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