Message Transfer Part (MTP)

The Message Transfer Part (MTP) is part of the Signaling System 7 (SS7) used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners.

MTP is formally defined primarily in ITU-T recommendations Q.701Q.702Q.703Q.704 and Q.705. Tests for the MTP are specified in theITU-T recommendations Q.781 for MTP2 and in Q.782 for MTP3. These tests are used to validate the correct implementation of the MTP protocol.

Different countries use different variants of the MTP protocols. In North America, the formal standard followed is ANSI T1.111. In Europe, national MTP protocols are based on ETSI EN 300-008-1

The SS7 stack can be separated into four functional levels:

  • Level 1 is the Signalling Data Link Functional Level (Data Link Level).
  • Level 2 is the Signalling Link Functional Level (Link Level).
  • Level 3 is the Signalling Network Functional Level (Network Level).
  • Level 4 is the MTP User and consists of SCCPISUPTUP, or any other MTP User.

Level 1 through Level 3 comprise the MTP, and Level 4 the MTP userMTP Level 3 is sometimes abbreviated MTP3MTP Level 2MTP2. MTP and SCCP are together referred to as the Network Service Part (NSP).

There is no one-to-one mapping of MTP Levels 1 through 3 onto the OSI model. Instead, MTP provides the functionality of layers 12 and part of layer 3 in the OSI model. The part of layer 3 of the OSI model that MTP does not provide, is provided by SCCP or other Level 4 parts (MTP users).

Signalling Data Link Functional Level

MTP Level 1 is described in ITU-T Recommendation Q.702, and provides the Signalling Data Link functional level for narrowband signalling links. For broadband signalling links, ITU-T Recommendation Q.2110 or Q.2111 describe the signalling data link function.

MTP1 represents the physical layer. That is, the layer that is responsible for the connection of SS7 Signaling Points into the transmission network over which they communicate with each other. Primarily, this involves the conversion of messaging into electrical signal and the maintenance of the physical links through which these pass. In this way, it is analogous to the Layer 1 of ISDN or other, perhaps more familiar, protocols.

MTP1 normally uses a timeslot in an E-carrier or T-carrier.

Signalling Link Functional Level

MTP Level 2 is described in ITU-T Recommendation Q.703, and provides the Signalling Link functional level for narrowband signalling links. For broadband signalling links, ITU-T Recommendation Q.2140 and Q.2210 describe the signalling link function referred to as MTP3b. The signalling link functional level may also be provided using the SIGTRAN protocol M2PA described in RFC 4165.

MTP2 provides error detection and sequence checking, and retransmits unacknowledged messages. MTP2 uses packets called signal units to transmit SS7 messages. There are three types of signal units: Fill-in Signal Unit (FISU), Link Status Signal Unit (LSSU), Message Signal Unit (MSU).

Access to the signalling link functional level’s service interface can be provided over SCTP by the SIGTRAN protocol M2UA, described in RFC 3331.

MTP Level 2 is tested using the protocol tester and test specifications described in Q.755Q.755.1Q.780 and Q.781.

Signalling Network Functional Level

MTP Level 3 is described in ITU-T Recommendation Q.704, and provides the Signalling Network functional level for narrowband signalling links and, with only minor modifications described inITU-T Recommendation Q.2210, for broadband signalling links. The functions of MTP Level 3 may also be replaced with the Generic Signalling Transport Service described in ITU-T Recommendation Q.2150.0 as provided by MTP3b (Q.2150.1), SSCOP or SSCOPMCE (Q.2150.2) or SCTP (Q.2150.3). MTP Level 3 functions can also be provided by using the IETFSIGTRAN M3UA protocol, described in RFC 4666, in IPSP mode.

MTP3 provides routing functionality to transport signaling messages through the SS7 network to the requested endpoint. Each network element in the SS7 network has a unique address, thePoint Code (PC). Message routing is performed according to this address. A distinction is made between a Signaling Transfer Point (STP) which only performs MTP message routing functionalities and a Signaling End Point (SEP) which uses MTP to communicate with other SEPs (that is, telecom switches). MTP3 is also responsible for network management; when the availability of MTP2 data links changes, MTP3 establishes alternative links as required and propagates information about route availability through the network.

Access to the signalling network functional level’s service interface (as described in Q.701) can be provided over SCTP by the SIGTRAN protocol M3UA, described in RFC 4666.

MTP Level 3 is tested using the protocol tester and test specifications described in Q.755Q.755.1Q.780 and Q.782.

MTP Users

Level 4 consists of MTP Users. The remaining components of the SS7 stack are all directly, or indirectly, MTP Users. Some examples of parts at Level 4 are SCCPISUP and TUP.[7] The services provided to MTP Level 4 by the MTP (that is, MTP to MTP Users) is described in ITU-T Recommendation Q.701.

 

Reference : http://en.wikipedia.org/wiki/Message_Transfer_Part

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Signalling Connection Control Part (SCCP) Protocol

The Signalling Connection Control Part (SCCP) is a network layer protocol that provides extended routingflow control, segmentation,connection-orientation, and error correction facilities in Signaling System 7 telecommunications networks. SCCP relies on the services of MTP for basic routing and error detection.

Routing facilities beyond MTP

Although MTP provides routing capabilities based upon the Point Code, SCCP allows routing using a Point Code and Subsystem number or a Global Title.

A Point Code is used to address a particular node on the network, whereas a Subsystem number addresses a specific application available on that node. SCCP employs a process called Global Title Translation to determine Point Codes from Global Titles so as to instruct MTP on where to route messages.

SCCP messages contain parameters which describe the type of addressing used, and how the message should be routed:

  • Address Indicator
    • Routing indicator
      • Route on Global Title
      • Route on Point Code/Subsystem Number
    • Global title indicator
      • No Global Title
      • Global Title includes Translation Type (TT), Numbering Plan Indicator (NPI) and Type of Number (TON)
      • Global Title includes Translation Type only
    • Subsystem indicator
      • Subsystem Number present
      • Subsystem Number not present
    • Point Code indicator
      • Point Code present
      • Point Code not present
  • Global Title
    • Address Indicator Coding
    • Address Indicator coded as national (the Address Indicator is treated as international if not specified)

Protocol classes

SCCP provides 5 classes of protocol to its applications:

  • Class 0: Basic connectionless.
  • Class 1: Sequenced connectionless.
  • Class 2: Basic connection-oriented.
  • Class 3: Flow control connection oriented.
  • Class 4: Error recovery and flow control connection oriented.

The connectionless protocol classes provide the capabilities needed to transfer one Network Service Data Unit (NSDU) in the “data” field of an XUDT, LUDT or UDT message. When one connectionless message is not sufficient to convey the user data contained in one NSDU, a segmenting/reassembly function for protocol classes 0 and 1 is provided. In this case, the SCCP at the originating node or in a relay node provides segmentation of the information into multiple segments prior to transfer in the “data” field of XUDT (or as a network option LUDT) messages. At the destination node, the NSDU is reassembled.

The connection-oriented protocol classes (protocol classes 2 and 3) provide the means to set up signalling connections in order to exchange a number of related NSDUs. The connection-oriented protocol classes also provide a segmenting and reassembling capability. If an NSDU is longer than 255 octets, it is split into multiple segments at the originating node, prior to transfer in the “data” field of DT messages. Each segment is less than or equal to 255 octets. At the destination node, the NSDU is reassembled.[3]

Class 0: Basic connectionless

The SCCP Class 0 protocol class is the most basic of the SCCP protocol classes. Network Service Data Units passed by higher layers to the SCCP in the originating node are delivered by the SCCP to higher layers in the destination node. They are transferred independently of each other. Therefore, they may be delivered to the SCCP user out-of-sequence. Thus, this protocol class corresponds to a pure connectionless network service. As a connectionless protocol, no network connection is established between the sender and the receiver.

Class 1: Sequenced connectionless

SCCP Class 1 builds on the capabilities of Class 0, with the addition of a sequence control parameter in the NSDU which allows the SCCP User to instruct the SCCP that a given stream of messages should be delivered in sequence. Therefore, Protocol Class 1 corresponds to an enhanced connectionless protocol with assurances of in-sequence delivery.

Class 2: Basic connection-oriented

SCCP Class 2 provides the facilities of Class 1, but also allows for an entity to establish a two-way dialog with another entity using SCCP.

Class 3: Flow control connection oriented

Class 3 service builds upon Class 2, but also allows for expedited (urgent) messages to be sent and received, and for errors in sequencing (segment re-assembly) to be detected and for SCCP to restart a connection should this occur.

Reference : http://en.wikipedia.org/wiki/Signalling_Connection_Control_Part

SIP Vs. SS7 Protocol

Signalling System No. 7 (SS7) is a set of telephony signaling protocols which are used to set up most of the world’s public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls. Other uses include number translation, local number portability, prepaid billing mechanisms, short message service (SMS), and a variety of other mass market services.

It is usually referenced as Signalling System No. 7 or Signalling System #7, or simply abbreviated to SS7. In North America it is often referred to asCCSS7, an abbreviation for Common Channel Signalling System 7. In some European countries, specifically the United Kingdom, it is sometimes called C7 (CCITT number 7) and is also known as number 7 and CCIS7 (Common Channel Interoffice Signaling 7). In Germany it is often called as N7 (Signalisierungssystem Nummer 7).

There is only one international SS7 protocol defined by ITU-T in its Q.700-series recommendations.[1] There are however, many national variants of the SS7 protocols. Most national variants are based on two widely deployed national variants as standardized by ANSI and ETSI, which are in turn based on the international protocol defined by ITU-T. Each national variant has its own unique characteristics. Some national variants with rather striking characteristics are the China (PRC) and Japan (TTC) national variants.

The Internet Engineering Task Force (IETF) has also defined level 2, 3, and 4 protocols that are compatible with SS7:

  • Message Transfer Part (MTP) level 2 (M2UA and M2PA)
  • Message Transfer Part (MTP) level 3 (M3UA)
  • Signalling Connection Control Part (SCCP) (SUA)

but use a Stream Control Transmission Protocol (SCTP) transport mechanism. This suite of protocols is called SIGTRAN.

AND

The Session Initiation Protocol (SIP) allows phone calls and similar communication sessions to be made over the Internet, private data networks, or cellular networks. It defines the messages that are sent between parties (signaling) which govern establishment, termination, and other essential elements of a call (or, more generally, a session, hence the name).

SIP is an IETF-defined signaling protocol and is widely used for controlling communication sessions such as voice and video calls over Internet Protocol (IP). The protocol can be used for creating, modifying and terminating two-party (unicast) or multiparty (multicast) sessions. Sessions may consist of one or several media streams, such as voice or video data.

Other SIP applications include video conferencing, streaming multimedia distribution, instant messagingpresence informationfile transfer and online games[citation needed].

SIP is an application layer protocol designed to be independent of the underlying transport layer; it can run on Transmission Control Protocol (TCP),User Datagram Protocol (UDP), or Stream Control Transmission Protocol (SCTP). It is a text-based protocol, incorporating many elements of theHypertext Transfer Protocol (HTTP) and the Simple Mail Transfer Protocol (SMTP).

 

Reference :
http://en.wikipedia.org/wiki/Signalling_System_No._7
http://en.wikipedia.org/wiki/Session_Initiation_Protocol

Session Control Protocol (SCP)

Several heavily used Internet applications such as FTPGOPHER, and HTTP use a protocol model in which every transaction requires a separate TCP connection. Since clients normally issue multiple requests to the same server, this model is quite inefficient, as it incurs all the connection start up costs for every single request.

SCP is a simple protocol which lets a server and client have multiple conversations over a single TCP connection. The protocol is designed to be simple to implement, and is modelled after TCP.

Services

SCP’s main service is dialogue control. This service allows either end of the connection to establish a virtual session over a single transport connection. SCP also allows a sender to indicate message boundaries, and allows a reciever to reject an incoming session.

Design goals

Unconfirmed service without negotiation.
SCP allows data to be sent with the session establishment; the recepient does not confirm successful connection establishment, but may reject unsuccessful attempts. This simplifies the design of the protocol, and removes the latency required for a confirmed operation.
Low overhead
SCP has a fixed overhead of 8 bytes per segment. This overhead is half the size of an IPNG address, and is only incurred once per segment, instead of once per packet.
Simple design
The session protocol should be simple enough to implement for a single application.

Protocol Description

Header Format:

Protocol Operation

Session ID allocation

Each session is allocated a session identifier. Session Identifiers below 1024 are reserved. Session IDs allocated by clients are even; those allocated by servers, odd.

Session establishment

A session is established by setting the SYN bit in the first message sent on that channel.

Graceful release

A session is ended by sending a message with the FIN bit set. Each end of a connection may be closed independently.

Disgraceful release

A session may be terminated by sending a message with the RST bit set. All pending data for that session should be discarded

Message boundaries

A message boundary is marked by sending a message with the PUSH bit set. The boundary is set at the final octet in this message, including that octet.

——

Reference : http://www.w3.org/Protocols/HTTP-NG/http-ng-scp.html