articles for telecom engineers

SS7

SS7 or Signalling System Number 7 is a set of protocols that describes a means of communication between telephone switches in public telephone networks. SS7 is a highly sophisticated and powerful form of Common Channel Signalling (CCS). The use of out-of-band signalling procedures offers considerable benefits over and above other signalling methods.

SS7 Signalling Network

The signalling network consists of a number of nodes interconnected by signalling links, with each link consisting of two PCM links (one for each direction). A number of links that interconnect two nodes directly are called a signalling link set. The topography of the network is such that there are at least two signalling paths and a maximum of eight signalling paths between any nodes in the network. This ensures that the network can survive the loss of one signalling path without customers being seriously affected. It also ensures that the nodes can split the traffic between the available signalling paths and thereby reduce the damage if there is a failure. In this context, failures include loss of signalling processors located at the nodes as well as loss of the physical link set.

Figure 3.2: System No 7 Signalling Network

An SSP (Service Switching Point) is the node directly serving the subscribers. For example, a telephone exchange for control of speech connections is an SPP.

An STP (Signalling Transfer Point) is used for transfer of signalling messages between network nodes. An STP can be a stand-alone unit or include an SSP.

An SCP (Service Control Point) is used for control of intelligent network services.

An SP (Signalling Point) is the name of a network node. For example, SSPs, STPs and SCPs are all SPs.

3.2.2 SS7 Levels

In common with many signalling protocols, SS7 is made up of a layered architecture. The lowest three layers together form the Message Transfer Part or MTP. This is responsible for the secure and reliable routing of messages. Higher layers have different functions and are implemented as required by the network. Call control is handled by one of a series of Layer 4 Call Control Protocols, such as ISUP or TUP. Other functions are built on top of another layer called SCCP.

The basic structure of SS7 levels is illustrated in figure 3.3.

Figure 3.3: The Basic Layers of SS7

MTP L1

MTP Layer 1 represents the physical layer. That is, the layer that is responsible for the connection of SS7 Signalling 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.

MTP L2

MTP2 L2 is a signalling link which together with MTP L3 provides reliable transfer of signalling messages between two directly connected signalling points.

Some of the parts and their functions are as follows:

BSN (Backward Sequence Number): This part is used to acknowledge message signal units which have been received from the remote end of the signalling link.

BIB (Backward Indicator Bit) and FIB (Forward Indicator Bit): These parts are used in the basic error control method to perform the signal unit sequence control and acknowledgement functions.

LI (Length Indicator Bit): This indicates the number of octets following the length indicator.

Checksum: Every signal unit has 16 check bits for error detection.

MTP L3

MTP L3 transfers messages between the nodes of the signalling network. It ensures reliable transfer of the signalling messages, even in the case of the failure of the signalling links and signalling transfer points. Therefore, the protocol includes the appropriate functions and procedures necessary both to inform the remote parts of the signalling network of the consequences of a fault, and appropriately reconfigure the routing of messages through the signalling network.

MTP L3 has mainly two parts:

Service Indicator: This part is used to perform message distribution and in some cases to perform message routing.

Sub-service Field: It contains the network indicator and two spare bits to discriminate between national and international messages.

SCCP

The SCCP offers enhancements to MTP L3 to provide connectionless and connection-oriented network services. It also provides address translation capabilities. MTP only uses point codes routing to determine the destination of messages. Although an individual signalling point can be identified by its point code, there may exist several components within the system to which different kinds of messages could be directed. MTP lacks the capability to distinguish between these, but SCCP can perform discrimination to ensure that messages reach their correct destination.

TCAP

TCAP enables the distribution of advanced intelligent network services by supporting non-circuit related information exchange between signalling points using the SCCP connectionless service. TCAP messages are contained within the SCCP.

There are two kinds of operations that are supported by TCAP. ?Dialogues? take place between TCAP and one of the higher layers that use its services. Each operation of these services may yield a result known as a component. When TCAP receives a message, it packages all of the separate components and sends them individually to the appropriate higher layer. A TCAP transaction occurs when all stored components relating to a dialogue are presented to SCCP for routing to the relevant TCAP.

ISUP

ISUP, ISDN User Part, describes signalling functions relating to the management and set-up of telephone calls that can include voice, non-voice and data transmission. It builds on the principles of the TUP and extends its capabilities to include full integration with the ISDN access network. This means that, calls which require end-to-end data transmission can be established across the network. ISUP also supports many additional services.

ISUP messages can convey a great deal more information than those specified in the TUP protocol and the rich set of messages and parameters is also subject to considerable national variations.

TUP

TUP, the Telephone User Part, was the first SS7 call control protocol which establishes a framework protocol for the exchange of messages designed to describe call set-up. These messages are used in the correct sequence to ensure that the relevant information is sent from the originating SP to the required destination and that the destination can signal back information regarding its availability to accept the call. Finally, when the parties involved complete their dialogue, information regarding the termination of the call is transmitted from both terminating SPs.

Specific messages have been defined for each of the relevant operations and the protocol requirements how these may be used and the information that they should transmit. Note that the protocol itself may not explicitly refer to the MTP layers. The security methods employed at MTP to ensure continuous operation are independent of the layer 4 procedures.

MAP, INAP, IS41

The Mobile Application Part (MAP), Intelligent Networking Application Part (INAP) and IS41 are layers of the SS7 stack that use TCAP to process their operations and procedures, and SCCP to transport their data to the relevant destinations. These layers allow the delivery of a wide range of key services above and beyond standard telephone exchange.

Third week I have examined the IP connections of the central unit and access servers and how these IP numbers are given to these systems. After that I have started to examine the new system which is Canvas Voicemail and SMS system. When I started examining these systems, the Voicemail part of the system had recently constructed. So I had a chance to explore the usage of this system. I had also have a chance to examine the defectives of the system and how these defectives can be corrected. In this week I have participate in a seminar which is about the Media Gateway. This is a test system which will enable conversation between a PSTN and a computer. This system is produced by a company named as Argela.

source: candidate of a great engineer. O Hasdeniz. Summer practise.