ISDN User Part Traffic Optimization in the SS7 Network

ISDN User Part Traffic Optimization in the SS7 Network

Rajarshi Sanyal (Belgacom International Carrier Services (BICS), Brussels, Belgium)
DOI: 10.4018/ijitn.2013100104
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The state of the art Long Term Evolution (LTE) mobile core networks predominantly use Session Initiation Protocol (SIP) and Real-time Transport Protocol (RTP) for transporting voice across networks. Voice-over-LTE (VoLTE) designed for LTE network based on an IP Multimedia System (IMS)) framework is the key to enriched voice communication services for the next generation networks. Emerging technologies like Rich Communication Suite, Web Real-Time Communication (WebRTC) will impart a new dimension to voice communications and harmonise voice and value added services. Many of the mobile networks and voice carriers evolved their network from traditional Time-Division Multiplexing (TDM) / Asynchronous Transfer Mode (ATM) to Internet Protocol (IP). Several interworking protocols have appeared for interexchange of Voice traffic between TDM and IP. SIP to ISDN User Part (ISUP) interoperability mechanisms have been devised. But, in spite of all the endeavours of the mobile networks move to the next generation, it will probably take some time for the networks to completely migrate from UMTS to LTE. As both the technologies will coexist, Circuit Switched Fallback (CSFB) and Single Radio Voice Call Continuity (SRVCC) are two of the most preferred options in the interim to switch from 4G to 3G when the coverage is lost. 3G networks rely still on circuit switching. It can be inferred that circuit switched networks piggybacking on ISUP for voice communication will continue. In this paper, we examine the ISUP signalling aspects to optimize the load sharing and streamline the performance of the traditional voice core networks. We will study the dependencies of the Circuit Identification Code (CIC) selection process on ISUP load sharing to derive our conclusions.
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1. Introduction

In a mobile network/voice carrier networks, the Mobile Switching Center (MSC), Gateway MSC (GMSC), National / International transit switches follow put of band signalling. This implies that the SS7 ISUP (ITU-T Recommendation Q.767, n.d.) related messages can be transported over another network element, namely the Signalling Transfer Point (STP).

In many of these networks, load distribution for ISUP link may not be perfect which will result in a demand for more provisioning of signalling links so that the load in the signalling nodes / links can be maintained below the threshold.

In this paper, we will study the various parameters which can be optimized to actuate a better balanced ISUP traffic distribution. We will also investigate the dependencies of ISUP load balancing with the hunting algorithm implemented for the voice circuits.

Analysis has been done on the various ISUP load-sharing algorithms available in a standard STP and the observations as well as proposals are presented. Rest of the paper is organized as follows.

  • In section 2, we provide a description of the load sharing aspects of ISUP messages

  • In Section 3 the SLS enhancement methods of STP are discussed

  • Section 4 highlights the criteria for choosing the right bit and the method

  • Section 5 provides a summary of our study results and the recommendation of appropriate method based on the network implementation scheme.

  • Section 6 brings out the the standard deviation of message distribution across the active links, for different ISUP load sharing Algorithms


2. Description Of The Load Sharing Aspects Of Isup Messages

In SS7 signalling, the signalling load distribution of the SS7 messages, ISUP or Mobile Application Part (MAP) is based on SLS (Signalling Link Selection) value in the Message Transfer Part (MTP) Routing Label (ITU-T Recommendation Q.751.1, n.d.).

ITU-T specifies SLS to be a 4-bit field (2^4 = 16 values); this in turn limits the load sharing of the SS7 messages to 16 links. Fig.1 demonstrates how it is achieved based on SLS.

Figure 1.



The ITU-T load share algorithm can actually load share evenly across a maximum of 8 links within a linkset (this comes from low order bit used for linkset determination, then the following 3 for SLC mapping making it 2^3 which equals 8 links). Hence load sharing can be achieved across two links, each containing 8 links, which makes a total of 8+8=16 links towards the mated pair STPs.

As per ITU-T recommendations, the Signalling Link Selection (SLS) for ISUP messages gets generated by taking the last four bits of the Circuit Identification Code (CIC) of the incoming ISUP message. The message is then relayed by the Network Element to the far end, using the SLS value formed.

But sometimes the default ITU-T algorithm might prove to be inefficient for load sharing in big complex networks. Some studies (Gradischnig, Krämer, & Tüxen, 2000; Angermayr, Schwarzbauer, & Tüxen, 2004) have been carried out on ISUP load sharing aspects where the load distribution is based only on the routing label.

In a practical network implementation, the CIC values populated in the incoming ISUP message might not form a specific pattern, because the incoming ISUP messages might arrive at the given Entity from different cross functional Network Elements, say from Integrated Local cum Transit Switches (ILT), external GSM Operators, MSCs, GMSCs, etc. Hence, any linear relationship between the last four bits of the CIC and the SLS value to be formed might not prove to be an efficient method of load sharing. Which is why, various alternatives need to be investigated.


3. Basic Sls Enhancement Methods In Stp

The following SLS Enhancement methods are available in any standard STP (Tekelec Eagle for example) for ISUP load distribution

  • Bit rotation method

  • Use of Other CIC bit method

  • Combination of Other CiC bit and Bit rotation method

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