Benefits of All-IP Deployment Using Ap Interfaces

1. Preface

As the common choice of international standard organizations, vendors and telecom operators, All-IP deployment has been becoming realistic for both mobile and fixed network. So what’s the driver behind this All-IP trend?　

• From end-user’s point of view, they want to enjoy more and more new services at lower cost, with little interest in technical novelty. In one word, they are only concerned of more services, and lower cost.
• From operator’s point of view, inexpensive basic voice service is always indispensable for carriers to survive in future.  Besides this, innovative value-added services are the cornerstone for carriers to prosper.

With All-IP and IMS, the dream of more services, less cost and even FMC (Fixed-Mobile Convergence) will come true.

For CDMA2000 network, with introduction of IP core network and IP RAN, the Ap interface (IP connectivity between CN and RAN) is the most important step toward All-IP and future IMS.

This paper will provide an overview of Ap interface and it’s benefits.

Ap interface is A interfaces based on IP network. That means MSCe/MGW will talk to BSC via IP network. The network diagram of Ap interface is shown in figure 1.

Figure 1. The network diagram of Ap interface

And features of Ap interfaces are as follows:

• A1p/A2p: Support IP interface and transport between BSC and MSCe/MGW, i.e. MAN or LAN Ethernet will be used to connect all BSC and MSCe/MGW, the connection between BSC and MGW/MSCe will not be point-to-point connection any more, thus facilitating the deployment (flat networking using IP cloud and no need to deploy Tandem MGW ).
• Vocoders will be able to move from BSC to MGW. And multiple codec will be supported in MGW: EVRC, QCELP, G.711 etc.
• Implementation of TrFO (Transcoder Free Operation), TrFO is the transport of compressed speech, from legacy mobiles, in a packet transport network through the elimination of unnecessary coding and decoding of the voice by intermediate elements in the bearer path. Transcoders may be associated either with the RAN or Media Gateway (MGW). By transporting only the compressed speech, TrFO achieves bandwidth efficiencies in the bearer stream and reduces round trip delays introduced by unnecessary transcoding. Also, TrFO may increase voice quality (less distortion). In one word, TrFO is a mechanism for configuring a call such that no transcoders are needed in the core network. That means the cost for transcoders will be saved.
• Implementation of Remote Transcoder Operation (RTO), RTO is a mechanism for configuring a call such that only a single transcoder resource is needed in the core network. E.g. To support a MS to PSTN call, a transcoder is inserted at the MGW or BSC. For a MS to MS call, where the codecs in the two MS differ, a transcoder is needed in the network. This eliminates the typical tandem use where two transcoders are used to transcode the two endpoints to a common but unique protocol for transport. RTO is just a special case of TrFO.

From operator’s point of view, Ap interface can significantly help operator lower CAPEX and OPEX.

The benefits of Ap are attributed to four aspects: cost reduction of MGW/BSC; cost savings for migration to All-IP network; cost savings for transmission bandwidth rental or equipment.

1）Ap interface deployment will provide smooth migration to All-IP network. With Ap interface, no need to replace any hardware when upgrading to IMS. For TDM deployment of BSC and CDMA softswitch, it will be difficult to upgrade to support All-IP network in commercial network, the service interruption will be inevitable. The migration is described as follows:

• All interface cards in A interface will have to be replaced. That means more than half of the interface cards of softswitch and BSC will be replaced.
• In order to implement TrFO and RTO, MGM  MGW will also be using IP connectivity, so TDM interface cards of MGM to MGW will also be replaced. As a result more than 80% TDM interface cards will be replaced by IP interface cards.
• Vocoder cards shall be moved from BSC to MGW, if Vocoder card is not common between BSC and MGW, then all vocoder cards will have to be replaced and old vocoder cards will have to be discarded. 
• Software upgrade on both BSC and MSCe/MGW will be inevitable.

2）For MGW and BSC, the cost reduction is due to less interface cards, less vocoders and less racks of equipment.

• For Ap interface, over 80% of TDM interface cards of BSC and MGW will be replaced by IP cards. As IP interface card his much higher throughput and processing capability (generally more than 4 times or even higher), so number of interface cards will be significantly reduced. Take ZTE’s equipment for example, one DTB support 32 E1, which can support 960 simultaneous calls. One IPI can support 4000 simultaneous calls. That means one IPI can be used to replace four DTB cards.
• With TrFO function, less vocoder cards will be used, generally up to 30-50% vocoder cards can be saved.
• As interface cards and vocoder cards reduced, the shelves and relating cards will also be reduced.  Less shelves means less racks, less space, less power consumption etc.

3）Cost savings for transmission is due to much higher bandwidth efficiency of All-IP network. For Ap interface, EVRC will be transparently transmitted from MS to MS or MS to terminated MGW instead of 64kbps PCM. Full-rate EVRC is 8kbps, not mention about half-rate and 1/8 rate. Following table is the bandwidth requirement for different cases. By using Ap interface, the bandwidth in the core network (BSC-MGW-MGW-BSC) can save as much as 80%. This can be converted into dollars of savings of transmission bandwidth rental or equipment.

• Suppose A interface is used, and we need to support simultaneous 10,000 voice subscribers (200,000 subscribers capacity, 0.05Erl/sub), the bandwidth requirement for BSC-MGW-MGW-BSC will be: 64kbps*10,000=640Mbps. It uses 320 E1 ports.
• If Ap is adopted, and we use a moderate compression (three frames in one packet), the bandwidth will be: 10,000*11.7kbps/sub=117Mbps, this will need two Fast Ethernet ports.
• Let’s compare the cost of 320 E1 ports and two 100Mbps Ethernet ports. The cost of two Ethernet ports maybe less than 5% of the cost 320 E1 ports. This will save transmission between MGW and BSC, MGW and MGW for both local call and long-distance call. It will definitely reduce the CAPEX and OPEX for transmission.

From end-user point of view, the Ap interface will provide better QoS for voice with lower cost. This is attributed to TrFO (Transcoder Free Operation).  TrFO is a mechanism for configuring a call such that no transcoders are needed in the network. As no transcoder is needed, it  reduces round trip delays introduced by unnecessary transcoding. Also, TrFO will significantly increase voice quality(less distortion).  It’s estimated that one transcoder operation will introduce around 10-20 ms delay and sacrifice around 10-20% voice information.

ZTE is the world’s first CDMA vendor to provide E2E All-IP solution using Ap interface (based on IOS5.0 of 3GPP2). This solution has been selected by Reliance Infocomm (the largest CDMA operator of India) and has been commercially launched in SLT (Sri Lanka Telecom), which is the largest full-service operator of Sri Lanka. With this solutions, the operator will lead the mobile / WLL market right away!

• Flexible network deployment, and flexible transmission; flat networking using IP cloud and no need to deploy Tandem MGW in IP network.
• Cost savings for transmission bandwidth between MGW and BSC(save up to 80%).
• Cost savings for vocoders (up to 30%-50%)
• Cost saving for interface cards (up to 80% interface cards), less shelves, less racks, less space and power consumption.
• Improved voice quality (less round trip delay, higher MOS)
• Smooth migration to All-IP network, no need to consider hardware replacement in the future.

TrFO (Transcoder Free Operation) is a mechanism for configuring a call so that no transcoders are needed in the network. TrFO achieves bandwidth efficiencies in the bearer stream and reduces round trip delays introduced by unnecessary transcoding.  Also, TrFO may increase voice quality.

RTO(Remote Transcoder Operation) is a mechanism for configuring a call so that only one transcoder resource is needed in the network. RTO is just a special case of TrFO.