Features of Femto Convergence Server Using SIP

Features of Femto Convergence Server Using SIP

Komal Garg

Banasthali University, Rajasthan, India

Abstract

Femtocell Convergence Server comes from the 3G(third Generation) CDMA (Code Division Multiple Access) standards. It is a network element which interworks between the CDMA core voice network and femtocells. In this paper, all the existing technology of femtocells with their characteristics are explained. In this technology, two main network models are used to support the circuit switched services. One is SIP/IMS (Session Initiaition Protocol/ Internet Protocol Multimedia System) network model and second is Legacy network models. The basic protocol used in this technology is SIP protocol. SIP is an signaling protocol, that controls the voice and video calls over Internet Protocol (IP). The Femto Convergence Server is a carrier-grade and scalable product that supports some standard interfaces including; standard mobile interfaces to HLR (Home Location Register), Supports SIP to existing IP infrastructure, Supports standard IMS interfaces to HSS (Home Subscriber Server).

Keywords: Femto Convergence Server (FCS), Session Initiation Protocol (SIP), Internet Protocol Multimedia System (IMS), Femto Access Point (FAP), Code Division Multiple Access (CDMA) and Real Time Transport Protocol (RTP).

1. Introduction

   For Femtocell convergence, using the home broadband network femto enables single number voice and messaging services delivered over the IP/SIP/IMS network to the femtocell. Some key values including; Voice convergence, Messaging convergence, Single number access across networks and devices, Femtocell handover & mobile dual ring. In Femtocell, Femto means 10-15 [4]. Femtocells are much smaller than the Macrocell cellular towers. A Femtocell i.e. based on the mobile network

   technology, is a low- power access point. It provides wireless voice and broadband services to customers in the home or offices. Through direct IP connections, Femtocell Convergence Server handles many thousands of femtocells, which are encrypted with IP sec and decoded by a IPSec Security Gateway [7].

   Femtocell has two interfaces; Air Interface and Internet Interface. Air interfaces are; UMTS (Universal Mobile Telecommunications System), CDMA2000, LTE (Long Term evolution). Internet interfaces are; DSL (Digital Subscriber Line) and Cable.

   Figure 1. Femtocell Network Architecture

   In Figure 1, the femtocell connected to the mobile operators network via a standard broadband connection. Usually, a single femtocell will deliver voice services to at least four to five users simultaneously within the home or offices, while allowing more to be connected to the cell and accessing services such as text.

   In Femto Convergence server, SIP request/response methods play an important role. Using SIP request/response method, SIP call flows of mobile operators network can be handled [1]. SIP requests are the codes used for communication. SIP request methods are shown in Table 1;

   6xx Global

   Request name	Description
   INVITE	A client is being invited to participate.
   ACK	The client has confirme the invite request.
   BYE	The call has been terminated by callee or caller.
   CANCEL	Cancels the pending requests.
   OPTIONS	Queries the capability.	server	for	its
   REGISTER	Registers the client according to the address in the To header with the server.
   PRACK	Provisional confirmantion/acknowledgeme nt.
   SUBSCRIB E	Subscribing the message for an Event of Notification.
   NOTIFY	Notify the subscriber of an event.
   PUBLISH	Publishes an event to a Server.
   INFO	Sends information that does not modify the session state in the middle of a session..
   REFER	Asks client to issue SIP request (call transfer.)
   MESSAGE	Sends instant messages using SIP.
   UPDATE	Without changing the state of the dialog modifies the state of a session.

   Request name	Description
   INVITE	A client is being invited to participate.
   ACK	The client has confirme the invite request.
   BYE	The call has been terminated by callee or caller.
   CANCEL	Cancels the pending requests.
   OPTIONS	Queries the capability.	server	for	its
   REGISTER	Registers the client according to the address in the To header with the server.
   PRACK	Provisional confirmantion/acknowledgeme nt.
   SUBSCRIB E	Subscribing the message for an Event of Notification.
   NOTIFY	Notify the subscriber of an event.
   PUBLISH	Publishes an event to a Server.
   INFO	Sends information that does not modify the session state in the middle of a session..
   REFER	Asks client to issue SIP request (call transfer.)
   MESSAGE	Sends instant messages using SIP.
   UPDATE	Without changing the state of the dialog modifies the state of a session.

   responses

   failure

   Table 2. SIP Response Methods

   The main attributes of Femtoce ls are; Uses mobile technology, Operates in licensed spectrum, Generates coverage and capacity, Over Internet-grade backhaul, Permits low prices, Fully managed by licensed operator, Self-organising and self-managing. There are many reasons why customers are demanding femtocells as the services an economies that can be provided by using them but there are many challenges in achieving these potential. Some difficulties are; Market challenges, Radio and Physical challenges, Network challenges, Regulatory challenges. The network architecture of femtocell supports some key requirements; Service Parity, Call Continuity, Security, Self-Installation & Simple Operational Management and Scalability [8].

   In any femtocell network architecture, there are three common network elements; FAP (Femtocell Access Point), SeGW (Security Gateway) and FMS (Femtocell Device Management System). So, femtocells are not simple standalone devices. But they should be integrated into the mobile operators network to ensure optimal performance and to enable seamless service across both femtocell and macrocell networks.

   Table 1. SIP Request Methods

2. Related work

   As the Femto Convergence Server is an IMS SIP Application server hat interfaces to the S-CSCF

   (Serving Call Session Control Function) using the

   Response Name	Description
   1xx	Provisional responses
   2xx	Successful responses
   3xx	Redirection responses
   4xx	Client failure responses
   5xx	Server failure responses

   Response Name	Description
   1xx	Provisional responses
   2xx	Successful responses
   3xx	Redirection responses
   4xx	Client failure responses
   5xx	Server failure responses

   SIP responses specify a reason phrase and a reason phrase is defined with each response code. SIP responses are shown in Table 2;

   ISC (IMS service Control) interfaces. FCS is the centralized SIP registration server [9].

   The main work related to Femto Convergence Server is to handle the mobile operators networks call flows using SIP request/response methods. Before the description of the SIP call flows, lets take a look at how SIP user agents register with a SIP registrar.

   Figure 2. Phase 1, SIP Registration

   After that 122 starts ringing & sends the response 180 Ringing to the proxy and then proxy will forward it to 121. The called user picks up the phone and his/her phone sends 200 OK. The body of the contains an SDP message so that the caller knows where to send his/her RTP stream after that proxy forwards the response to the caller. The caller (121) confirms the receipt of 200 OK with ACK message

   then proxy forwards the ACK t 122. At this point,

   Figure 3. Phase 2, SIP Registration

   This is how SIP user agents register with a SIP registrar in two phases shown in Figure 2 & Figure 3. In Phase 1, registrar server will immediately reply

   the call has been established and both parties can start sending their RTP streams.

   with 100 Trying (a provisional response). It

   indicates that the user agent needs to authenticate. So it responds with 401 Unauthorized (Client failure response). In second phase, the registrar server will again reply with 100 Trying and will compare the two MD5 (Message Digest) hashes. If they match, the registrar will reply with 200 OK and insert to the location database.

   After Registration, lets take a look at a typical SIP call flows shown in Figure 4, Figure 5 & Figure 6.

   Figure 5. 180 Ringing

   We will take a scenario with a SIP involved [5].

   proxy server

   For example; a user at the SIP telephone with no. 121 dials the no. 122, then 121 does nott know the IP address of 122 but it knows the SIP proxy IP then user agent will compose an INVITE request & send it to the proxy. The body of the INVITE request carries an SDP (Session Description Protocol) message, the called party will need to send its RTP (Real Time Transport Protocol) stream to the caller. Then the SIP proxy responds with 100 Trying and then forwards the INVITE request to the target telephone.

   Figure 4. INVITE Request

   Figure 6. 200 OK

   When one of the users hangs up, his/her phone sends the request BYE and proxy forwards the message to other party. The other party responds to BYE request with 200 OK then both parti s will stop sending RTP data and the call is over [6].

   By using these types of call flows, mobile network features can be handled. In this paper, MO calls, MT calls and supplementary features of mobile network will be discussed.

3. FCS Network Models and Call Scenarios

   Femtocells are the devices that deliver the both Circuit Switched and Packet Data Services when connect to a mobile operator core network. For Circuit Switched services, there are two distinct models; SIP/IMS model and Legacy Network model shown in Figure 7 [15].

   Figure 7. Femtocells Network Architectures for supporting voice

   SIP/IMS Model: In this model, the femtocells connects to a new core network of the mobile operator. Femtocells behave towards the SIP/IMS network like a SIP/IMS client by transporting the voice traffic over RTP and by converting the circuit- switched 3G signaling to SIP/IMS signaling. The main components of SIP/IMS model are; Femto Access Point with SIP/IMS client, SIP/IMS core network and Femto Convergence Server.

   Over Femtocells, the SIP/IMS model is a forward looking approach for delivering services. It can also be used to offer converged fixed-mobile services to both fixed-line phones and mobile devices. As FCS is the main key component of SIP/IMS, support of active handoff is done through FCS. It acts as a peer MSC (Mobile Switching Center) in the mobile core network.

   Legacy Model: In this model, the femtocell connects directly to the existing mobile operator core network. For UMTS femtocells, 3GPP standards use the legacy core network model. The main components of Legacy network Model are; Femtocell Access Point, Femtocell Network Gateway and Security Gateway. When SIP/IMS network is not already in place then the Legacy network is easier to deploy, since it allows the operator to reuse the existing mobile core network. In this model, support of active handoff is done through the legacy MSC. Currently, the legacy network model is not supported in CDMA femtocell standards.

   In CDMA networks, via the security gateway, femtocells can connect to the PDSN (Packet Data Serving Node). This is possible because without any modifications, existing PDSNs can handle a very large number of femtocells.

   The main feature of the femtocell is that it supports the emergency calling services. It supports emergency services even for those users who are not authorized to use the femtocell.

   The basic features of the femto convergence server are; Voice & Messaging Services, Subscriber Registration & Authentication, Supplementary services, Session handoff, Regulatory services and VoIP & IMS Support.

   The main focus is on the following sections: MO (Mobile Origination) Call, MT (Mobile Termination) Call, Call Waiting, Call Barring, Call Hold, and Three-way Call.

   Call Setup & Disconnect: In successful call setup & disconnect;

   • User A calls User B.

   • User B answers the call.

   • User B hangs up.

     Call Setup & Hold: In successful call setup & hold;

   • User A calls User B.

   • User B answers the call.

   • User B puts User A on hold.

   • User B takes User A off hold.

     Simple Hold: In successful simple hold;

   • User A calls User B.

   • User B answers the call.

   • User B places User A on hold.

   • User B takes User A off hold.

   • The call continues.

     Call Hold with Consultation: In successful call hold with consultation;

   • User A calls User B.

   • User B answers the call.

   • User B places User A on hold.

   • User B calls User C.

   • User B disconnects from user C.

   • User B takes User A off hold.

   • The original call continues.

     Call Waiting: In successful call waiting;

   • User A calls User B.

   • User B answers the call.

   • User C calls User B.

   • User B accepts the call from User C.

   • User B switches back to User A.

   • User B hangs up, User A ends the call.

   • User C notifies the User B of remaining call.

   • User B continues the call and answers the notification.

     Three-way Calling: In successful Three-way calling;

   • User A calls User B.

   • User B answers the call.

   • User B puts User A o hold.

   • User B calls User C.

   • User C answers the call.

   • User B takes User A off hold.

   So these are some basic positive scenarios that is developed and tested in this paper.

4. My Contribution

   Improvement of Femto Convergence Server already being done by various technologies and protocols. I am just using these technologies to improve the mobile network features using SIP protocol by developing and testing. My main part is to firstly understand the technology accurately. Area of this paper focused only on the successful delivery to the customer. The mobile network features requires testing before delivering it to customer like Functional Testing, Feature Testing, Automation Framework Update, New Automated Tests, Network and IP Redundancy Testing, Regression and Stability Testing.

   To achieve this goal, some software specifications are required. For backend, C/C++ programming language is used and for testing part, TCL/TK scripting language is used. Generally, for Automation, scripts are written in TCL/TK scripting language. Emacs/Vim for editor, G++ compiler, inhouse automation tools for testing, SIPP for call scenarios and clearcase for code repository and MySQL for database and Valgrind for Memory leaks. To achieve the call scenarios perfectly, Software Development Life Cycle Model (Figure 8) and

   Software Testing Life Cycle Models (Figure 9) were used.

   Software Life Cycle Model:

   Figure 8. Software Development life Cycle

   This model explains how software is developed.

   Analysis: Analyzes the end-user information needs. Design: Describes desired features including screen layouts, process diagrams, pseudocode and other documentation.

   Implementation: The coding is written here. Testing: Collects all the code together into a testing environment and then checks for errors, interoperability and bugs.

   Maintenance: This is the longest stage, it maintains all the changes, correction and additions.

   By using this cycle, I developed the call scenarios using C programming language. After Developing the features testing was done manually and automation both.

   Software Testing life cycle Phases:

   Figure 9. Software Testing Life Cycle

   This model explains how testing can be done. Firstly for test planning, test cases are written. After that design the framework for testing then construction of test cases and after final testing there will be post implementation. The Test Objectives will include the following sections; Mobile Origination Call (MO

   Call), Mobile Termination Call (MT Call), Call Waiting, Call Barring, Call Hold, 3-Way Call. The testing will be done via test automation framework

   which will be enhanced to support the required

   configuration changes at FCS and to support the respective call flows.

   Wireshark, network analyzer is used to analyze the call scenarios.

   Figure 12. Automation Framework

   This is the framework that explains how automation can be done. Using telnet, SIPp uses the smoketest

   written and with the help of SIP protocol it is

   automated on the femto convergence server.

   Figure 10. Call Establishment flow using Wireshark

   Figure 11. Call Termination using Wireshark

   Automation Framework: TCL/TK scripting language used in automation framework. It is use for testing the end-to-end scenario using sip and ss7 interfaces.

   SIPp: SIPp is a free Open Source test tool for SIP protocol and used for performance testing and traffic generation.

   Figure 13. SIPp scenario

   So my main role is to develop the call scenarios and to test them perfectly without any bug before delivering it to the customer.

5. Conclusion

From this area of research, I conclude that femtocells have increasing number of high data rate applications and have the potential to provide high quality network access to indoor user at low cost. The main purpose is to provide the correct information about mobile networks to the Intelligence agencies. The CALEA (Communications Assistance for Law Enforcement Act) standards purpose is to enhance the ability of law enforcement, an Intelligence agencies to conduct electronic survelliance by allowing Federal Agencies to monitor, broadband internet, VoIP traffic, and all telephone in real time.

This paper is helpful for the beginners to get an idea about the SIP call flows & their features, FCS technology, their merits and demerits. The future work is to improve the efficiency of this proposed work and wanted to add SMS features also.

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