A Study of Throughput for Iu-CS and Iu-PS Interface in UMTS Core Network

Transcription

1 A Study of Throughput for Iu-CS and Iu-PS Interface in UMTS Core Network Ye Ouyang Howe School of Technology Management Stevens Institute of Technology Hoboken, NJ, USA M. Hosein Fallah, Ph.D, P.E. Howe School of Technology Management Stevens Institute of Technology Hoboken, NJ, USA Abstract Current literature provides many practical tools or theoretical methods to plan and dimension GSM or UMTS radio networks but overlooks the algorithms of network plan and dimensioning for core networks. This paper introduces an algorithm for traffic and throughput dimensioning for UMTS core network. A case study is provided to verify the algorithms created for UMTS core network. This paper is aimed at helping wireless carriers plan and dimensioning their G core networks. Keywords-UMTS; core network; circuit switch; packet switch; throughpu; network plan, network dimension. I. ARCHITECTURE OF UMTS CORE NETWORK As mobile operators evolve their networks to UMTS or even LTE, they will look to minimize cost and maximize subscriber usage. Therefore, a new problem appears: how to correctly plan and dimension the emerging UMTS core networks (CN) with a new flat and all-ip structure to avoid configuring unnecessary network resources and maintain a high quality of service (QoS) to subscribers? Meanwhile, the dimension algorithms for UMTS CN should be significantly differentiated from the traditional design philosophy for circuit switched (CS) and time division multiplexing (TDM) networks such as 2G GSM and CDMA networks. The core network (CN) is the heart of a mobile communication network. The CN plays an essential role in the whole mobile network system to provide such important capabilities as mobility management, call and session control, switching and routing, charging and billing. With a logical division, the CN in Universal Mobile Telecommunications System (UMTS) is classified into the circuit switched domain (CS) including such logical Network Entities (NE) as Mobile Switching Center Server (MSC Server or MSS), Media Gateway (MGW), Visitor Location Register (VLR) integrated in MSS physically, Home Location Register (HLR), Authentication Center (AUC), Equipment Identity Register (EIR) and the packet switched domain (PS) including NEs: Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). Below is a short description on the NE of MGW, MSS and SGSN. As the core NE of the CN in UMTS, MSC Server is a functional entity that implements mobile call service, mobility management, handover, and other supplementary services. MSC Server provides Nc interface to connect with its peer MSC Server, Mc interface with MGW, C/D interface with HLR, A interface with 2G Base Station Controller (BSC), and the optional Gs interface with SGSN. A MGW in UMTS implements bearer processing functions between different networks. MGW provides Iu-CS interface to connect with Radio Network Controller (RNC) in Radio Access Network (RAN), Nb interface with its peer MGW, E interface with 2G MSC, Mc interface with MSC Server, A interface with BSC, and Ai interface with PSTN. SGSN is responsible for the delivery of data packets from and to MSs within its serving area. Its interfaces include Iu-PS interface connecting to RNC, Gn/Gp interface to GGSN, Gr interface to HLR, Gs interface to MSC Server or MSC, Gd interface to Short Message Center (SMC), and Ga interface to Charging Gateway. In order to accurately design and dimension the UMTS CN, this paper will develop the algorithms of traffic and throughput for the CN NEs. The analysis will be based on the traffic and throughput generated or absorbed in the interfaces of CN mentioned above. II. THROUGHPUT ALGORITHMS OF UMTS CORE NETWORK Wherever Times is specified, Times Roman or Times New Roman may be used. If neither is available on your word processor, please use the font closest in appearance to Times. Avoid using bit-mapped fonts if possible. True-Type 1 or Open Type fonts are preferred. Please embed symbol fonts, as well, for math, etc. Since Iu-CS, and Iu-PS interface are newly developed in UMTS CN, this section 2 is focused on the algorithms for the new interfaces. The calculation of TDM based traffic for the other interfaces such as A to E and Gb interface, since they have been existing in GSM CN, is still based on traditional algorithm: multiply total traffic (Erlang) and traffic proportion to obtain the traffic distribution for each NE and each link /09/$ IEEE 49

2 A. Iu-CS Interface Iu-CS interface locates between MGW and RNC to establish the voice channel and transport the Radio Access Network Application Part (RANAP) signaling message [4]. The transmission medium in Iu-CS interface is ATM in R4 and is suggested to be replaced by IP from UMTS R5. As per References [1] and [2], the interface Iu-CS consists of user plane based on ATM Adaption Layer 2 (AAL2) and control plane based on AAL5. The protocol stack of Iu-CS interface is shown in Table I. In CS voice user plane, Iu Interface User Plane Protocol (Iu-UP) stands on the top layer and follows by AAL2 and ATM. Reference [] defines the PDU format for Iu-UP in which we are able to obtain the overhead of Iu-UP frame = Frame Control Part (FCP) + Frame Check Sum Part (FCSP). Typical Iu-UP Packet Data Unit (PDU) formats are Iu-UP PDU type 0, 1 and 14 in which both FCP and FCSP occupies 2 bytes respectively. One exception is FCSP is 1 byte for type 1 defined to transfer user data over the Iu UP in support mode for pre-defined SDU sizes when no payload error detection scheme is necessary over Iu UP. But this scenario is not usually adopted for the reason that error detection is always needed in transmission. Generally we obtain the overhead of Iu-UP frame = FCP + FCSP = 2+2=4 bytes. This value is used for the following calculation. Radio Network Control Plane MM/SM/CC RANAP TABLE I. Transport Network Control Plane ALCAP IU-CS UMTS PROTOCOL STACK Circuit Switching Data User Plane Application TAF RLP CS Voice User Plane AMR Codec SCCP STC MTP-D MTP-D SSCF NNI SSCF NNI Iu UP SSCOP SSCOP AAL2-SAR SSCS AAL5 AAL5 AAL2 ATM AAL2 below the layer of Iu-UP provides bandwidth - efficient transmission of low-rate, short and variable packets in delay sensitive applications. So it is the ideal bearer medium for the circuit switching service of UMTS. From Reference [2] and [6], AAL2 can be subdivided into two layers: the Common Part Sub-layer (CPS) and the Service Specific Convergence Sub-layer (SSCS). The later is normally void so only CPS is considered in our case. The structure of the AAL2 CPS PDU is given in the following illustration. From the PDU structure, we obtain the Start Field=8 bits=1bytes=1 Octet; AAL2 Header= =24 bits= bytes= Octets. In addition, the ATM cell is 5 bytes and the header of ATM cell is 5 bytes. TABLE II. AAL2 CPS PDU Start field AAL2 CPS-PDU payload OSF SN P AAL2 PDU payload PAD 6 bits 1 bit 1 bit 0-47 bytes AAL2 CPS PDU TABLE III. AAL2CPS PDU PAYLOAD AAL2 Header Information Payload CID LI UUI HEC Information payload 8 bits 6 bits 5 bits 5 bits 1-45/64 bytes AAL2 PDU Payload The AMR (Adaptive Multi-Rate) codec encodes narrowband ( Hz) signals at variable bit rates ranging from 4.75 to 12.2 kbps. We adopted mode 7 with a codec speed at 12.2kbps for voice signal and use 64 kbps as the codec speed for video call service in our case. The following Table summarizes the necessary parameters for Iu-CS interface. Size (Octet s) TABLE IV. Iu- UP Ov erh ead AAL 2 Start Field OVERHEAD OF PROTOCOLS IN IU-CS INTEFACE AAL 2 Head er AT M Head er AT M Cell AMR Payloa d (at 12.2 kbps) G.711 Payload (at 64kbps) TABLE V. CODEC PARAMETERS Codec Type Codec Speed (kbps) Payload per Frame (Octets) Speech Frame (ms) AMR. Type AMR_SID. Type Not a fixed value G Video Type, e.g. G Based on the conditions obtained above, we are able to give the functions for the voice channel bandwidth in Iu-CS interface. Without the Voice Activity Detection (VAD) technique, a single channel bandwidth in Iu-CS is given by BW NonVAD = SP E AMR / AAL2 (1) where SP AMR denotes the codec speed of AMR, obtained from Table V, E AAL2 denotes the efficiency of AAL2 encapsulation. It is given by formula 2 below. From Table II, Channel Identification is 8 bits, meaning 2 8 =256 CIDs are available. However CID 0 is not used and CID from 2 to7 are reserved, so only from 8 to 255, 248 CIDs are actually provided for AAL2 user. E AAL2 = NCID PFrame /( NATMcell SATMcell) (2) where NCID denotes the number of CID, P Frame denotes the payload of frame in Table V, N ATMcell denotes the number of ATM cells, obtained by formula 4, S ATMcell denotes the size of ATM cell which is 5 octets. () P Codec = SCodec F Speech /8 50

3 where F Speech denotes the speech frame in Table V. NATMcell = ( HIuUP + HAAL2 + PCodec) NCID /( SATMcell HATMcell SFAAL2) (4) where H IuUP denotes the header of IuUP, H AAL2 denotes the header of AAL2, P Codec denotes the payload of Codec obtained from formula, H ATMcell denotes the header of ATM cell which is 5 octets, SF AAL2 denotes the start field of AAL2 obtained from Table IV. Then substituting the known parameters from Table II to V into the conditions in formula 1 to 4 to obtain BW Non- VAD=16.95kbps. With the VAD technique, the codec speed of a AMR_ Silence Descriptor (SID) = 1.8kbps, we obtain BW VAD =5kbps. BW VAD = SP E SID / AAL2 (5) where SP SID denotes the codec speed of AMR SID, obtained from Table V. So the BW Voice Channel is given by ( FVAD) + BWVAD FVAD BW Voicechannel = BWNonVAD 1 (6) where F VAD denotes VAD factor: the ratio of silence time in a call to the total time of call, Similarly the bandwidth of single channel for video call service is provided below BW Videochannel = SPVideo / EAAL2 (7) where SP video denotes the codec speed of video call, obtained from Table V. In Iu-CS interface, the major throughput is generated by voice service and video call service. At last the total bandwidth of Iu-CS interface is provided by PVoice ErlVoU / BH BWVoicechannel BWIuCS = NS F Re dudancy PVideo ErlViU / BHBW / + Videochannel (8) where N S denotes the number of G subscribers in RNC. P Voice denotes the percentage of subscribers using voice call to total subscribers. Normally it s 100%. P Video denotes the video call service penetration rate. Erl VoU/BH denotes the average voice call traffic in Erlang per user per busy hour. Erl ViU/BH denotes the average video call traffic in Erlang per user per busy hour. F Redundancy denotes redundancy factor which prevents the network from traffic overflow. Normally set it 0.7. B. Iu-PS Interface Iu-PS interface, logically a part of interface Iu, is between RNC and SGSN. Similar to Iu-CS interface, it also consists of user plane and control plane: AAL5 protocol is responsible for transporting the messages in both control and user plane. Signaling Connection Control Part (SCCP) is adopted to transfer signaling messages in control plane. The protocol stack of Iu-PS interface is shown below. TABLE VI. PROTOCOL STACK AND HEADER SIZE OF IU-PS INTERFACE User Plane Header Size (Octets) Iu-UP 4 GTP-U 12 UDP 8 IP 20 AAL5 ATM 5 Total 52 From Table VI, the Number of ATM Cell is given by SPacket + HIuUP + HGTP NATMCell = /(5 HATM ) HUDP HIP H AAL5 (9) in which S Packet denotes the average size of packet data, H IuUP denotes the header of Iu-UP packet which is obtained H IP denotes the header of IP packet which is obtained from Table VI, H AAL5 denotes the header of AAL5 packet which is obtained H ATM denotes the header of ATM cell which is obtained Finally we obtain the bandwidth of Iu-PS interface. BWIuPS = NS ThUser / BH ( NATMCell 5 / SPacket) 8 /( 600 F Re dundancy) (10) where N S denotes the number of G GPRS subscribers, Th User/BH denotes the average throughput per user per busy hour, N ATMCell denotes the number of ATM Cells. C. Summary of Section 2 Section 2 provides the algorithms of throughput for the Iu interface in UMTS CN. The algorithms for the other interfaces such as A, C, E, Gb, Gs, Gi, Gs and Gc interface are still the same with those in GSM/GPRS stage. In the control plane of Iu-CS and Mc interface, throughput of RANAP protocol may also be considered in dimensioning the CN topology. Section 2A for Iu-CS interface only considers the main sources of throughput. Throughput generated by RANAP may be accumulated onto the result of formula 8. III. CASE STUDY A mobile operator intends to build a new G UMTS CN in the area with heavy traffic loading to replace the legacy GSM systems. The plan is to provision one MSC Server to control three MGWs in the three areas with high traffic loading. Each MGW supports 100,000 G subscribers in its local area. MSS supports 00,000 G subscribers. The traffic model is shown in Table VII. Based on the formulas in section 2, we obtain the results below. 51

4 Figure 1. CN topology 75 Mbps BWIuCS1 BWIuCS2 BWIuCS Threshol d 15 Iu-CS Interface Throughput Trial 0 T1 T2 T T4 T5 T6 Busy Hour Figure 2. Iu-CS interface throughput trial 100% BWIuCS1 = 100, % = 66.79Mbps = BWIuCS 2 = BWIuCS / 0.7 TABLE VII. TRAFFIC MODEL Parameter Value Notes Network Volume 00,000 G subscribers Local 1 Volume 100,000 G subscribers Local 2 Volume 100,000 G subscribers Local Volume 100,000 G subscribers Redundancy factor 0.7 Range: Voice traffic per Sub at BH Decided by Video traffic per Sub at BH historical data, VAD Factor 0.5 engineering TrFO rate 100% experience or Video Call penetration rate 10% carriers BHCA Call per sub 1.5 request. Inter-office call rate 50% Call fail rate (Call fail tone played) 1% BHCA Handover per sub 0.5 BHCA Inter-officeHOper sub 0.1 Inter-office call rate 50% Figure 2 display the result from a trial which records the average throughput of three Iu-CS interface in 6 selected time frames (busy hour). It shows the real-time throughput in Iu-CS interface is below the designed threshold value: Threshold 1=66.79Mbps when F Redundancy =0.7. Threshold 2=58.44Mbps when F Redundancy =0.8. Threshold =51.95Mbps when F Redundancy =0.9. IV. SUMMARY AND CONCLUSION The current literatures introduced many applied methods and tools to plan and design G radio networks. Not much 52

5 effort, however, has been focused on the evolution of the core network. This paper illustrated the encapsulation, delivery and transport process of traffic and messages in UMTS CN. Based on the traffic flow and message flow, the algorithms and formulas to calculate the throughput for each interface and route are provided. Since some parts in the message packet are optional, the message size, header size and overhead size are suggested values in dimensioning the UMTS CN. The actual values vary from different vendor s equipments. While the deployment of UMTS radio access networks receives considerable attention, the UMTS core network has emerged as a critical element in the delivery of next generation mobile broadband services. As such, the algorithms provided in the paper are and will benefit mobile operators to address the issues in network dimension and plan while positioning them for future technologies. The study needs to be extended further to network dimensioning and planning towards R5 and on to R8 phase with IP Multimedia Sub-system (IMS) and System Architecture Evolution (SAE) NEs integrated into UMTS core network. REFERENCES [1] GPP TS , Technical Specification Group Radio Access Network: UTRAN Overall Description. [2] ITU-T I.6.2, B-ISDN ATM Adaptation Layer Specification: Type 2 AAL Series I: Integrated Services Digital Network- Overall Network Aspects and Functions-Protocol Layer Requirements. [] GPP TS , Technical Specification Group Radio Access Network: UTRAN Iu interface user plane protocols.. [4] GPP TS 25.41, Technical Specification Group Radio Access Network: UTRAN Iu interface Radio Access Network Application Part (RANAP) signaling. [5] ITU-T I.6.5, B-ISDN ATM Adaptation Layer Specification: Type 5 AAL - Series I: Integrated Services Digital Network Overall Network Aspects and Functions - Protocol Layer Requirements. [6] ITU-T I.66.2, AAL type 2 service specific convergence sublayer for narrow-band services. 5