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 youyang@stevens.edu

M. Hosein Fallah, Ph.D, P.E. Howe School of Technology Management

Stevens Institute of Technology Hoboken, NJ, USA

hfallah@stevens.edu

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 3G 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

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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.

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 [3] 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.

TABLE I. IU-CS UMTS PROTOCOL STACK

Radio Network Control Plane

Transport Network Control Plane

Circuit Switching Data User Plane

CS Voice User Plane

MM/SM/CC Application

AMR Codec

RANAP

TAF ALCAP RLP

SCCP STC

Iu UP MTP3-D MTP3-D

SSCF NNI SSCF NNI 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=8+6+8+5=24 bits=3 bytes=3 Octets. In addition, the ATM cell is 53 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 (200-3400 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.

TABLE IV. OVERHEAD OF PROTOCOLS IN IU-CS INTEFACE

Iu-UP Overhead

AAL2

Start Field

AAL2

Header

ATM

Header

ATM

Cell

AMR Payload (at 12.2 kbps)

G.711 Payload

(at 64kbps)

Size (Octet

s)

4 1 3 5 53 31 40

TABLE V. CODEC PARAMETERS

Codec Type Codec Speed (kbps)

Payload per Frame (Octets)

Speech Frame (ms)

AMR. Type 7 12.2 31 20 AMR_SID. Type

8 Not a fixed value 5 160

G.711 64 40 5 Video Type, e.g.

G.729 64 40 5

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

2/ AALAMRNonVAD ESPBW = (1)

where SPAMR denotes the codec speed of AMR, obtained from Table V, EAAL2 denotes the efficiency of AAL2 encapsulation. It is given by formula 2 below.

From Table II, Channel Identification is 8 bits, meaning 28=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. )/(2 ATMcellATMcellFrameCIDAAL SNPNE ××= (2) where NCID denotes the number of CID, PFrame denotes the payload of frame in Table V, NATMcell denotes the number of ATM cells, obtained by formula 4, SATMcell denotes the size of ATM cell which is 53 octets.

8/SpeechCodecCodec FSP ×= (3)

where FSpeech denotes the speech frame in Table V. ( )

)/( 2

2

AALATMcellATMcellCID

CodecAALIuUPATMcell

SFHSNPHHN

−−×++=

(4) where HIuUP denotes the header of IuUP, HAAL2 denotes the header of AAL2, PCodec denotes the payload of Codec obtained from formula 3, HATMcell denotes the header of ATM cell which is 5 octets, SFAAL2 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 BWNon-

VAD=16.95kbps. With the VAD technique, the codec speed of a AMR_

Silence Descriptor (SID) = 1.8kbps, we obtain BWVAD=5kbps.

2/ AALSIDVAD ESPBW = (5) where SPSID denotes the codec speed of AMR SID, obtained from Table V.

So the BWVoice Channel is given by ( ) VADVADVADNonVADelVoicechann FBWFBWBW ×+−= 1

(6) where FVAD 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

2/ AALVideoelVideochann ESPBW = (7) where SPvideo 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

dudancyelVideochannBHViUVideo

elVoicechannBHVoUVoiceSIuCS F

BWErlPBWErlP

NBW Re/

//⎟⎟

⎠

⎞⎜⎜⎝

⎛×+

××=

(8) where NS denotes the number of 3G subscribers in RNC. PVoice denotes the percentage of subscribers using voice call to total subscribers. Normally it’s 100%. PVideo denotes the video call service penetration rate. ErlVoU/BH denotes the average voice call traffic in Erlang per user per busy hour. ErlViU/BH denotes the average video call traffic in Erlang per user per busy hour. FRedundancy 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 3 ATM 5 Total 52

From Table VI, the Number of ATM Cell is given by

)53/(

5ATM

AALIPUDP

GTPIuUPPacketATMCell H

HHHHHS

N −⎟⎟⎠

⎞⎜⎜⎝

⎛

+++++

=

(9) in which SPacket denotes the average size of packet data, HIuUP denotes the header of Iu-UP packet which is obtained from Table VI, HIP denotes the header of IP packet which is obtained from Table VI, HAAL5 denotes the header of AAL5 packet which is obtained from Table VI, HATM denotes the header of ATM cell which is obtained from Table VI,

Finally we obtain the bandwidth of Iu-PS interface. ( )

( )dundancy

PacketATMCellBHUserSIuPS

FSNThNBW

Re

/

3600/8/53

×××××=

(10) where NS denotes the number of 3G GPRS subscribers, ThUser/BH denotes the average throughput per user per busy hour, NATMCell 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 3G 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 3G subscribers in its local area. MSS supports 300,000 3G subscribers. The traffic model is shown in Table VII. Based on the formulas in section 2, we obtain the results below.

Figure 1. CN topology

Figure 2. Iu-CS interface throughput trial

32

3

3

1

79.66

7.0/1085005.0%10

1017025.0%100000,100

IuCSIuCS

IuCS

BWBWMbps

BW

===

⎟⎟⎠

⎞⎜⎜⎝

⎛

×××+

××××=

TABLE VII. TRAFFIC MODEL

Parameter Value Notes Network Volume 300,000 3G subscribers Local 1 Volume 100,000 3G subscribers Local 2 Volume 100,000 3G subscribers Local 3 Volume 100,000 3G subscribers

Redundancy factor 0.7 Range: 0.7-1 Voice traffic per Sub at BH 0.025 Decided by

historical data, engineering

experience or carriers’ request.

Video traffic per Sub at BH 0.005 VAD Factor 0.5 TrFO rate 100%

Video Call penetration rate 10% BHCACall per sub 1.5

Inter-office call rate 50% Call fail rate (Call fail tone played) 1%

BHCAHandover per sub 0.5 BHCAInter-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 FRedundancy=0.7. Threshold 2=58.44Mbps when FRedundancy=0.8. Threshold 3=51.95Mbps when FRedundancy=0.9.

IV. SUMMARY AND CONCLUSION The current literatures introduced many applied methods

and tools to plan and design 3G radio networks. Not much

0

15

30

45

60

75

T1 T2 T3 T4 T5 T6

Iu-CS Interface Throughput Trial

BWIuCS1

BWIuCS2

BWIuCS3

Threshold3

Mbps

Busy Hour

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] 3GPP TS 25.401, Technical Specification Group Radio Access

Network: UTRAN Overall Description. [2] ITU-T I.363.2, B-ISDN ATM Adaptation Layer Specification: Type

2 AAL Series I: Integrated Services Digital Network- Overall Network Aspects and Functions-Protocol Layer Requirements.

[3] 3GPP TS 25.415, Technical Specification Group Radio Access Network: UTRAN Iu interface user plane protocols..

[4] 3GPP TS 25.413, Technical Specification Group Radio Access Network: UTRAN Iu interface Radio Access Network Application Part (RANAP) signaling.

[5] ITU-T I.363.5, 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.366.2, AAL type 2 service specific convergence sub-layer for narrow-band services.