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AbstractThis paper discusses the use of Carrier Ethernet for

the transport of UMTS radio access network as a alternativesolution for the gradual migration towards pure IP-based RAN.By means of Pseudo-Wire technique, the ATM service isemulated over the underlying Ethernet network. Within thiswork, the performance of such Carrier Ethernet based UTRANis evaluated and compared to the ATM-based UTRAN of UMTSRelease 99, in particular the transport efficiency, the delay and

packet losses of the Iub. Another contribution of this paper is toinvestigate the parameter settings to provide a guideline for theoptimum configurations.

Keywords-Carrier Ethernet, UTRAN, Iub, Pseudo-Wire, PWE3

I. INTRODUCTION

Third generation (3G) mobile communication systems, in

particular the Universal Mobile Telecommunication Systems

(UMTS), are expected to have an intensive growth in the next

few years caused by a continuously increasing number of

mobile subscribers and operative networks all over the world,

as well as by a dramatically growing traffic demand for data

applications like video streaming, web and multimediaservices. This in turn requires the Universal Terrestrial Radio

Access Network (UTRAN) to offer much higher transport

capacity supporting the evolved UMTS radio interface and

HSDPA (High Speed Downlink Packet Access) as well as

HSUPA (High Speed Uplink Packet Access) services [1][2].

But adding ATM capacity by leasing additional E1/T1 lines

leads to a linear increase of the operation expense. Therefore,

the usage of cost efficient IP as an alternative transport

technology is becoming a trend of UTRAN evolution. IP also

facilitates the integration of different radio access technologies

operating over a common IP backbone and therefore eases the

development of heterogeneous network access. However,substantial expenditures have been invested ATM-based

transport networks with numerous NodeBs with ATM-based

interfaces. Thus a smooth introduction of IP asks for a gradual

evolution towards IP. Therefore, an intermediate migration

solution is needed to integrate cost-efficient IP based transport

alternatives to reduce the cost per bit-rate within the radio

access network, and to allow backward compatibility and

interworking of RANs with different transport technologies.

In this context, Carrier Ethernet [3] has already established

itself as a very cost-effective way of addressing the rapidly

increasing bandwidth demands of new services. It is also a

viable solution of converged fixed-mobile access networks, as

well as a flexible and reliable way for enabling heterogeneous

access networks and "all IP" 3G mobile networks.

The deployment of Carrier Ethernet for UTRAN is realized

by establishing Pseudo-Wires in the backhaul network. This

technique is standardized by the IETFs Pseudo Wire

Emulation Edge-to-Edge (PWE3) working group defining

various types of Pseudo-Wires to emulate traditional and

emerging services such as ATM or frame relay over Packet

Switched Network (PSN) [4][5][6]. Despite many technical

advantages and low costs of implementing Carrier Ethernet as

transport in the UTRAN, there are two major performance

challenges that need further investigation [3]: (1) The delay is

often an issue of paramount importance in UTRAN networks,

not only due to its impact on service quality, but also because

some signaling and control protocols cannot tolerate additional

delay. The transport network must deliver the frame on time to

the base stations for transmission over the air, excessivelydelayed frames are discarded [8]. This leads to strict delay and

delay variation requirements on the UTRAN transport

network. (2) The QoS challenge in Ethernet networks is

mainly associated with the fact that Ethernet was designed as a

connectionless technology. Therefore, predefining a path for a

service, and pre-allocating bandwidth along this path is

considered impossible. Standard QoS mechanisms are possible

to prioritize between packets belonging to different traffic

classes, but this cannot really guarantee an end-to-end QoS.

Recently, many proposals have been made by mobile

operators to use Carrier Ethernet as the transport network in

UTRAN by means of establishing an Ethernet Pseudo-Wire.

However, to the best of authors knowledge, there have notbeen many investigations and analyses on the performance of

the Carrier Ethernet-based UTRAN, especially regarding its

transmission efficiency, delay QoS and requirement of the

transport network bandwidth compared to the ATM-based

UTRAN of UMTS Rel99. In the present work we study the

performance of Carrier Ethernet-based transport in UTRAN,

in particular the experienced transport delay, jitter and packet

loss, and achievable bandwidth efficiency on the Iub interface

(between RNC and NodeB) of single Pseudo-Wire scenario.

Xi Li1, Yongzi Zeng

1, Bjoern Kracker

2, Richard Schelb

2, Carmelita Goerg

1, Andreas Timm-Giel

1

1 Communication Networks, University of Bremen, Germanyemail: [xili | yzeng | cg| atg]@comnets.uni-bremen.de

2Nokia Siemens Networks GmbH & Co. KG, Germany

email: [bjoern.kracker | richard.schelb]@nsn.com

Carrier Ethernet for Transport in UMTS Radio

Access Network: Ethernet Backhaul Evolution

978-1-4244-1645-5/08/$25.00 2008 IEEE 2537

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The analysis is carried out from simulations under a high-

loaded mixed voice and data traffic scenario, and compared to

the ATM-based UTRAN of UMTS R99. Usually the network

operators can tune a number of parameters in Pseudo-Wire to

control the end-to-end delay and jitter. In this paper, the

impact of different parameter settings is also investigated and

hence the optimum PWE configuration is determined.

The remainder of the paper is organized as follows: SectionII introduces the Carrier Ethernet-based UTRAN structure and

protocol stacks. Section III addresses the major performance

metrics and QoS criteria. Section IV describes the applied

traffic model and simulated network scenario. Section V

presents the simulation results and the performance analysis.

The end conclusions the paper and discusses the future work.

II. CARRIER ETHERNET-BASED UTRANNETWORK

A. Network Structure and Protocol Stack

According to the PWE3 reference model in IETF draft [5],

the network structure of UTRAN is reorganized to deploy the

Pseudo-Wire replacing the conventional ATM transportnetwork layer (TNL) with an Ethernet network, as illustrated

in Figure 1. Both the NodeB and RNC are Customer Edges

(CEs), which are not aware of using an emulated ATM service

over Ethernet. The NodeB and RNC are connected to the

transport Ethernet network via two intermediate PWE capable

routers (ATM_IP_router) which contain dual interfaces for

ATM and Ethernet. Such routers are located at the edge of the

ATM network and the Ethernet network, hence also called as

Provider Edges (PEs), which establish a tunnel emulating

ATM service over the Ethernet network for the corresponding

CEs. Between these routers, an Ethernet Pseudo-Wire is

established. ATM cells coming from CEs will be encapsulated

into Ethernet PDUs within the routers and then carried acrossthe underlying Ethernet network. After the Ethernet packets

arrive at the egress port of the Ethernet network, they are

decapsulated into the ATM cells and then forwarded to their

destination. Figure 1 also shows the involved protocol layers.

At the user plane of the RNC or NodeB, higher layer data

entering the UTRAN, e.g. packets of speech data from an

AMR codec, is carried via the Frame Protocol (FP) PDUs

through the Iub interface. These FP PDUs are segmented into

AAL2 packets and transmitted as ATM cells to the ATM

links. In the ATM-IP router, the ATM cells are received from

either RNC or NodeB through an ATM Virtual Circuit (VC).

At the ATM interface of the router, the ATM cells are

captured and delivered to the PWE layer. Here the ATM cells

are concatenated into a PWE payload and PWE protocol

overheads and control information (e.g. specifying the ATMservice to be emulated in this case) are added. The

encapsulated PWE frames are then sent downwards through

UDP, IP and Ethernet. At last Ethernet packets are created and

transmitted via the Ethernet link to other side. A reverse

process occurs at the router of the other end, where PWE

payloads are retrieved and the carried ATM cells are extracted

and sent via the ATM link to destination node.

B. PWE Parameters

Pseudo-Wire solutions allow network operators to control

two parameters that can affect the PWE frame size and

resultant delay and jitter. Nc: maximum number of ATM cells

allowed to be concatenated into one PWE frame; Tc:maximum waiting time for the concatenation of ATM cells

into a PWE frame; this determines the maximum waiting time

if less than Nc PDUs are in the buffer. Each Ethernet frame

includes a dedicated header, so a large setting of Nc and Tc

minimizes the overhead per service PDU, which in turn results

in higher efficiency. Nevertheless, the larger these parameters

are configured, the higher is the additional delay and delay

variance inferred by PWE. This can be seen in Figure 2. Thus,

by means of these parameters the overhead and the resulting

quality impact have to be carefully balanced to achieve a

suitable Iub delay not exceeding its inherent delay boundary.

III. PERFORMANCE METRICS AND QOS CRITERIAS

A. Delay

As mentioned above, UTRAN has to fulfill stringent delay

and delay variation requirements in order to assure the QoS

according to the types of services as well as to protect the

delay-intolerant signaling and control protocols. As shown in

Figure 1, the major PWE delay contributors at the Iub include:

Figure 1: Protocol Stack of Carrier Ethernet- ased UTRAN using PWE

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PWE Concatenation Delay: the time consumed by theconcatenation process, where the ATM cells areencapsulated into PWE frame in PWE concatenation buffer.This delay depends on the inter-arrival time of ATM cellsas well as the setting of Nc and Tc.

PWE Encoding Delay: the time after a complete PWEframe is formed until the delivery of the PWE packets tothe Ethernet link. This delay consists of two parts: (1) The

processing time for encoding a PWE packet to an Ethernetpacket. (2) The time spent waiting in the Ethernet bufferuntil the Ethernet packet is transmitted. This is directlyrelated to the Iub bandwidth on the Ethernet link.

Ethernet Switch Delay: the time spent within theintermediate Ethernet Switch between two routers.

Additionally, the FP PDU Delay needs to be measured. It is thedelay to transport one FP PDU from the NodeB to the RNCover the TNL. It includes the delay of the ATM part as well asthe above mentioned three PWE delay components. For eachdelay, the average delay and jitter are observed in thesimulations. The jitter is defined as the deviation between themaximum and minimum delay of every 100ms.

2 4 6 8 102

2.5

3

3.5

4

4.5

5

Delay(ms)

Tc (ms)

Impact of Tc

2 4 6 8 100.3

0.35

0.4

0.45

0.5

0.55

0.6

BandwidthEfficiency

bandwdith efficiency

delay

2 4 6 8 10 12 14 16 18 20 220

2

4

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8

Delay(ms)

Nc

Impact of Nc

2 4 6 8 10 12 14 16 18 20 220.3

0.35

0.4

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0.5

0.55

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BandwidthEfficiency

bandwidth efficiency

delay

Figure 2: Impact of Nc and Tc on the delay and efficiency

B. Bandwidth Efficiency

The Bandwidth Efficiency is defined as the Information-to-

Carrier Packet Length ratio. For the ATM-based UTRAN, this

is upper layer IP throughput over the ATM link throughput;

and for the PWE-based UTRAN it is the IP throughput over

Ethernet link throughput. The effective bandwidth utilization

in the RAN, especially on the last mile links towards theNodeBs, is directly related to the transport costs. High

bandwidth efficiency reduces the RAN expenses.

C. Packet Loss

Due to the need for synchronous data transfer in the

UTRAN downlink, excessively delayed FP PDUs will be

discarded at the NodeB, as they cannot be sent over the air

interface during the allocated time slot. It is an indication of

congestion of the Iub link. Following the packet loss ratio is

also referred to as delayed FP PDU ratio.

D. Throughput

The Throughput refers to the throughput of the physical

link. For the ATM-based UTRAN, the ATM link throughput

is measured. In Carrier Ethernet-based UTRAN the

throughput is measured for both ATM link and Ethernet link.

E. QoS Criteria of UTRAN

In the present work, the acceptable FP PDU delay value

considered for 99% of the transmissions for voice service is

10ms and 30ms for data services. Moreover, for both, voice

and data services, the FP PDU delay jitter should be less than

15ms for 99% of the transmissions. These requirements need

to be satisfied to determine the required Iub link bandwidth.

IV. SIMULATION DESCRIPTION

A. Simulation Scenario

In this paper, we study only single Pseudo-Wire scenario as

shown in Figure 1. It consists of 1 NodeB, 1 RNC and 2

ATM_IP routers for interworking with ATM and IP dual-stack, within which the PWE en- and decapsulation algorithms

are implemented. Between the two routers, an Ethernet switch

is used. In this scenario, two Ethernet links with a link-rate of

1Gbps and 100Mbps are configured for RNC and NodeB side

respectively, but the leased Ethernet bandwidth for the Iub

interface is 2.09 Mbps. Both RNC and NodeB are connected

to their ATM-IP routers via a 2Mbps ATM E1 link. The

reserved Iub bandwidth at the Ethernet part is set to a slightly

higher value than the bandwidth for ATM in order to

incorporate a margin for the overheads of PWE.

B. Traffic Model

In this paper, a mix of data and voice traffic is evaluated. Forthe packet switched data services (web browsing) the ETSI

traffic model [9] and for voice services a traffic model

specified in the MOMENTUM project [10] is used.

Traffic Model: web application

Parameter Distribution and value

Reading Time Geometric distribution

Mean = 5 second

Page Size Pareto distribution

Mean = 25 Kbyte

Traffic Model: voice

Parameter Distribution and value

Voice Codec Adaptive Multi Rate (AMR)

Silence / Speech period Exponential distribution

Mean = 3 secondSession duration Exponential distribution

Mean = 120 second

Table 1: Traffic Model

For web application a packet switched Radio Access Bearer

(RAB) of 64kbps is used on both uplink and downlink, andvoice is transmitted with AMR and circuit switched RAB of

12.2 kbps. In this configuration, there is a strict AAL2 priority

for voice over web traffic at the RNC and NodeB to guarantee

the delay requirements of the voice users.

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V. PERFORMANCE ANALYSIS

In this section the results of the performance analysis of

Carrier Ethernet-based UTRAN under a PS dominant traffic

scenario are presented. They include the resultant delay, jitter

and transport efficiency on the Iub interface and compare it to

the ATM-based UTRAN. The capacity of the NodeB is

2Mbps supporting three cells each serving around 8 voice

users and 12 web users. The generated ATM link throughputis around 1.4Mbps and the total traffic load consists of 82%

web traffic. Different PWE parameter settings (Nc, Tc) are

investigated to determine the optimum PWE configuration.

.

(27, 15ms ) (22, 8ms ) (16, 5ms ) (9, 3ms )1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

(Nc, Tc)

Throughput(Mbps)

Link Througjput vs. (Nc, Tc)

ATM:ATM Link Throughput

PWE:ATM Link Throughput

PWE:Ethernet Link Throughput

Figure 3: Link Throughput

Figure 3 shows the link throughput over different (Nc,Tc)

combinations. It can be seen that with the decrease of (Nc,Tc)

settings the Ethernet link throughput in the Carrier Ethernet-

based UTRAN is increased, whereas the ATM link throughput

maintains constant as it is not influenced by the PWE

configurations. The gap between the ATM link throughput and

Ethernet link throughput represents the overhead of the

transport network layer. The increased Ethernet link

throughput by smaller (Nc, Tc) is because that more PWEoverhead are generated to carry the same amount of user data

since smaller Nc and Tc settings lead to smaller PWE frame

sizes. The corresponding bandwidth efficiency is compared in

Figure 4. The efficiency of the Carrier Ethernet-based

UTRAN is in general lower than the ATM based, independent

of the (Nc, Tc) settings. This is due to the extra PWE and

Ethernet overheads. Besides, while Nc and Tc decrease, the

efficiency of Carrier Ethernet-based UTRAN declines as a

consequence of larger overheads (refer to Figure 2). However,

the average FP PDU delays of web traffic and voice are

significantly improved when the Nc and Tc settings are

decreased, as illustrated in Figure 5. Because with lower Tc

and Nc values, the number of concatenated ATM cells and themaximum encapsulation time are lower, the PWE

concatenation delay is reduce being the major contributor of

the FP PDU delay. It is also visible that the FP PDU delay in

the ATM-based UTRAN is lower than in the PWE

configuration. The extra delay for PWE is due to additional

protocol overheads for the encapsulation of ATM cells

requiring a longer transmission delay; in addition extra

concatenation and switching delays are generated in the

routers and Ethernet switch.

(27,15ms) (22, 8ms) (16, 5ms) (9, 3ms)50

55

60

65

70

75

80Bandwidth Efficiency

(Nc, Tc)

B

andwidthEfficiency(%)

ATM

Carrier Ethernet

Figure 4: Bandwidth Efficiency

(27,15ms) (22, 8ms) (16, 5ms) (9, 3ms)0

1

2

3

4

5

6

7

8

9

10

11

12

13

14Average FP PDU delay

(Nc, Tc)

FP

PDUdelay(ms)

ATM (web)

Carrier Ethernet (web)

ATM (voice)

Carrier Ethernet (voice)

Figure 5: Average FP PDU Delays

Figure 6 depicts the ratio of delayed FP PDUs that needed

be discarded at the base station in the Carrier Ethernet-based

UTRAN. It is observed that with decreased (Nc, Tc) values,

the delayed FP PDUs of web traffic increases, though the

average FP PDU delay is reduced (Figure 5). The reason is

that smaller values of Nc and Tc cause a higher Ethernet linkthroughput as seen in Figure 3. With higher link throughput, it

is more probable to experience the link congestion which leads

to abrupt delay variations at the transmission buffer of the

Ethernet layer, where the shaping rate for the Iub interface is

restricted to 2.09Mbps in our scenario. This contributes to a

larger PWE encoding delay variation. On the other hand, as

voice traffic has higher priority than web traffic, the

experienced FP PDU delay variations for voice is much

smaller and therefore it obtains much lower delayed FP PDU

ratio than web traffic. The extreme case of voice is under large

(Nc, Tc) setting (Nc=27, Tc=15ms). In this case, there is a

significantly larger delayed FP PDU ratio. The reason is that

the configured Tc of 15ms is longer than the voice delayconstraints of 10ms. Hence with Nc of 27 the PWE3 needs to

wait for 27 ATM cells to be encapsulated into a PWE frame

before the timer expires. If the offered traffic is not able to

always assure 27 ATM cells to arrive for the concatenation

before the timer expires, the timeout probability is relatively

high. So there is a higher chance for a longer concatenation

delay which leads to a relatively higher probability of

instantaneous FP PDU delay larger than 10ms. So in general it

is suggested to set Tc below the delay constrains required at

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the Iub. Another important delay QoS criterion is jitter. In this

investigation, the FP PDU delay jitter of both voice and data

services should be less than 15ms. Figure 7 shows the ratio of

FP PDU delay jitter exceeding 15ms boundary. As it can be

seen, the FP PDU delay jitter of voice is lower than that of

web traffic, not only because it has a smaller FP PDU size but

also because it has higher priority in the transmission. For both

web and voice traffic, the experienced jitter is decreased withlower Tc and Nc, as smaller Tc and Nc settings reduce the gap

between min. and max. values of the PWE concatenation

delay. Here the rise of jitter for voice at (Nc=9, Tc=3ms) is

mainly caused by a higher PWE encoding delay variation as a

result of the increased Ethernet link throughput. In the ATM-

based UTRAN the obtained delayed FP PDU ratio and the FP

PDU delay jitter are lower than with the Carrier Ethernet-

based UTRAN. This is due to the lower transport overhead in

the ATM transport and the resulting lower link throughput.

With smaller ATM cells the traffic is additionally less bursty.

(27, 15ms) (22, 8ms) (16, 5ms) (9, 3ms)0

0.5

1

1.5

2

2.5

(Nc, Tc)

delayedFPPDUratio(%)

delayed FP PD U ratio (Carrier Etherent)

web traffic

voice

Figure 6: Delayed FP PDU ratio

(27, 15ms) (22, 8mms) (16, 5ms) (9, 3ms)0

2

4

6

8

10

12

(Nc, Tc)

ratioofFPPDUdelayjitter>15ms(%)

ratio of FP PDU delay jitter > 15ms (Carrier Ethernet)

web traffic

voice

Figure 7: FP PDU Delay Jitter

VI. CONCLUSION

In this paper, we discuss the deployment of Carrier Ethernet

for the transport network in UTRAN, and give a quantitative

evaluation on its performance, in particular the experienced

delay, jitter and packet loss, and achievable bandwidth

efficiency on the Iub interface. Simulation results show that

using Carrier Ethernet by means of Pseudo-Wire (PWE) to

replace the ATM-based transport in UTRAN, the achieved

transmission efficiency becomes lower due to the extra PWE

and Ethernet overheads. In addition, the delays and jitters

experienced at the Iub interface tend to increase as a result of

larger transport packet format as well as the extra

concatenation and switching delays in the intermediate routers

and switches. However, by adjusting PWE configurations it is

possible to find a tradeoff between efficiency and the delay

performance in the Carrier Ethernet-based UTRAN. From thepresent investigations, it is found that the larger setting of Nc

and Tc can result in higher bandwidth efficiency but also

induce longer average delay and larger jitter. While with small

values of (Nc,Tc) pairs, larger PWE overhead will be

generated which leads to higher link throughput and in turn

increased probability of congestion. This causes more FP

PDUs discarded. Therefore, the setting of Nc and Tc should be

configured in a medium range, not too large and also not too

small. The network operation has to consider which factor is

more important to the specific traffic scenario, is it more

efficiency-critical or time-critical, in order to decide the

optimal (Nc,Tc) pair. Besides, the Ethernet bandwidth should

be configured higher than the ATM link for accommodatingthe additional PWE and Ethernet overheads. In this work we

only study single Pseudo-Wire scenario. Further work will

consider network scenario of multiple NodeBs where either

each NodeB sets up individual Pseudo-Wire or a group of

NodeB share a common Pseudo-Wire to the RNC, in this case

PWE encoding delay and Concatenation Delay are strongly

dependent on the number of Pseudo-Wires and how many

NodeBs served by one Pseudo-Wire.

ACKNOWLEDGMENT

This work is carried out within the research project Mature(Modeling and Analysis of the Transport Network Layer in theUTRAN Access Network REsearch). The partner of this workis the Nokia Siemens Networks GmbH & Co. KG, Germany.

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Wireless Europe, January 2006[3] Converging an ALL IP mobile transport on Carrier Ethernet networks,

White Paper by Siemens Communication Fixed Networks Access, 2006.

[4] IETF RFC 3916: Requirements for Pseudo-Wire Emulation Edge-to-Edge(PWE3), September 2004[5] IETF RFC 3985: Pseudo Wire Emulation Edge-to-Edge (PWE3)

Architecture, March 2005[6] IETF, RFC 4717: Encapsulation Methods for Transport of Asynchronous

Transfer Mode (ATM) over MPLS Networks, December 2006[7] Steve Byars, Using Pseudo-Wires for Mobile Wireless Backhaul over

carrier Ethernet, Blueprint Metro Ethernet, 13 February 2006[8] 3GPP TS 25.402, Synchronization in UTRAN state 2

[9] ETSI, Universal Mobile Telecommunications System (UMTS): Physicallayer aspects of UTRA High Speed Downlink Packet. 3GPP TR 25.848v3.2.0, 2001-03

[10] IST-2000-28088 Project: Models and Simulations for Network Planningand Control of UMTS (MOMENTUM)

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