Integrating SDH and ATM in UMTS (3G) Access Networks

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Horsebridge Network Systems Ltd, 1 Pate Court, North Place, Cheltenham, GL50 4DY England

Tel:+44 (0)1242 530630 Fax: +44 (0) 1242 530660 E-Mail info@horsebridge.net www.horsebridge.net

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White Paper

December, 2008

The information contained in the documentation and/or disk is proprietary and is subject to all relevant copyright, patent, and other laws protecting intellectual property, as well as any specific agreement protecting ECI Telecom's rights in the aforesaid information Neither this document nor the information contained in the documentation and/or disk may be published, reproduced, copied, modified or disclosed to third parties, in whole or in part, without the express prior written permission of ECI Telecom In addition, any use of this document, the documentation and/or the disk, or the information contained therein for any purposes other than those for which it was disclosed, is strictly forbidden ALL RIGHTS NOT EXPRESSLY GRANTED ARE RESERVED BY ECI TELECOM

Any representation(s) in the documentation and/or disk concerning performance of ECI Telecom product(s) are for informational purposes only and are not warranties of product performance or otherwise, either express or implied ECI Telecom's standard limited warranty, stated in its sales contract or order confirmation form, is the only warranty offered by ECI Telecom

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CONTENTS

Contents

Introduction 7

Role of ATM in 3G Access Networks 8

Introduction of ATM Switching into the Access Network 8

STM-1 Interfaces in the RNC 8

Savings in Bandwidth 8

Lower Bandwidth Consumption 8

Granularity of Bandwidth Allocation 9

Statistical Multiplexing Based on Peak vs Sustained Rate 9

Multiplexing Based on Usage Statistics 10

Higher Savings 10

Deploying a 3G Access Network 11

Deployment over Pure TDM Transmission 11

Co-location of ATM Switches and RNCs 12

ATM Concentration Devices in the Access Network 12

The Cost of Using ATM Switches in the Access 13

The Extra Costs of Maintaining IMA Groups 14

The Dilemma 14

ECI Telecom’s Solution for 3G Access Networks 15

The XDM Architecture 15

The ATS (ATM Traffic Switch) Concept 16

Traffic Concentration from Several Node Bs into One Unchannelized VC-4 17

Advantages of the ATS vs a Standalone ATM Switch 18

Savings in Equipment 18

Operational Savings 19

IMA Flexibility 19

Cost Flexibility 20

The XDM ATS Card as a Node B Concentrator 22

Canonical Concentration of Node B Traffic into VC-4s 22

Application Scalability 23

Sparse Deployment of Node Bs 23

Increased Bandwidth Demand 24

Savings on Intermediate Bandwidth 24

Combined VC-4 and IMA Aggregation 25

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CONTENTS

Integration of 2G and 3G Traffic 26

TDM Multiplexing of 2G and 3G Traffic 26

Conclusion 27

About ECI Telecom 28

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CONTENTS

List of Figures

Figure 1: TDM-based access network 11

Figure 2: ATM switch co-located with the RNCs 12

Figure 3: ATM switches in the access network 12

Figure 4: ATM concentration with an external ATM switch 13

Figure 5: Schematic view of the XDM architecture 15

Figure 6: ATS card architecture 16

Figure 7: Concentration of 72 E1s into a single VC-4 17

Figure 8: Concentration of E1s into VC-4s 22

Figure 9: Configuration for a low number of Node Bs 23

Figure 10: Configuration for increased bandwidth demand 24

Figure 11: Concentration of Node B traffic into IMA groups 24

Figure 12: Combination of VC-4 and IMA concentration 25

Figure 13: TDM multiplexing of 2G and 3G data 26

List of Tables Table 1: Two-layer implementation versus integrated implementation 18

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CONTENTS

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INTRODUCTION

Introduction

The deployment of cellular UMTS (Universal Mobile Telecommunications

Systems, better known as 3G) is one of the most difficult challenges facing service

providers’ network planning experts today They must juggle immature

technologies, limited financial resources and uncertain future market demand

Furthermore, they must reduce capital and operational expenses and keep network

costs to a minimum in order to make 3G services economical while still providing

for network upgrades on demand Since the actual demand for 3G services is still

unknown and network design must provide a cost-effective solution for both

optimistic and pessimistic scenarios, cost structures must be flexible

Given the uncertainties of the services to be offered, bandwidth demand,

applications, and so on, networks must be as cost-effective as possible in their

initial, low-level usage phase Equipment costs, as well as expenditure on leased

bandwidth and radio frequencies, must be kept to a minimum, yet allowing these

networks to provide for fast growth and cost-effective bandwidth increase

3G access networks are based on two distinct technologies: transmission and ATM

Conventional 3G access infrastructures implement these technologies over two

separate network layers Although network design is simple, it is expensive and

inflexible

In line with its tradition of responding to customer needs, ECI Telecom’s Optical

Networks Division offers an innovative concept: integration of SDH and ATM in

the same hardware fully optimized for 3G access networks ECI Telecom’s

solution is not only far more economical than any other solution on the market

today; it is also flexible and scalable, providing for future expansions in network

coverage and capacity

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ROLE OF ATM IN 3G ACCESS NETWORKS

Role of ATM in 3G Access Networks

A cellular access network connects Node Bs to RNCs (Radio Network Controllers)

via the Iub interface The Iub interface is a complex set of protocols handling all

aspects of Node B-to-RNC communications, including media, signaling, and OAM

(Operation, Administration, and Maintenance) over ATM ATM in turn can be

transported over various TDM links

In practice, most Node B connections range from a fractional E1 to several E1s

bundled as an ATM IMA (Inverse Multiplexing over ATM) group RNC

connections are usually either E1s or STM-1s

Early releases of the 3G standard defined the Node B-to-RNC connection as purely

a TDM connection In the ATM layer, Node Bs and RNCs were connected via a

direct ATM link, without intermediate ATM switching The definition provides the

following functions:

Independence of the underlying transmission layer

Definition of groups of several TDM links as one logical link using the ATM

IMA mechanism

Ability to carry voice and data over the same link

Implementation of statistical multiplexing between different applications on

the same Node B while maintaining QoS (Quality of Service)

Introduction of ATM Switching into the Access Network

Release 4 of the 3G standards formally stipulated how to perform ATM switching

in the access network, and how to provide the QoS guarantees required for the

successful operation of 3G applications

ATM switching in the access network provides two major advantages:

The ability to configure RNCs with STM-1 interfaces instead of E1s, thus

drastically reducing the cost of the RNC

Savings in bandwidth consumption

STM-1 Interfaces in the RNC

Current deployments demonstrate that it is not economical to deploy E1 links in the

RNC STM-1, on the other hand, has proved to be a far less expensive solution,

even with the cost of intermediate ATM switching

Savings in Bandwidth

ATM switching in the access supports ATM concentration, providing finer

granularity and statistical multiplexing benefits This results in savings in the

network bandwidth requirements

Lower Bandwidth Consumption

ATM switching reduces bandwidth consumption, thus saving operating costs The

following sections describe how to attain these savings

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Granularity of Bandwidth Allocation

In a TDM-based network, the link between the Node B and the RNC has a

granularity of E1 Although fractional E1 connections are feasible, these are

usually reserved for sub-E1 rates

This bandwidth allocation is part of the basic design of the Node B However,

ATM concentration in the access network can improve bandwidth utilization For

example, if the peak traffic to/from a Node B is estimated to be 3 Mbps, then two

E1 interfaces (4 Mbps) must be allocated to the Node B at the TDM level On the

other hand, an ATM switch concentrating traffic from 10 such Node Bs can

concentrate from 40 Mbps (10 x 2 x E1) to 30 Mbps (10 x 3 Mbps, or only 15 E1s)

without violating the basic bandwidth allocation rule of 3 Mbps per Node B

Statistical Multiplexing Based on Peak vs Sustained Rate

An ATM link can contain many ATM virtual circuits, each with its own

parameters The main parameters are peak cell rate and sustained cell rate

The peak rate controls the maximum permissible cell rate, whereas the sustained

rate is the average connection rate A Node B may transmit at the peak rate for a

short period of time only (controlled by the maximum burst size), which is

typically lower than 50 milliseconds Over longer intervals, traffic must be

controlled by the sustained cell rate, typically much lower In the real world, only a

few Node Bs transmit at the peak rate, whereas the majority transmits at the

sustained rate

ATM concentration in the access layer enables maintaining the peak rate of the

connection at a high level, thus ensuring short delays As the number of Node Bs

transmitting concurrently at the peak rate can be statistically bounded, bandwidth

must be allocated for the sustained rate for all Node Bs, with the peak rate allocated

to only some As a result, bandwidth consumption is significantly lower

Obviously, the possibility exists (though chances are extremely low) that all Node

Bs send a burst of traffic simultaneously with the resulting loss of ATM cells This

can easily be computed based on the ATM policing and shaping mechanisms, thus

guaranteeing cell rate in compliance with 3G standards

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Multiplexing Based on Usage Statistics

Bandwidth allocation per Node B is based on the maximum concurrent bandwidth

demanded by users served by the specific Node B While it is desirable to provide

full service to all users in any scenario, this is economically impossible

As with any mass service, statistical assumptions about overall usage can safely be

made For example, in GSM (Global System for Mobile Communication, also

known as 2G) voice-based deployments, network design assumes that not all

subscribers will make a call at the same time If they do, some will be rejected, as

network capacity planning takes into consideration the distribution of user

demands

3G services are subject to the same design considerations In effect, due to the

bursty nature of data, network planning must rely on the statistical nature of usage

patterns Unlike ATM statistical multiplexing (which allows users to send high rate

traffic over short periods of time and then forces them to reduce the rate), usage

statistical multiplexing is based on the assumption that not all subscribers use the

network concurrently Consequently, this multiplexing method may vary with

changes in usage patterns As it is the nature of data to adapt the application to the

available bandwidth, usage-based multiplexing can be implemented even if the

service level is sometimes degraded

Higher Savings

Reducing bandwidth consumption is always a recommended approach However,

depending on network structure and design, the rationale behind this reduction

varies from service provider to service provider

When using leased-lines or licensing radio frequencies to build a network, lower

bandwidth consumption obviously translates into direct savings in operational

expenses This reduction involves more than only the monthly costs of leasing the

lines and radio frequencies When bandwidth consumption is reduced, the entire

access network becomes smaller Service providers can then manage a smaller

transmission network with less expensive interfaces, less equipment cards, and less

manpower

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DEPLOYING A 3G ACCESS NETWORK

Deploying a 3G Access Network

A 3G cellular access network can be deployed in one of the following

configurations:

RNCs with E1 ports connected to the Node Bs via a pure-TDM transmission

network

RNCs with STM-1 ports and Node Bs with E1 ports In this configuration,

ATM switches deployed along the connection convert E1s originating in the Node Bs to STM-1s, by:

Co-locating ATM concentration devices with the RNC

Placing ATM concentration devices inside the access network The following sections describe the advantages and disadvantages of each

approach

Deployment over Pure TDM Transmission

ATM switching in the access network is recommended, but it is not technically

mandatory It is possible to build a network connecting an E1 port from a Node B

directly to an E1 port from the RNC This approach, however, lacks the advantage

of using STM-1 ports in the RNC and ATM concentration in the network that

results in savings in bandwidth and network costs

Figure 1: TDM-based access network

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Co-location of ATM Switches and RNCs

A second alternative is to deploy an ATM switch co-located with the RNC In this

scenario, the access network carries TDM connections from the Node Bs to the

ATM switch; the switch concentrates E1s into a single STM-1, which in turn is

connected to the RNC

RNCs can thus be configured with STM-1 ports, resulting in a more economical

network structure However, bandwidth consumption in the access network is still

based on the peak demand of every Node B, without ATM concentration

Figure 2: ATM switch co-located with the RNCs

ATM Concentration Devices in the Access Network

The deployment of ATM switches in the access network is therefore the most

efficient and cost-effective implementation of 3G in these networks The switches

concentrate traffic from Node Bs into VC-4 containers, enabling an economical

RNC configuration

Figure 3: ATM switches in the access network

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The Cost of Using ATM Switches in the Access

ATM access networks are necessary, but expensive, as ATM is an expensive

technology Moreover, the installation of a simple ATM switch for traffic

concentration includes the addition of ATM hardware, as well as support of a

significant number of PDH and SDH interfaces

Figure 4 depicts a typical scenario in which STM-1 links concentrate traffic from

Node Bs In this example, the site concentrates traffic from a channelized STM-1

with 52 active channels, with 20 E1s from local Node Bs

Figure 4: ATM concentration with an external ATM switch

The total number of E1s is 72 and therefore a channelized STM-1 is no longer

sufficient Concentration must be performed at the ATM level, as TDM

concentration results in the need for additional STM-1s – clearly a waste of

bandwidth for only the 9 E1s in the second STM-1

As already described, an ATM switch can easily compress traffic originally carried

on 72 E1s into one VC-4 However, as shown in Figure 4, the ATM switch must

connect to 72 E1 ports and one STM-1 port Therefore, to enable ATM

concentration, the following components must also be added:

An ATM switch with 72 E1 interfaces and one STM-1 interface

An additional STM-1 interface in the transmission network

Additional 72 E1 interfaces in the transmission network

N OTE: In theory it is possible to add only 52 new E1 interfaces and connect the 20 local interfaces directly to the ATM switch From the management viewpoint, however, this is not recommended, as these 20 links cannot be controlled by the transmission network’s management system

The above configuration is extremely expensive and casts a shadow on the

cost-effectiveness of ATM concentration in the access network Clearly, a far more

economical solution is required

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The Extra Costs of Maintaining IMA Groups

IMA is a low level protocol transporting ATM over multiple E1 links It configures

multiple physical links as a single ATM link, and adds and drops physical links

without affecting traffic

The capability to add and drop TDM capacity from the IMA link without affecting

ATM traffic is extremely powerful but costly IMA is implemented at the hardware

level, and therefore all links in the same IMA group must reside on the same

interface card In many cases, the assignment of IMA groups to a single interface

card is restricted, as all E1s belonging to the same IMA group must be processed

by the same ASIC

These limitations make the network-planning scenario virtually impossible

Continuing with the example in Figure 4, let us assume that the 72 E1 ports

originate in 36 Node Bs, where each Node B is connected via an IMA group of two

E1s Let us also assume there are plans to upgrade the Node B links to an IMA

group of four E1s In this case, the operator has two choices:

Deploy an ATM switch with 72 E1 interfaces and upgrade them when traffic

volume increases, or

Deploy an ATM switch with 144 interfaces, leaving room for future IMA

expansion The first option is extremely complicated When upgrading the network connection

of the Node Bs from two to four E1 links, the new links must all be allocated to the

same interface card At some point in time, a new ATM interface card will be

needed, requiring a rewiring of the physical cables The resulting upgrade is a

complicated traffic-affecting cable management procedure

The second option of reserving ports for future use is simpler, but requires

investing in equipment that will not be used until needed, if at all The deployment

of ATM interfaces based on future upgrade plans is not economically justifiable

due to uncertain changes in the traffic volume

In conclusion, assigning E1 links to IMA groups when planning future upgrades

places the cellular operator in an impossible situation In addition, upgrade

procedures are extremely complex and demand traffic-affecting cable changes The

alternatives are expensive and require initial rollouts for unpredictable future

scenarios

The Dilemma

When deciding on the deployment of ATM in the access network, operators are

faced with a dilemma: they cannot afford not to do it, but they cannot afford to do

it either

ATM concentration in the access in essential for maintaining affordable operational

costs, thus facilitating a reasonable cost structure for 3G services On the other

hand, the cost of deploying a future-proof ATM network is high, and hence capital

expenses may rule this option out

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