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

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

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 [email protected] www.horsebridge.net�

Integrating SDH and ATM in UMTS (3G) Access Networks White Paper December, 2008

© Copyright by ECI Telecom, 2008. All rights reserved worldwide.

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CONTENTS

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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

CONTENTS

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Integration of 2G and 3G Traffic..................................................................................26

TDM Multiplexing of 2G and 3G Traffic.............................................................. 26

Conclusion .....................................................................................................................27

About ECI Telecom .......................................................................................................28

CONTENTS

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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

CONTENTS

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INTRODUCTION

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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.

ROLE OF ATM IN 3G ACCESS NETWORKS

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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.



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.



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.

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



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



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

NOTE: 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.



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.

ECI TELECOM’S SOLUTION FOR 3G ACCESS NETWORKS


ECI Telecom’s Solution for 3G Access Networks

The cellular communications industry is and has always been an important ECI Telecom target market. With this in mind, the company developed an innovative and unrivaled solution for deploying 3G access networks. It is tailored specifically for the needs of the cellular industry and has a reasonable cost structure.

The solution is based on the integration of SDH and ATM into a single platform – the XDM®. This integration provides outstanding cost-effective flexibility and future-readiness. This unique ATM solution is configured for the needs of 3G access networks, and optimized both in terms of cost and features.

The XDM Architecture To better understand the ECI Telecom ATM solution, it is necessary to first understand the XDM, the company’s flagship MSPP (MultiService Provisioning Platform), designed for cellular and metro networks.

The XDM supports a myriad of TDM and optical capabilities. The key feature enabling ATM implementation is the full VC-12 granularity of the XDM matrix that supports a full range of PDH and SDH interfaces from E1 to STM-64. The XDM can cross connect any E1 to any other E1 over these interfaces, without limitations.

Figure 5: Schematic view of the XDM architecture



The ATS (ATM Traffic Switch) Concept In the ECI Telecom ATM solution, the ATS is an interface card that connects to the system’s matrix rather than to the physical interface ports.

The ATS is in fact an ATM switch. However, rather than being equipped with physical ports, it features SDH/PDH connections that are transported by the matrix from any physical interface.

Figure 6: ATS card architecture

The ATS supports three types of ATM ports:

VC-4 ports from STM-1 interfaces or VC-4s on any higher order virtual container

E1 ports from any physical E1 interface or any E1 channel from any other interface

IMA groups of multiple E1s

ATM switching has no limitations, and traffic can be switched at the ATM level from/to any other port.



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

Let us continue with the example of concentration of ATM traffic over 52 E1s that are part of an aggregate STM-1 port, together with 20 E1s connected as tributaries. In the non-integrated ATM solution, an ATM switch is required as well as 72 E1 and one STM-1 interfaces. In addition, 72 new E1 and one STM-1 interfaces must be added to the SDH equipment.

With the integrated XDM ATS solution (Figure 7), the matrix routes 52 E1 interfaces to the incoming STM-1, and the 20 PDH E1 interfaces to the ATS card. The ATS card serves as an ATM switch, aggregating the 72 E1 ports into one VC-4. The matrix then routes the resulting VC-4 to the STM-1 port.

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

A single device therefore accomplishes two tasks: managing the SDH transport layer and concentrating ATM traffic. Technically, this is equivalent to an SDH node connected to an ATM switch, but the integration of the two functions in one box is less expensive and more flexible.



Advantages of the ATS vs. a Standalone ATM Switch The XDM integrated SDH/ATM solution is more compact (requires less boxes), flexible, cost-effective and easier to manage. The following sections describe these benefits in more detail.

Savings in Equipment

Table 1 shows the BOM (Bill of Materials) required to concentrate 52 aggregated E1s and 20 tributary E1s into one unchannelized VC-4 over two layers, vis a vis the equipment required with the XDM ATS solution.

Table 1: Two-layer implementation versus integrated implementation

Separate SDH and ATM layers Integrated SDH/ATM solution

One SDH ADM, including: 20 x E1 tributary ports 72 x E1 ports connected to the ATM switch 2 x STM-1 aggregate ports 1 x STM-1 port connected to the ATM switch

One ATM switch, including 72 x E1 ports 1 x STM-1 port

One XDM MSPP with ATM capabilities, including:

20 x E1 tributary ports 2 x STM-1 aggregate ports 1 x ATS card

The integrated approach offers the same solution with only a fraction of the equipment: A single card in the integrated SDH/ATM solution replaces an ATM switch containing an enclosure, power supplies, a backplane, a matrix, and interface cards. In addition, the SDH system is smaller as it does not require additional interfaces for connection to the ATM switch.

NOTE: It is sometimes technically possible to connect the ATM switch directly to the network and not through SDH equipment. In this particular example, 20 tributaries from the Node Bs can be connected directly to the ATM switch. However, this constitutes a huge burden on management and administration, as not all Layer 1 connections are managed via the transmission system.

This drastic reduction in equipment is due to the integration of ATM and SDH into the same hardware. When implementing each technology with a different box, the standard telecom connection between them requires its own hardware, connectors, and cables. When the two technologies are integrated in the same box, the interface between the SDH and ATM components is via internal hardware buses. This results in higher port density and lower costs. Since the ATS does not have physical ports, it can support 126 E1 interfaces (configured in up to 84 IMA groups) in a single card. No other ATM system on the market reaches this density.



Operational Savings

The operational costs of the integrated solution are obviously much lower than that of maintaining two separate layers. The following factors play a major role in this cost reduction:

A single management system manages both network layers, thus reducing overhead costs

Only one type of equipment is deployed; different layers are supported by different interface cards in the same equipment

Smaller footprint

Reduced power consumption

Less physical cabling, as management-based provisioning replaces physical connections

IMA Flexibility

As described in the section entitled The Extra Costs of Maintaining IMA Groups, planning access network IMA groups can be a discouraging task. A minimalist design that deploys the number of ATM E1 ports presently needed means that future expansions will require cabling changes. This is inconceivable as far as the network planner is concerned. On the other hand, future-proof designs must allow for the provisioning of empty ports for future IMA expansion. Since predicting future expansion is always risky, present financial expenses based on assumptions of future demands are usually ruled out, and justifiable so.

The ATS card integrated in the XDM is the ideal solution. Unlike conventional ATM switches, the ATS can combine any set of E1 ports, including ports residing in different PDH interface cards, into one IMA group. This is due to the VC-12 granularity of the XDM matrix. In our example, the 20 tributary E1 ports can be arranged into 10 IMA groups (each containing traffic from one Node B), with two E1s each. In the future, the existing IMA groups may be upgraded to three or four E1s, or even more.

With the conventional approach, it is necessary to reserve empty ATM E1 ports for future use. With the integrated XDM ATS solution, new E1s arriving from each Node B can be connected to a new PDH interface card. The XDM matrix then routes all E1s from the same Node B to the same destination, even if they are connected to different PDH interfaces.



Cost Flexibility

When designing a network, cost is a primary factor. Network planning, however, should not be seen as an expenditure, but rather as a means to generate revenues. Ideally, network design should accommodate the demands of all users, while generating maximum revenue. In other words, the optimum design should be attained at minimum cost.

But unlike other network planning scenarios where demand may be known in advance, it has as yet been impossible to foresee the increase in the demand for 3G services. A network design should therefore account for a variety of scenarios. Ideally, it should follow these guidelines, usually referred to as “cost flexibility”:

The initial design to be a small low-cost network that facilitates rollout and handles sparse usage.

With increasing demand to upgrade the network so that revenues are never jeopardized by insufficient capacity.

Unfortunately, trying to cater for both needs – small initial investment and easy cost-effective upgrade with increasing demand is not always feasible.

When measuring the cost of a specific network solution, cost flexibility – that is, the ability to adjust the cost of the network to actual demand – is an extremely important factor. At the outset assumptions are made about network current usage, as well as possible scenarios for network upgrades.

The major consideration, therefore, is not the correlation between capacity and demand, but that between the cost of the network and the revenues it produces. To maintain a feasible economical model, the cost of the network must be offset by the revenues obtained from the services it provides.

Even after making the correct assumptions, service demand may lag behind or exceed initial expectations. But, there should never be loss of revenue due to insufficient capacity. It therefore emerges that network capacity should be calculated so that it guarantees no loss of revenue.

When opting for a particular network design, a capacity upgrade strategy must be predicted. Unfortunately, upgrades for a future-ready network always come at a price and includes:

Buying bigger enclosures that provide for the future addition of more cards

Installing high capacity interfaces even if not required in the first phase

Leaving empty ports for future IMA expansion

Using high-instead of low-density interface cards that would need to be replaced

This results in the all-familiar dilemma: deploy a network today with the minimum configuration required and pay a high price for future upgrades, or build a future-ready network today, incurring costs that may prove to be a waste of money in the future.



The XDM is a build-as-you-grow™ and pay-as-you-grow platform, and the ATS card was designed with these premises in mind. Since the ATS is an interface card (not a standalone box), the cost of adding more ATM capacity to the network is substantially lower. Specific network design is consequently more economical, and future upgrades are simpler, easier, and more cost-effective. There are no concerns as to what size of ATM switch should be installed, but rather how many ATS switching cards are required if and when demands change.

THE XDM ATS CARD AS A NODE B CONCENTRATOR


The XDM ATS Card as a Node B Concentrator

This section reviews several network scenarios in which the XDM platform is used to implement 3G access networks. In each application the compactness and flexibility of the ATS solution enables an optimal network solution.

Canonical Concentration of Node B Traffic into VC-4s A typical ATS application consists of the concentration of E1s from Node Bs into VC-4s carried to the RNC. Figure 8 shows a schematic view of this implementation.

Figure 8: Concentration of E1s into VC-4s

In this application, Node Bs send traffic via E1s, either as E1 ATM or as groups of IMA E1s. The ATS cards terminate these E1s at the ATM level and concentrate them into a single unchannelized VC-4 carrying ATM traffic from all Node Bs.

Each ATS card then functions as a concentrator ATM switch, carrying all VCs from the incoming VC-4 to the outgoing VC-4, and adding local traffic from Node Bs.

It is important to note the features making this application so flexible:

The E1s from the Node Bs can be treated as ATM UNI E1s or be combined into IMA groups

The E1s may be either physical ports connected to the XDM PDH interface cards, or a channel in the XDM SDH interfaces

IMA groups may contain any type of E1s, even those that reside in a mixture of PDH/SDH interfaces



Application Scalability

Each network solution should scale from the initial phase of sparse deployment and a low number of users, to full network coverage for mass usage. In each phase, the network should provide the services required while maintaining a reasonable ratio between expenses and revenues.

When translating this scalability into concrete terms, three factors define network demands:

Number of users and resulting capacity requirements

Number of Node Bs in the network

Bandwidth availability

The first factor is directly related to the bandwidth of the links carrying concentrated traffic. Even with huge Node B deployment, if the number of users is low, the actual bandwidth required is minimal. In our example, a single VC-4 can concentrate traffic from a very large number of Node Bs since the number of end-users that actually generate traffic is low.

The second factor (number of Node Bs in the network), relates directly to the quantity of ATS cards required as each ATS card can support a certain number of E1 ports and IMA groups from the Node Bs.

The third factor (bandwidth availability) determines the requirement for intermediate concentration. If it calls for carrying traffic through a specific pipe, additional ATS cards will be required to concentrate the traffic at the ATM level and meet this demand.

The next sections describe how this canonical design can easily be adapted to actual scenarios based on changing network needs.

Sparse Deployment of Node Bs

It is reasonable to assume that the initial network deployment consists of a few Node Bs. Thus, installation of an extra ATS card at every intermediate point would be an unnecessary and superfluous expense. The solution would then be to deploy ATS cards only in some locations and carry and connect Node Bs traffic in intermediate points via the SDH concentration links.

Figure 9: Configuration for a low number of Node Bs

In this scenario, the ATS card can use both E1s from local PDH interfaces and E1s carried from remote E1 ports over SDH channels.

A typical network deployment can start with this scheme that is sufficient when only a few Node Bs are connected to each site. Later, as more Node Bs are deployed, additional ATS cards can be installed at intermediate points.



Note also that only the management-based provisioning needs to be modified when a new ATS card is added at an intermediate port. It is not necessary to reconnect or reroute cables or physical devices. This is another example of the future-readiness of the ATS versus an external ATM switch, which makes the upgrade a tedious task entailing the installation of new equipment and rerouting of physical cables.

Increased Bandwidth Demand

In Figure 8, traffic concentration from Node Bs into a VC-4 is accomplished over a single VC-4 trail. As the actual number of users increases, more and more bandwidth is required. A simple reconfiguration of the network (as shown in Figure 10) caters for the required increase in capacity.

Figure 10: Configuration for increased bandwidth demand

The same number of Node Bs with the same number of ATS cards therefore serves additional bandwidth towards the RNC.

Note again that the increase in network capacity is achieved without adding a single new card or interface. A simple management operation routes traffic from the Node Bs over two VC-4s instead of one.

Savings on Intermediate Bandwidth

A capacity of one VC-4 may be excessive for the demands in the first phases of network deployment. The ATS can accommodate actual demand using large IMA groups.

IMA groups are usually used to carry traffic from Node Bs. In this example IMA groups of two to eight E1s are sufficient. The ATS, however, can support larger IMA groups of up to 32 E1s. These large IMA groups may be used for traffic concentration, consuming only the needed bandwidth instead of occupying an entire VC-4. Figure 11 illustrates this concept.

Figure 11: Concentration of Node B traffic into IMA groups

In this configuration, the benefits of ATS flexibility are enormous:

3G traffic consumption is controlled by the E1 granularity of the XDM matrix.



Modification of the size of the IMA groups is a simple management provisioning operation (unlike the external ATM switch solution, which requires new E1 ports).

Upgrade from an IMA-based solution to a VC-4-based solution is management-controlled. With external ATM switches, the upgrade would involve changes to the hardware configuration of the switches.

Combined VC-4 and IMA Aggregation

VC-4s and IMA groups can also be combined into a single network design, thus accounting for the capacity vs. cost paradigm. Figure 12 shows how IMA group concentration can be added when the existing VC-4 has been fully utilized.

Figure 12: Combination of VC-4 and IMA concentration

Similarly, the initial deployment can consist of a single IMA group and later, when traffic increases, it can be concentrated into a VC-4.

INTEGRATION OF 2G AND 3G TRAFFIC


Integration of 2G and 3G Traffic

John Donne (1572-1631) said, “No man is an island”. By the same token, no network is an island.

This paper has dealt with 3G access networks only. However, the following important question arises: How do 3G and 2G networks share resources? Once again, this is not a technical issue but an economical one: To reduce costs, 3G networks must share resources with existing 2G deployments.

TDM Multiplexing of 2G and 3G Traffic This section describes how 2G traffic can populate only parts of the STM-1 links, while 3G traffic can be concentrated into an IMA group populating the remaining links.

Figure 13 shows how 2G TDM traffic is carried over standard TDM links. The E1 granularity of the XDM matrix is ideal for cellular applications, as it provides a flexible and convenient way of building 2G access networks.

At the same time, the ATS card concentrates ATM traffic from multiple Node Bs into one IMA group. The 3G IMA group and 2G traffic can therefore share the same channelized STM-1. Furthermore, any change in bandwidth allocation between the two networks is effected via management, unlike external ATM switches that require changes in physical cabling.

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

CONCLUSION


Conclusion

The deployment of 3G cellular networks is one of the greatest challenges facing the telecommunications industry today. While the technological foundations have been laid, implementation continues to be elusive. This is due to several factors:

Network infrastructure and operational costs

Types of services offered

Treatment of different network layers

Changes in the number of users and user bandwidth demand

The XDM ATS is a revolutionary design that integrates two technologies into a single layer, thus providing a powerful and cost-effective solution for the implementation of 3G access networks.

This technological integration is the key to drastic cost reductions when compared to any other solution for separate ATM and SDH layers. Furthermore, it can flexibly be configured to suit many scenarios, adapting the network to changes in deployment, number of users, and increases in capacity.

The XDM ATS approach not only supports the deployment of cost-effective networks, but also provides a scalable solution from initial deployment to full mass usage. Cellular operators no longer need to invest money based on vague future predictions, and are no longer limited by network upgrades when the need arises.

ABOUT ECI TELECOM


About ECI Telecom

ECI Telecom is a leading global provider of intelligent infrastructure, offering platforms and solutions tailored to meet the escalating demands of tomorrow's services. Our comprehensive 1Net approach defines ECI’s total focus on optimal transition to Next-Generation Networks, through the unique combination of innovative and multi-functional network equipment, fully integrated solutions and all-around services.

For more information, please visit http://www.ecitele.com.

http://www.ecitele.com/

1Net defines ECI’s total focus on facilitating our customers' optimal transition

to Next-Generation Networks, through the unique combination of innovative and

multi-functional network equipment, fully integrated solutions and all-around services

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 [email protected] www.horsebridge.net�

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

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