Inverse Multiplexing for ATM Technical Material

8/10/2019 Inverse Multiplexing for ATM Technical Material

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Reduce operating costs by using available bandwidth more

efficiently. Generate new revenue opportunities from customizedbandwidth services with minimal incremental investment.

Inverse Multiplexing for ATMTechnical Brief

This technical brief describes how service providers can use the IMA supporton their existing Lucent CBX 500 Multiservice WAN Switches andPacketStar PSAX Multiservice Media Gateway devices to:

Generate new revenue opportunities by offering flexible bandwidth services

Recoup costs and start generating profits in as little as three months

Support converged services for users with both ATM and non-ATM interfaces

Reduce transmission costs and relieve congestion in wireless and wireline network backbones

Bridge the Bandwidth Gap UsingInverse Multiplexing for ATM (IMA)to Step Up Services From DS1/E1 to DS3/E3


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Document Abstract

This paper describes IMA, its implementation on Lucent multiservice switches and media gateways, and its

potential network service and infrastructure applications. The paper also contains a cost-justification example

illustrating how a single IMA module added to an existing Lucent switch or media gateway can generate enough

new service revenues to recoup the total cost of ownership of the module and start generating profits in as little

as three months. This paper is intended for network designers who are considering using IMA to increase service

density, flexibility, and bandwidth efficiency on existing ATM networks.


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1. Introduction ...............................................................................................................................................4

Tried and true, maybe, but leased lines can only do so much ...................................................................................4

Enter IMA .................................................................................................................................................................4

IMA Ensures Reliability.............................................................................................................................................4

2. Understanding the IMA specification...........................................................................................................5

Examining IMAs physical and logical architecture ...................................................................................................5

Maintaining ATM cell order ......................................................................................................................................6

Supporting ATM functions over multiple interfaces in the IMA group.....................................................................8

3. Understanding the Lucent solution............................................................................................................10

IMA support on the CBX 500 Multiservice WAN Switch ......................................................................................10

IMA support on the PacketStar PSAX Multiservice Media Gateways....................................................................12

Multivendor Interoperability...................................................................................................................................14

Unified Navis network management .....................................................................................................................14

4. IMA applications .......................................................................................................................................15

Increasing the port density of existing DS1 or E1 services ......................................................................................15

Providing flexible bandwidth services to ATM users ...............................................................................................15

Providing flexible bandwidth services for non-ATM users ......................................................................................16

Supporting converged services with links below DS3 ............................................................................................17

Reducing the cost of network backhaul .................................................................................................................18

5. Conclusion................................................................................................................................................20

Appendix A: Acronyms..............................................................................................................................20

Table of Contents


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1. Introduction

The DS1/E1 and DS3/E3 leased line infrastructure installed over the last several decades has created large

revenue streams for service providers and fundamentally changed how enterprises do business. There are many

reasons for this success. As a digital system based on Time Division Multiplexing (TDM), leased lines provide

extremely reliable transport at the highest quality of service (QoS) even when high QoS is not required.

Leased lines support demanding real-time applications such as voice and video, as well as applications with less

stringent QoS requirements, such as data traffic based on IP or frame relay.

Tried and true, maybe, but leased lines can only do so much

The leased line architecture has a significant shortcoming a huge bandwidth gap between entry-level leased

lines (1.5 Mbps for a DS1, 2 Mbps for an E1) and second-tier leased lines (45 Mbps for a DS3, 34 Mbps for an

E3). In addition, while DS1 and E1 lines have been deployed ubiquitously, more expensive DS3 and E3 leased

lines are still not available in all locations.

The result is that enterprises requiring more than 1.5 or 2 Mbps of bandwidth face awkward and expensive

choices. Enterprises can either remain at their current DS1 or E1 level and take steps internally to reduce the

amount of traffic sent over the leased line, or immediately jump to leasing a DS3 or E3 line and hope that

business traffic eventually justifies the increase in cost.

Today, some enterprises have sidestepped this problem by leasing one or more additional DS1 or E1 lines from

their service provider and designating each line to carry a specific type of traffic (voice, video, or data) or to

support specific applications. This incremental approach can be cost-effective for enterprises that require up to

eight DS1 (12 Mbps) or nine E1 (18 Mbps) circuits, however, it is not efficient for service providers to offer.

When service providers reserve circuits for specific applications or traffic types, they sacrifice the ability to use

ATM to consolidate and efficiently manage diverse traffic types across those circuits.

Enter IMA

The ATM Forum (www.atmforum.com), the principal standards body governing ATM protocols, developed the

Inverse Multiplexing for ATM (IMA) Version 1.1 specification to address this issue.1 With IMA, service providers

can save money by using two or more parallel physical DS1 or E1 circuits as if they were a single logical interface

creating interim bandwidth steps between entry level DS1 and E1 and the much more expensive DS3 and E3.

All ATM QoS classes and traffic management functions continue to operate over the IMA-aggregated links.

IMA can help service providers save money by using two or more parallel physical DS1 or E1 circuits as if they

were a single logical interface creating customized bandwidth steps between entry-level DS1 and E1 and the

much more expensive DS3 and E3. All ATM QoS classes and traffic management functions continue to operate

over the IMA-aggregated links.

IMA Ensures Reliability

In addition, because an IMA group continues to transmit traffic as long as at least one of its constituent links is

working, service providers can offer their customers additional protection against individual facility outages.

When a link in an IMA group fails, IMA shifts the traffic automatically to the remaining links. Depending on

their specific IMA equipment, service providers can use ATM QoS and priority schemes to ensure that the IMA

group shifts higher priority traffic first increasing the reliability of higher value, QoS-sensitive services. Thisapproach stands in stark contrast to a service-specific setup that uses separate links for voice, video, and data

traffic. In a service-specific arrangement, if one link fails, the customer loses all traffic sent on that link.

The Lucent CBX 500 Multiservice WAN Switch and PacketStar PSAX Multiservice Media Gateway devices offer

comprehensive IMA support on a variety of interfaces.

1 The complete IMA v1.1 specification is available at http://www.atmforum.com/standards/approved.html.


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2. Understanding the IMA specification

This section describes IMAs physical and logical architecture and how IMA preserves ATM cell order and

supports native ATM functions when splitting transmissions over multiple links.

Examining IMAs physical and logical architecture

As shown in Figure 1, at the physical level, an IMA deployment simply looks like a set of parallel DS1 or E1

lines. In the diagram, the multiservice switch uses IMA to provide various speed ATM User to Network Interface

(UNI) connections to CPE devices at two customer sites and to a DSLAM at a third. Although IMA operates at a

logical level to give these locations access to cumulative bandwidth that falls between DS1 and DS3, the IMA

deployments are physically constructed using a group of parallel DS1 lines.

Figure 1 Service providers can use IMA-enabled multiservice switches to create scalable ATM links in DS1 or E1increments. Such links offer customizable ATM bandwidth that grows as end user needs grow.

At the logical level, IMA bundles the parallel physical DS1 (or E1) lines into a single logical ATM interface,

known as the IMA group. The network operator associates the IMA group with a UNI or Network-to-Network

Interface (NNI) inside the IMA-enabled devices at both ends of the virtual link just as the operator would

associate both ends of a non-IMA link with either a UNI or NNI.

Figure 2 shows the same network as Figure 1, but from the perspective of the higher-level ATM functions. From

this perspective, IMA transforms each set of parallel physical DS1 lines into a single virtual link that has an

aggregate bandwidth equal to the sum of the bandwidth of the individual links.

Customer Premises

Customer Premises

3 DS1s give 4.5 MbpsATM UNI

DSL

2 DS1s give 3 MbpsATM UNI

5 DS1sgive

7.5 MbpsATM UNI

MultiserviceSwitches

CPE

CPE

DSLAM

IMA Physical Layer

DigitalCross-connectSystem (DCS)


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Figure 2 IMA enables a simplified logical view of parallel DS1 lines. A single UNI (or NNI) is associated with the IMA

virtual link at both ends of the connection. Any configuration that is performed on non-IMA UNIs (or NNIs) can be

performed on the UNI associated with the IMA virtual link.

IMA bundles the separate physical lines into a single logical interface, enabling flexibly-sized (and appropriately

priced) access services, without sacrificing the ability to use ATMs QoS and traffic management functions to

efficiently manage diverse traffic types across multiple circuits.

Maintaining ATM cell order

ATM networks must maintain cell order when transmitting data. A device receiving ATM cells assumes that it

has received the cells in the same order in which they were originally transmitted. This assumption simplifiesATM transmission significantly while decreasing the need for cell header bits. ATMs approach contrasts with

many other link layer and network layer protocols in which receiving devices must take additional steps to stitch

together frames or packets in the correct order to reconstruct the data stream.

Preserving cell order in native ATM links

Figure 3 shows how the network preserves cell order when it transmits native ATM cells over a single DS1 link.

As the diagram shows, there is a logical separation between the ATM layer and the physical layer framing. The

ATM layer transmits cells through its associated UNI or NNI logical port at a rate that does not exceed the known

rate of its logical port (1.5 Mbps in Figure 3). The ATM layer leaves all framing and other details to the lower

level functions.

The lower-level framing function preserves the ATM cell order as it packs the cells into DS1 frames for

transmission through the DS1 WAN link. At the other end of the DS1 WAN link, the receiving device simply

unpacks the ATM cells in the correct order.

Customer Premises

Customer Premises

UNI with 4.5 Mbpsvirtual link

DSL

UNI with 3 Mbpsvirtual link

UNI with 7.5 Mbpsvirtual link

MultiserviceSwitches

CPE

CPE

DSLAM

IMA Logical Layer

DigitalCross-connectSystem (DCS)


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Figure 3 When IMA is not in place, the DS1 framing function preserves the cell order within each DS1 frame.

Preserving cell order when using IMA

With IMA, several individual DS1 links act as if they were a single logical link. Cells carrying data, voice, or video

traffic are split across all of the links in the IMA group. However, the ATM network still requires that the cells

arrive at the receiving device in the same order in which they were transmitted.

To preserve cell order, the IMA specification uses a strict round-robin scheduling algorithm to distribute the cells

over the bundled DS1 or E1 links. As Figure 4 shows, when the IMA round robin receives cells from the ATM

layer, it distributes them sequentially among the individual physical links in the IMA group. The IMA round

robin sends the first cell along the first DS1, the second cell through the second DS1, and the third cell through

the third DS1. When it has distributed one cell to each link, the round robin starts the cycle over again, sending

the fourth cell to the first DS1, the fifth cell to the second DS1, and so on.

ATM Transmission without IMA

ATM layer transmits cellsthrough its associated UNI

UNI 1.5 Mbpslogical port

ATM cell order is preservedinside DS1 frame

Receiver simply unpackscells in correct order

DS1 WAN link

DS1 frame

ATM cells

DS1 framing

function packs cellsin DS1 frame

4

3

2

1

4 3 2 1


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Figure 4 IMA preserves the ATM cell order with strict round robin cell distribution. Whether IMA is in place or not,

the ATM layer has no need to know the details of physical framing.

This predictable round robin pattern enables the receiving IMA function to reliably reassemble the cell stream from

multiple DS1 links. The ATM layer in the transmitting and receiving devices does not know that IMA is in place.

Instead the ATM layer simply transmits cells as they become available up to the maximum rate of its associated UNI

logical port in this case, 4.5 Mbps, the aggregate bandwidth of the three DS1 links in the IMA group.

Supporting ATM functions over multiple interfaces in the IMA group

IMAs ability to preserve ATM cell order is only part of the reason why ATM attributes such as traffic

management and QoS control continue to function reliably over IMA links. The other key element is IMAs link

management and framing capabilities. This section provides a conceptual overview of IMA framing and how

IMA uses the cells within IMA frames to regulate cell transmission rates (and cell delay) while managing the

IMA group and its constituent links.

Understanding IMA framing

An IMA process resides at each end of an IMA link. The two IMA processes communicate using IMA frames to

control data flow and manage the logical IMA group as well as its constituent DS1 (or E1) physical links. Each

IMA frame contains a fixed number of ATM cells. There are typically 128 cells in each IMA frame, although

operators can also set the frame size to 32, 64, or 256 cells, depending on the specific IMA implementation.

As the ATM layer hands cells to the IMA process for transmission over the WAN, the IMA process distributes

them in round robin fashion among the constituent links, counting them, and grouping them into IMA frames

that span all of the constituent DS1 or E1 links. When the IMA process reaches the IMA frame size limit, it

begins creating a new IMA frame. Figure 5 illustrates this process for an IMA group of three DS1 links.

ATM layer

UNI 4.5 Mbpslogical port ATM cell order is preserved by

distributing cells in DS1 framesin strict round robin order

Receiver simply unpacks cells instrict round robin mode topreserve cell order

DS1 WAN link 2

DS1 frame

ATM cells

DS1 framer

4

3

2

1

2

DS1 WAN link 3

DS1 frame

DS1 framer 3

DS1 WAN link 1

DS1 frame

DS1 framer 4 1

IMA

roundrobin

ATM Transmission with IMA


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Figure 5 The two IMA processes associated with this IMA group communicate using IMA frames that span across the

three DS1 lines used to create the virtual 4.5 Mbps connection.

Regulating cell transmission rates and cell delay

Each IMA frame contains ATM data cells, ATM idle cells, and a few ATM cells marked as IMA Control Protocol

(ICP) cells. When an IMA process receives a frame, it hands off the ATM data cells to the ATM layer. The IMA

process uses the ICP cells, which are identified by a special code in the cell header, to communicate with each

other, control data flow, and manage both the IMA group and its constituent links.

Each IMA frame contains primarily ATM data cells. IMA frames include ATM idle cells only when there are no

ATM cells ready to transmit that is, when the cell rate offered by the ATM layer for transmission over the

WAN is less than the virtual link cell rate. This use of idle cells is known as cell rate decoupling. Because the WAN

transmits cells at full speed even when the real ATM cell traffic is at a lower speed, the two rates are decoupled.

Each IMA frame also contains one ICP cell per constituent DS1 or E1 link. The ICP cells are always transmitted in

the same location on a given DS1 or E1 link; however, the ICP cells can be transmitted in different locations on

different links. For example, one constituent link may always transmit the ICP cell as the fourth cell in the IMA

frame, while another link in the same IMA group may always transmit its ICP cell as the twelfth cell in the IMA

frame. In Figure 5, the DS1 WAN link 1 always transmits its ICP cell in the second cell of the IMA frame, while

DS1 WAN link 2 transmits its ICP cell in the first cell of the IMA frame, and DS1 WAN link 3 transmits its ICP cell

in the last cell of the IMA frame. (Figure 5 shows the ICP cells as highlighted cell boxes.)

Because it inserts one ICP cell on each DS1 link per frame, the IMA process can introduce some delay into the

cell flow. However, the standard includes smoothing buffers that remove the delay and support virtual circuits of

all QoS classes, including those requiring minimal cell delay and predictable cell delay variation. In addition, the

IMA standard includes bit stuffing and other techniques that allow predictable cell delay variation even when the

different DS1 links use different clocks and have different delay variations.

Managing individual links

The IMA specification relies on ICP cells to monitor the status of the links in the IMA group and dynamically

adjust the bandwidth available on the IMA link. This function is critical to IMAs ability to bundle multiple DS1

or E1 links into a persistent virtual connection that can transmit ATM cells in both directions.

Note: The ATM layer, the UNI, and the DS1 framers shown in Figures 3and 4 have been removed from this diagram for clarity.

DS1 WAN link 2

ICP ICP

1272 5 2

DS1 WAN link 3

ICP

1283 6 3

DS1 WAN link 1

ICP

1261 4 1

IMA

roundrobin

IMA frame 2in the making

IMA frame 1 ready fortransmission, containing

cells 1 to 128

Three DS1frames ready for

transmission


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During normal operations, individual links in the IMA group can be added, removed from service, or simply fail

affecting the amount of bandwidth available for the IMA group. The IMA specification requires the IMA

group to dynamically adjust to these changing conditions without requiring operators to stop and restart the

overall traffic flow. The two IMA processes use ICP cells to communicate the status of the constituent links as

well as the IMA group as a whole. The IMA processes use the ICP cells to remove failed links from the IMA

group, and to restore recovered links to the IMA group.

This dynamic response to changing network conditions improves the fault tolerance on the IMA link. If oneconstituent link in the IMA group fails, the throughput on the link will drop, but the IMA group will

automatically shift the traffic to the remaining links. The IMA group continues to transmit traffic as long as at

least one of its constituent links remains operational. Similarly, if an operator restores a failed link to the IMA

group or adds a new link, the IMA processes automatically adjust to the IMA groups new maximum bandwidth

rate again without interrupting the existing traffic flow.

3. Understanding the Lucent solution

The Lucent CBX 500 Multiservice WAN Switch and the PacketStar PSAX 4500, PSAX 2300, PSAX 1250, and

PSAX 1000 Multiservice Media Gateways all support IMA. These Lucent products support a wide range of DS1,

E1, channelized DS3/1 and channelized STM-1/E1 IMA modules to give service providers provide the flexibility

to deploy customizable bandwidth services or reduce transmission costs in wireless and wireline backbonenetworks. This section briefly describes the IMA support on the Lucent CBX 500 and PacketStar PSAX product

lines.2 (For more information on the potential applications for the Lucent IMA solutions, see Section 4.)

IMA support on the CBX 500 Multiservice WAN Switch

The CBX 500 Multiservice WAN Switch supports a 3-Port Channelized DS3/1 with IMA ATM module and a

1-Port Channelized STM-1/E1 with IMA ATM module.

IMA implementation on the CBX 500 Multiservice WAN Switch complies with the ATM Forum IMA v1.1

specification and is backward compatible with v1.0. Following are just a few characteristics worth noting on how

the CBX 500 switch supports IMA. The remainder of this section describes the modules framing procedures,

flow control capabilities, bandwidth allocation mechanism after-link failure, and port densities.

Internal framing ensures full support for ATM traffic management and QoS

The IMA process on each end of the IMA link distributes ATM cells for transmission over the WAN in a strict round-

robin fashion among the constituent DS1 links. The 3-Port Channelized DS3/1 with IMA ATM module operates at

DS3 speeds so its implementation requires an extra framing step. Each DS3 port on the module connects to an

M13/DS3 framer. The 28 DS1 circuits within each channelized DS3 port then connect to one of four DS1 framers.3

Each IMA group supported on the module is engineered using DS1 links that use a common DS1 framer. When the

DS1 framer passes information to an IMA processor, the IMA processor assembles the channels to pass along to the

ATM plane for flow control before transmitting them onto the switch fabric. Once it hits the switch fabric, the traffic

looks and acts like any other ATM circuit with associated QoS.

Flow control enables end-to-end QoS

Both the 3-Port Channelized DS3/1 with IMA and the 1-Port Channelized STM-1/E1 with IMA ATM moduleshave an integrated flow control processor (FCP) subsystem to deliver end-to-end Available Bit Rate (ABR) flow

control as well as two forms of closed-loop flow control to intelligently move congestion to the network edge. In

addition, the FCP subsystem uses per-VC queuing on non-real-time circuits to grow virtual circuit (VC) rates

when possible or to intelligently throttle down the VC rates as needed.

2 Lucent Stinger DSL Access Concentrators also support IMA. More information on the Lucent Stinger DSLAM product

line can be found at http://www.lucent.com/solutions/broadband_access.html.

3 The first three DS1 framers handle eight of the DS1 circuits. The fourth handles only four DS1s, for a total of 28 DS1 circuits.


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Bandwidth allocation procedures protect high-value services after individual link failures

If one link in an IMA group fails, IMA automatically shifts traffic to the remaining links in the group. As long as

the available bandwidth does not fall below the configured minimum bandwidth threshold, the IMA group

continues to transmit traffic without interruption at a reduced rate. The IMA implementation on the CBX 500

Multiservice WAN Switch enables service providers to use ATMs QoS and priority schemes to determine how

bandwidth is allocated after link failures to protect higher-value, QoS-sensitive services.

To understand how this works, suppose a service provider is using an IMA group that contains three DS1 links.When one link in the group fails, the throughput for the IMA group drops from 4.5 Mbps to 3 Mbps, but the

IMA group continues transmitting. If the service traffic carried on the group requires less than 3 Mbps of

bandwidth, then there is no need to reallocate resources among the services.

If the service traffic exceeds 3 Mbps, the egress buffers for the IMA group back up until they reach the discard

level. At this point, the IMA group has a negative available bandwidth, causing all circuits configured on the link

to compete for bandwidth. The IMA implementation on the CBX 500 Multiservice WAN Switch does not delete

or bounce circuits. Instead, the CBX 500 redistributes the remaining bandwidth among the circuits according to

their QoS level giving higher priority circuits preference over lower priority ones.

High port densities for increased scalability

Each CBX 500

3-Port Channelized DS3/1 with IMA ATM module has a capacity of up to 42 IMA groups or up to30 IMA groups on each CBX 500 1-Port Channelized STM-1/E1 with IMA ATM module.4 Alternatively, any of

the DS1 or E1 links on the channelized modules can be defined as non-IMA links and used as any other DS1 or

E1 ATM link supported by the CBX 500 Multiservice WAN Switch to create extremely dense DS1 or E1 ATM

services. The two CBX 500 channelized IMA modules offer a higher port density and lower per-port cost than

multiple port DS1 or E1 cards. (See Figure 6.)

Figure 6 The 3-Port Channelized DS3/1 with IMA and 1-Port Channelized STM-1/E1 with IMA ATM modules combine

high port density with low per-port costs.

Service providers use standard digital cross connect devices to access the 28 DS1s on each physical DS3 port on the

CBX 500 3-Port Channelized DS3/1 with IMA ATM module and the 63 E1s on the CBX 500 1-Port Channelized

STM-1/E1 with IMA ATM module. The cross-connects simply break out the channelized DS1 or E1 lines from the

unified DS3 or STM-1 line. This system allows extremely high DS1/E1 port density from the CBX 500

Multiservice WAN Switch.

CBX 500 Switch Description

Capacity

3-Port Channelized DS3/1with IMA ATM module

1-Port Channelized STM-1/E1 with IMA ATM module

IMA-enabled E1 or DS1 interfaces per port 28 DS1s 63 E1s

IMA-enabled E1 or DS1 interfaces per module 84 DS1s 63 E1s

IMA-enabled E1 or DS1 interfaces per chassis5 1,176 DS1s 882 E1s

IMA-enabled E1 or DS1 interfaces per rack5 2,352 DS1s 1764 E1s

IMA groups per module 42 IMA groups 30 IMA groups

IMA groups per chassis 588 IMA groups 420 IMA groups

IMA groups per rack 1,176 IMA groups 840 IMA groups

Virtual circuit support per module 16,384 virtual circuit endpoints 16,384 virtual circuit endpoints

4 This 30 IMA group number includes 27 IMA groups containing two E1s and three IMA groups containing three E1s.

5 Assumes that each chassis is fully loaded with the 3-Port Channelized DS3/1/0 with IMA ATM module or 1-Port

Channelized STM-1/E1with IMA ATM module.


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Dynamic changes to IMA groups minimize service interruptions

Unlike competing solutions, the IMA implementation on the CBX 500 Multiservice WAN Switch enables service

providers to add or remove links from existing IMA groups without tearing down the bundle. This gives service

providers the flexibility to dynamically resize bandwidth for specific customers without interrupting current

services. For example, CBX 500 IMA modules can be used to create IMA groups containing only a single DS1 or

E1 link. When customers are ready to upgrade to a higher level of service, more DS1 or E1 facilities can be added

to the existing IMA group to dynamically increase the available service bandwidth.

IMA support on the PacketStar PSAX Multiservice Media Gateways

As with the CBX 500 Multiservice WAN Switch, the IMA implementation on Lucent PacketStar PSAX

Multiservice Media Gateway devices complies with the ATM Forum IMA v1.1 specification. In addition, PSAX

gateways comply with both variants of the earlier ATM Forum IMA v1.0 specification.

IMA implementation on the PacketStar PSAX Multiservice Media Gateway devices complies with ATM Forum

IMA specifications. Four aspects of the PSAX implementation make it an especially cost-effective choice for

service providers:

The wide range of available IMA modules

The manner in which PSAX reallocates bandwidth after individual link failures

The ability to dynamically add or remove links from IMA bundles without interrupting services

The flexibility to use any PSAX module with any PSAX chassis

These features are described below.

A wide range of IMA modules for finely tuned IMA services

The PacketStar PSAX Multiservice Media Gateway family currently supports five IMA modules:

6-port DS1 IMA module

6-port E1 IMA module

Medium-density 12-port DS1 IMA module

High-density 21-port E1 IMA module

1-port Channelized DS3 IMA module

Two additional modules, a 1-port Channelized OC-3 IMA module that supports up to 84 DS1s and a 1-port

Channelized STM-1 that supports up to 63 E1s, will be available in a future release.

This broad range of IMA-capable modules gives service providers the flexibility to deploy equipment that

provides the appropriate amount of port density for each location in their network. The following chart outlines

the port densities on each currently available module.

The CBX 500switch and PacketStar PSAX gateway IMA

implementations give service providers the flexibility to dynamically

resize bandwidth without interrupting service.


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Figure 7 The IMA modules available for the PacketStar PSAX product line combine high port density with low per-

port costs. The port densities in this table reflect a redundant configuration that uses one of the available I/O slots to

house a second CPU instead of an additional IMA module.

Responding to link failures within the IMA group

As with other IMA implementations, IMA groups can be configured on PacketStar PSAX Multiservice Media

Gateway devices to continue transmitting traffic over the remaining links when one or more links in an IMA

group fail. This forces existing services to compete for a smaller amount of available bandwidth, so the PSAX uses

its Connection Admission Control (CAC) algorithm to predictably allocate the remaining bandwidth.

To understand how this works, suppose an IMA group containing three DS1 links (providing 4.5 Mbps aggregate

bandwidth) is created. If one link fails, the IMA group automatically continues to transmit traffic over the

remaining two links. When notified of the link failure, CAC adapts the maximum available bandwidth for the

IMA group to 3 Mbps. If the current services transmitted over the IMA group require more than 3 Mbps, CAC

deletes SVC or SPVC connections as necessary to reduce the IMA groups bandwidth requirements. CAC can

delete SVCs and SPVCs without user intervention. If it does delete an SVC or SPVC connection, CAC generates a

trap to notify operators of the event. If after deleting the SVCs and SPVCs, the remaining PVC services still

require more than 3 Mbps of bandwidth, PSAX generates a trap to alert the network administrator to delete

specific PVCs to reduce bandwidth requirements for services carried over the link to comply with the nominal

CAC limit.

When the failed link is restored to the IMA group, CAC automatically adjusts the maximum available bandwidth

for the IMA group back to 4.5 Mbps again without interrupting any existing services. During this process,

CAC automatically restores deleted SPVC connections. Deleted SVC connections, however, must be re-initialized.

Description

Capacity

6-Port DS1IMA module

6-Port E1IMA module

12-Port DS1IMA module

21-port E1IMA module

1-PortChannelized DS3IMA module

IMA-enabled interfaces per port 6 DS1s 6 E1s 12 DS1s 21 E1s 28 DS1s

IMA-enabled interfaces per PSAX 1000chassis6

24 DS1s 24 E1s 48 DS1s 84 E1s 112 DS1s


60 DS1s in19 shelf

84 DS1s in23 shelf

60 E1s in 19 shelf

84 E1s in 23 shelf

120 DS1s in19 shelf

168 DS1s in23 shelf

210 E1s in19 shelf

294 E1s in23 shelf

280 DS1s in19 shelf

392 DS1s in23 shelf


90 DS1s 90 E1s 180 DS1s 315 E1s 420 DS1s


90 DS1s 90 E1s 180 DS1s 315 E1s 420 DS1s

IMA groups per module Up to 3 IMAgroups

Up to 3 IMAgroups

Up to 6 IMAgroups

Up to 10 IMAgroups

Up to 14 IMAgroups

Virtual circuit support per module 32,000 32,000 32,000 32,000 32,000

6 Assumes a PSAX 1000 with four redundant slots is fully loaded with that particular module.

7 Assumes a redundant PSAX 1250 is fully loaded with that particular module. There are 10 redundant slots in the 19 shelf

and 14 redundant slots in the 23 shelf.

8 Assumes a PSAX 2300 with 15 redundant slots is fully loaded with that particular module.

9 Assumes a PSAX 4500 with 15 redundant slots is fully loaded with that particular module.


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Dynamically adding new links to an IMA group for service flexibility

As with the CBX 500, service providers can add or remove links from an existing IMA group without taking

them out of service. To be able to add future links to an IMA group, the IMA group is established with extra

ports assigned to the group, but without active facilities for those ports. When the actual facilities are later linked

to the ports, CAC discovers the new facilities and restores them automatically to the IMA group increasing

the maximum available bandwidth for the IMA service.

Interchangeable modules protect capital investments and improve operating efficiency

All PacketStar PSAX IMA modules are interchangeable, and therefore, service providers can install them on any

PacketStar PSAX 4500, PSAX 2300, PSAX 1250, or PSAX 1000 Multiservice Media Gateway device as needs

dictate. This helps service providers protect their initial capital investments, ensure interoperability, and improve

operating efficiency.

Multivendor Interoperability

The IMA implementation on both the CBX 500 multiservice WAN switch and Lucent PacketStar PSAX

Multiservice Media Gateway devices complies with the ATM Forum IMA v1.1 specification. Consequently, any

PSAX Multiservice Media Gateway can interoperate with the CBX 500 Multiservice WAN Switch or any third-

party solution that supports IMA v1.1 or v1.0.

Unified Navis network management

The Lucent Navis iOperations suite of software products supports the Lucent IMA solution and offers the

integrated, role-based systems service providers need to manage the entire lifecycle of their multiservice,

multivendor optical, packet, voice, data and mobile networks. Navis iEngineer software helps service providers

configure, build, and maintain their networks, evolve them to include new technology, and enable seamless

introduction of profitable new services, such as customizable bandwidth services based on IMA. The Navis

iEngineer family includes the award-winning NavisCore Element Management System, which provides

sophisticated, standards-based management for multiservice devices in frame relay, ATM, and IP networks on a

single platform. Lucent has industry-leading switches combined with Navis network management software,

enabling service providers worldwide to deliver high-performance ATM, frame relay, IP, and private line services

from a single network infrastructure.


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4. IMA applications

IMA opens up crucial new revenue and cost-saving opportunities for service providers with Lucent multiservice

switches already deployed in their network. This section surveys a few of the major applications for Lucent IMA

modules, including:

Higher-density DS1 and E1 services

Flexible bandwidth services for ATM and non-ATM users

Low-speed converged services

More efficient low-speed trunking between network sites.

Increasing the port density of existing DS1 or E1 services

The IMA modules available for the CBX 500 Multiservice WAN Switch and PacketStar PSAX Multiservice

Media Gateways can be used for IMA-enabled services, or they can be used to support higher density ATM

services at standard DS1 and E1 speeds. This offers several advantages to the service provider.

First, service providers can replace multiple lower-density modules on existing CBX 500 Multiservice WAN

Switches or PacketStar PSAX Multiservice Media Gateways with a single high-density IMA card. This frees up

slots on existing chasses for other services. For example, four 8-Port T1/E1 ATM modules installed on a singleCBX 500 Multiservice WAN Switch can replace all four cards with a single 3-Port Channelized DS3/1 ATM with

IMA module freeing up three slots, while increasing the port density of their DS1 services from 32 ports to 84

ports. By using existing equipment more efficiently, the need to acquire and deploy additional multiservice

switches can be postponed.

Second, service providers can configure any DS1 or E1 circuit supported on the cards as an IMA group with a

single IMA link. Configuring the links in this way allows seamless upgrades of existing DS1 or E1 links to higher

speed services without tearing down the original link or interrupting existing services. Instead, additional links

can be dynamically added to the existing IMA group to increase its available bandwidth. The flexibility of the

Lucent solution contrasts with competing IMA implementations that require service providers to interrupt

services by tearing down the circuit, deleting the existing logical port and creating a new IMA group to change

the available bandwidth.

Finally, service providers applying for tariff permission to implement IMA in a given region can use the IMA

modules to offer high-density DS1 or E1 services while the applications are being processed. Once the tariff

applications are approved, the new services can be immediately turned up.

Providing flexible bandwidth services to ATM users

The most obvious service application for IMA is to offer flexibly sized bandwidth services between DS1 and

DS3 or E1 and STM-1.Service providers can use the IMA modules in the CBX 500 Multiservice WAN Switch

and PacketStar PSAX Multiservice Media Gateways to offer scalable, flexible IMA services in 1.5 Mbps or

2 Mbps increments. For example, if a customer has enough voice, video, and data traffic to consume between

5 and 6 Mbps of bandwidth, depending on traffic bursts, an IMA-based service bundling four DS1s or three E1s

together can be provided to offer a 6 Mbps service. Without IMA, the the customer is forced to purchase a

more costly DS3 (45 Mbps) or STM-1 (155 Mbps), both of which may provide far more bandwidth than thecustomer truly needs.

Free-up slots on existing chasses for other services by replacing

multiple lower-density modules with a single high-density

IMA module.


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Built-in IMA capabilities on the IMA modules available for the Lucent CBX 500 Multiservice WAN Switch and

the PacketStar PSAX Multiservice Media Gateway family aggregate multiple DS1 or E1 ATM cell streams into a

single logical interface or IMA group. The CBX 500 Multiservice WAN Switch modules can be used to create

aggregated IMA groups using two to eight DS1s or two to nine E1s. Alternatively, service providers can use the

IMA modules available for the PacketStar PSAX Multiservice Media Gateway family to create aggregated IMA

groups using anywhere from two to 21 E1 links or two to 28 DS1 links. This allows service providers the

flexibility to offer a broad range of interim bandwidth services for direct ATM interfaces (see Figure 8 below).

Figure 8 Service providers can use the modules available for the Lucent CBX 500 Multiservice WAN switch and

PacketStar PSAX Multiservice Media Gateway family to create a wide range of bandwidth services to help bridge the

gap between entry-level DS1 or E1 access services and more expensive DS3, E3, or STM-1 services.

Providing flexible bandwidth services for non-ATM users

In addition to supporting direct ATM interfaces, service providers can use IMA support on the CBX 500

Multiservice WAN Switch with ATM Forum IMA v1.1-compliant customer premise equipment (CPE) to provide

access to flexible, higher bandwidth services to end-users over familiar, non-ATM interfaces, such as FrameRelay, Ethernet, private line, and voice. To offer this service, each DS1 or E1-based IMA group on the CBX 500

switch is associated with an ATM UNI interface to an IMA-capable CPE, such as a member of the PacketStar

PSAX Multiservice Media Gateway family. This configuration eliminates the complexities of ATM access for the

end-user, while preserving ATMs flexible and reliable service management structure. Figure 9 illustrates this

application using the CBX 500 3-Port Channelized DS3/1 with IMA ATM module.

Switch Module IMA groups per moduleConstituent links |perIMA group

Range of potentialbandwidth services10

CBX 500 MultiserviceWAN Switch

3-Port Channelized DS3/1with IMA ATM module

Up to 42 groups 2 to 8 DS1 links 1.5 to 12 Mbps,in 1.5 Mbps increments

1-Port ChannelizedSTM-1/E1 with IMAATM module

Up to 30 groups 2 to 9 E1 links 2 to 18 Mbps,in 2 Mbps increments

PacketStar PSAXMultiservice MediaGateway family

6-port DS1 IMA module Up to 3 groups 2 to 6 DS1 links 3 to 9 Mbps,in 1.5 Mbps increments

6-port E1 IMA module Up to 3 groups 2 to 6 E1 links 4 to 12 Mbps,in 2 Mbps increments

Medium Density 12-portDS1 IMA module

Up to 6 groups 2 to 12 DS1 links 3 to 18 Mbps,in 1.5 Mbps increments

High Density 21-port E1IMA module

Up to 10 groups 2 to 21 E1 links 4 to 42 Mbps,in 2 Mbps increments

1-port Channelized DS3IMA module11

Up to 14 groups 2 to 28 DS1 links 3 to 42 Mbps,in 1.5 Mbps increments

10 Any DS1 or E1 link on this module can be designated as a non-IMA link or as an IMA group with only one constituent link

designated to provide 1.5 Mbps or 2 Mbps access services.

11 Supports 28 DS1 interfaces.


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Figure 9 The 3-Port DS3/1 Channelized ATM with IMA module delivers bandwidth between 1.5 and 45 Mbps for end

user connections and network links. The I/O module supports direct ATM interfaces, as well as non-ATM interfaces

from certain Frame Relay, Ethernet, private line, and voice equipment located on the customer premise.

Supporting converged services with links below DS3

In the past, enterprises that required more than one type of service from a service provider would also requiremore than one way to access those services. Each access method carried a specific type of traffic (i.e. voice or

Frame Relay or IP traffic) to support a specific business application, but when circuits are reserved for specific

applications or traffic types, this sacrifices the ability to use ATM to consolidate and efficiently manage diverse

traffic types across those circuits.

Offering converged access over ATM enables service providers to use the inherent strengths of ATM to simplify

enterprise WAN access. To offer the service, the service provider uses IMA to create a single access pipe with

enough bandwidth to support all WAN access needs (voice, video, SNA, LAN, Frame Relay, ATM, and other data

services). ATMs strict QoS and traffic management supports converged voice, video, and data services over the

IMA link. With IMA, service providers can once again take advantage of ATMs statistical multiplexing and load

balancing capabilities to efficiently manage edge bandwidth.

The following two diagrams illustrate the potential impact. In the first diagram, the service provider is usingthree separate circuits to handle voice, video, and data traffic. Circuit 1, which handles only voice traffic, is at

70% capacity. Circuit 2, which handles only video traffic, is at 20% capacity. Circuit 3, which handles data traffic,

is at 80% capacity. Since the data traffic continues to grow steadily, the service provider is currently evaluating

whether to deploy a fourth circuit to handle the data traffic overflow.

Figure 10 Using three separate circuits to handle specific types of traffic without IMA.

Circuit 1Voice only

Circuit 3Data only

Circuit 2Video only

70% 80%20%


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In the next diagram, the links are bundled into a single IMA group to carry voice, video and data traffic. This

enables the service provider to use the isolated bandwidth on Circuits 1 and 2 to support data traffic overflow. By

using IMA to logically consolidate the existing circuits, the service provider has freed up an entire circuit to

support other customers relieving congestion at the network edge. In addition, since the IMA group continues

working even if one of the constituent links fails, fault tolerance for the converged services is also improved.

Figure 11 Supporting converged services using an IMA group. IMAs more efficient bandwidth management enables

the service provider to eliminate the third link. Because IMA uses a round-robin scheduler to balance the load, the

remaining two links operate at 85% capacity.

Service providers can offer customers a single integrated bill and a single circuit to support all services, which

leads the industry to refer to converged access over IMA as a one pipe, one bill service.

Reducing the cost of network backhaul

Service providers can also use IMA within the network backbone to create efficient low-speed trunks in areas

where DS3 or STM-1 lines are not cost-effective or simply not available. With IMA, additional DS1 or E1 lines

can provide added incremental bandwidth as demand grows without sacrificing the ability to use ATMs QoS,

traffic management, and statistical multiplexing capabilities to efficiently manage diverse traffic types across

multiple circuits.

The following diagram illustrates how a wireless service provider can use IMA on any PacketStar PSAX

Multiservice Media Gateway for cost-effective backhaul from the cell site. As the diagram shows, service

providers can use the PSAX to provide efficient cell site backhaul for a variety of wireless networks including

CDMA, TDMA, GSM, and AMPS as well as 3G voice and data services. PSAX provides service providers with the

flexibility to migrate from GSM to UMTS without upgrading their backhaul equipment. PacketStar PSAXMultiservice Media Gateways also offer unique, low-cost scalability. PacketStar PSAX 1000 can support smaller

cell sites, then evolve to a higher capacity PacketStar PSAX 2300 as demand grows. All PSAX chassis use an

interchangeable set of modules, so service providers can use equipment from existing backhaul applications to

scale their networks. Finally, using the IMA capabilities on PSAX media gateways to provide appropriately sized

backhaul pipes for existing wireless services can significantly reduce recurring facilities costs.

Circuit 1Data, Voice & Video

Circuit 3No longer needed

Circuit 2Data, Voice & Video

100% 70%


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Figure 12 Wireless service providers can use the IMA modules available for the PacketStar

PSAX Multiservice MediaGateway family to reduce cell site backhaul costs throughout their networks.

Similarly, the IMA capabilities on the CBX 500 Multiservice WAN Switch can reduce facilities costs at radio

network control (RNC) and node hub sites within their wireless networks. The following diagram illustrates this

application. In the diagram, the service provider uses the IMA capabilities on the CBX 500 switch to avoid

managing physical E1 links at the RNC site. This gives the service provider more flexibility to manage bandwidth

and complete re-homing procedures. The IMA capabilities on the CBX 500 Multiservice WAN Switch allow

ATM traffic to be aggregated on the remote side of the node hub site and help reduce leased line costs by utilizing

trunks at speeds lower than DS3.

Figure 13 Wireless service providers can use IMA capabilities on the CBX 500 Multiservice WAN Switch to simplify

bandwidth management and re-homing procedures at RNC sites.

Wireline service providers can use IMA in a similar fashion to create cost-effective, interim-speed trunks between

access and edge devices (or edge and core devices). When deployed intelligently throughout wireless and

wireline networks, IMA can help service providers reduce their facilities costs and more efficiently leverage the

available bandwidth in their network backbones.

IMA Group

IMA

ST

M1

CBX500

MuxNode

B

OpticalTransportNetwork

OpticalTransportNetwork

RadioNetwork

Controller

E1s Chan.STM1

Chan.STM1

Chan.STM1


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5. Conclusion

As this paper demonstrates, adding one or more IMA modules to existing Lucent CBX 500 Multiservice WAN

Switches or PacketStar PSAX Multiservice Media Gateways can be a cost-effective way to generate new revenue

opportunities from the customized bandwidth and converged services markets. At the same time, implementing

IMA on existing Lucent multiservice switches enables service providers to increase service density and use the

bandwidth already in their network more efficiently reducing facilities costs and relieving congestion in

network hubs and at network access points.

Appendix A: Acronyms

The following table lists the acronyms used in this paper.

Acronym Meaning

ABR Available Bit Rate

CDMA Code Division Multiple Access

CPE Customer Premise Equipment

FCP Flow Control Processor

ICP IMA Control ProcessorICP IMA Control Protocol

IMA Inverse Multiplexing for ATM

MR Monthly Revenue Potential

NNI Network to Network Interface

PVC Permanent Virtual Circuit

QoS Quality of Service

RNC Radio Network Control

SNA Systems Network Architecture

SVC Switched Virtual Circuit

TCO Total Cost of Ownership

TDM Time Division Multiplexing

UNI User to Network Interface

VC Virtual CircuitWAN Wide Area Network

Specifications subject to change without notice. Contact your

Lucent representative for more information on availability and

upgrades. Lucent reserves the right to change, modify, transfer

or other wise revise this publication without notice.

To learn more about our comprehensive portfolio, please

contact your Lucent Technologies Sales Representative,

Lucent Business Partner or visit our web site at

http://www.lucent.com/solutions/core_switching.html.

CBX 500, Navis, NavisCore, PacketStar and Stinger are

registered trademarks of Lucent Technologies, Inc.

Copyright 2003

Lucent Technologies Inc.

All rights reserved

MSS v1 3/03

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