02c_atm services_dn03481353_2_en_global_pdf_paper_a4_d.pdf

ATM Services

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ATM Services

Contents

Contents 3

1 ATM services 5

2 Virtual connections in ATM 7

3 Statistical multiplexing 11

4 ATM layer service categories 13

5 Usage/Network Parameter Control (UPC/NPC) 15

6 Traffic shaping 17

7 Buffer management 21

8 Scheduling 23

Related Topics 27



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Contents



ATM Services

1 ATM services

ATM services provide a mechanism to ensure that the Quality of Service (QoS)remains as high as possible while the operator is able to utilise the networkcapacity in an efficient way. The bandwidth of ATM connections can be allocatedflexibly according to application needs. The set of actions taken to achieve thisgoal in ATM networks is called traffic management. ATM traffic managementincludes the following functions: traffic shaping, policing, buffer managementand scheduling.

One of the strengths of ATM is that it can use statistical multiplexing and providea communication with a bit rate that is individually tailored to fit the actual need,including variable bit rates. ATM also provides a two-level connection hierarchy:Virtual Path (VP) and Virtual Channel (VC) level connections. This two-levelhierarchy can be used for providing efficient traffic engineering in ATMnetworks.

The benefit of statistical multiplexing is based on the fact that the behaviour ofmany applications is bursty by nature. In other words, the amount of requiredbandwidth varies by time. For a group of bursty connections, less bandwidthneeds to be reserved than if bandwidth reservation was based on the peak rate ofthe connections. This makes it possible to save transmission bandwidth in somecases.

The virtual path concept enables the grouping of virtual channels that sharecommon paths through the network. These ATM connections can then bemanaged as a smaller number of virtual paths instead of a larger number ofindividual virtual channels.



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ATM services



ATM Services

2 Virtual connections in ATM

Logical connections in ATM are referred to as virtual connections. A two levelrouting hierarcy is defined for ATM virtual connections: virtual path connections(VPC) and virtual channel connections (VCC). A virtual channel identifier (VCI)identifies a particular virtual channel (VC) link for a given virtual pathconnection. Furthermore, a virtual path identifier (VPI) identifies a particularvirtual path (VP) link. A specific value of VPI and VCI is assigned each time avirtual channel is switched in the network so the identifiers have only localmeaning.

VPC can be seen as a bundle of VCCs that have the same end points. Thus, all thecells flowing in a single VPC can be switched together. Virtual paths are used forbundling a number of virtual channels into a larger stream which is routedthrough ATM switches, that is, cross-connection and switching can be done on aVPC level instead of the individual VCC level. The figure below shows therelationship between the virtual channel, the virtual path and the transmissionpath.

Figure 1. Relationship between the transmission path, VP and VC

VC and VP switching

The following figures show the concepts of VC and VP switching. VP switchesterminate VP links. A VP switch translates incoming VPIs to the correspondingoutgoing VPIs according to the destination of the VPC whereas VCI valuesremain unchanged. VC switches terminate VC links and also VPC connections. AVC switch must, therefore, switch both VPs and VCs, so that both VPI and VCItranslations are performed.

VCVP

VCVP

VCVP

Transmission path

VPVC

VPVC

VPVC



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Virtual connections in ATM

Figure 2. VC and VP switching

Figure 3. VP switching

VC switch/cross-connect

VP switch/cross-connect

VCI 21

VCI 22

VCI 24

VCI 23

VCI 21

VCI 22

VCI 21

VCI 22

VCI21

VPI1

VPI3

VPI2

VCI22

VCI23

VCI24

Endpointof VPC

VPI 4 VPI 5

VPI 1

VPI 3

VPI 2

VP switch/cross-connect

VCI 23

VCI 24

VCI 21

VCI 22

VCI 25

VCI 24

VCI 21

VCI 22

VCI 23

VCI 24

VCI 25

VCI 24

VPI 1

VPI 2

VPI 3

VPI 4

VPI 5

VPI 6



ATM Services

Advantages of having VPCs

. Simplified network architecture: By grouping the virtual channels into avirtual path, the entire groups can be routed through the network based onthe VPI.

. Increased network performance and reliability: The network deals withfewer, aggregated entities.

. Segregation of traffic: A form of priority control can be implemented bysegregating traffic types requiring different Quality of Service (QoS) todifferent virtual paths.

. Reduced processing and short connection-setup time: Much of the work isdone when the VPC is set up. By reserving capacity on a VPC inanticipation of later call arrivals, new VCCs can be established byexecuting simple control functions at the end points of the VPC; no callprocessing is required at transit nodes. Thus, adding new VCCs to anexisting VPC involves minimal processing which decreases the connectionsetup time.



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ATM Services

3 Statistical multiplexing

The number of cells the users of the ATM network generate depends on theamount of information they send. The amount of network resources required by auser changes constantly. When these resources are shared among several users itis very unlikely that all users send data at their peak cell rate simultaneously. Thismeans that the network operator can either reduce the resources required for acertain amount of traffic or accommodate more traffic with the same amount ofresources. This is called statistical multiplexing.

The figure below shows an example of statistical multiplexing. The picture on theleft shows the required amount of bandwidth when the capacity of eachconnection is reserved according to the peak cell rate. The following picture onthe right shows the statistical multiplexing gain, when the principle of statisticalmultiplexing is used in the bandwidth reservation.

Figure 4. Statistical multiplexing gain

Statistical multiplexing is most efficient when the peak cell rate of eachmultiplexed connection is small compared to the total capacity of the physicallink, traffic is bursty and traffic sources are independent.

When Virtual Paths are used, two levels of multiplexing exist: VC level and VPlevel. At the VC level, VCs are statistically multiplexed on a VP. At the VP level,VPs are either deterministically or statistically multiplexed on a physical link.

R1

R2

R3

C1

C2

C3

Gain

requiredbandwidth

C1

C2

C3



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Statistical multiplexing

If VPs are deterministically multiplexed, they do not share the bandwidthreserved for them with the other VPs on the same link. The sum of the reservedbandwidths of the VPs cannot exceed the bandwidth of the link. If VPs arestatistically multiplexed, they share the bandwidth nominally reserved for themwith the other VPs on the same link. VPs do not have strictly reservedbandwidths.



ATM Services

4 ATM layer service categories

The traffic in ATM networks has different demands as to the Quality of Service(QoS). One way to fulfill these requirements is to create traffic classes, alsoknown as service categories. All virtual connections within the same traffic classhave the same kind of QoS requirements. For information on QoS, see ATMquality of service.

Service categories

Constant Bit Rate (CBR) and Unspecified Bit Rate (UBR) service categories aresupported according to the ATM Forum Traffic Management SpecificationVersion 4.1. For more information on service categories, see Traffic ManagementSpecification Version 4.1, ATM Forum.

. CBR is used by applications which request a static amount of bandwidththat is continuously available during the lifetime of the connection. Thisstatic amount of bandwidth has a peak cell rate value. In the CBRcapability, the source can emit cells at the peak cell rate at any time and forany duration and the QoS commitments still pertain. Cells that are delayedbeyond the value specified by maxCTD (cell transfer delay) are assumed tobe of significantly reduced value to the application.

. The real time Variable Bit Rate (rt-VBR) service category is intended forreal time applications that transmit at a rate which varies over time. Rt-VBR connections are characterised in terms of a Peak Cell Rate,Sustainable Cell Rate, and Maximum Burst Size. Cells that are delayedbeyond the value specified by maxCTD are assumed to be of significantlyreduced value to the application. The rt-VBR service category is currentlynot supported.

. The non-real time Variable Bit Rate (nrt-VBR) service category is intendedfor non-real time applications that have bursty traffic characteristics. It hasthe same traffic parameters as rt-VBR. However, no strict delay boundariesare associated with this service category. The nrt-VBR service category iscurrently not supported.

. UBR is meant for non-real-time applications that do not have strictrequirements for delay and delay variance. As It does not have aguaranteed QoS, it is also known as "best effort" traffic.



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ATM layer service categories



ATM Services

5 Usage/Network Parameter Control (UPC/NPC)

Traffic policing, also known as usage/network parameter control (UPC/NPC), isused for monitoring the compliance of virtual connections to agreed trafficcontracts. Traffic contract defines traffic descriptors (for example, peak cell rate)for each connection. UPC is applied in User-Network Interface (UNI) and NPC isperformed in network node interface.

Traffic policing is defined as a set of actions taken by the network to monitor andcontrol the amount of incoming ATM traffic. The main purpose is to protectresources from misbehaviour which can affect the Quality of Service (QoS) ofother already established connections. This is done by detecting violations ofnegotiated traffic contract and taking appropriate actions. At the ATM cell level,actions may include cell passing, cell tagging (only for CLP=0 cell stream) andcell discarding. Usage/network parameter control ensures that the QoS objectivesare met for all compliant connections.

Network elements support a dual leaky-bucket policing algorithm defined byATM Forum. Policing can be enabled or disabled on a per virtual connectionbasis. Traffic policing parameters are configured via Element Managementinterface when the ATM Permanent Virtual Connection (PVC) is created.

The policing service is available only for CBR connections in network interfaceunits. Policing of CBR VCCs applies to CLP=0+1 cell stream, and the non-conforming cells are always discarded.



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Usage/Network Parameter Control (UPC/NPC)



ATM Services

6 Traffic shaping

Network elements may introduce supplementary cell delays that shape theoutgoing traffic streams. Need for traffic shaping raises from multiplexingtogether bursty traffic coming from different sources. Output schedulingdetermines which queued cell is chosen to be transmitted out to the interface.

Shaping generally allows an increase in the network utilization by means ofgreater multiplexing gain. The behaviour of the whole public network is morepredictable after traffic shaping since the traffic behaves more smoothly.

Generally shaping means that there is a reduction in the use of Cell DelayVariation (CDV) because cell spacing is used in the outgoing line. If outgoingtraffic has not been shaped, multiplexing usually causes different transmissiondelays and, hence, CDV tolerance value is larger than 0. This causes bursts to thenetwork cloud and leads to poor behaviour of the whole network. Shaping is,therefore, an important feature from the whole network point of view.

Figure Generic placement of policing and shaping represents generic placementof policing and shaping.



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Traffic shaping

Figure 5. Generic placement of policing and shaping

In the figure above you can see that if traffic shaping has not been done foroutgoing traffic in the network element x, traffic will be bursty in the networkelement y. Therefore, there may be a high cell loss probability in the receivingnetwork element y.

In this network element the VC level traffic shaping capability has beenimplemented for CBR VCCs in all units that originate traffic. VC level trafficshaping is always enabled for CBR VCCs, and cannot be applied to UBRVCCs.

VP (VP connection termination point) level traffic shaping has been implementedin all network interface unit types in egress direction. VP level traffic shaping is aconfigurable function that can be enabled or disabled. However, enabling VPlevel shaping is usually mandatory in public ATM networks that police theincoming traffic VP level (for example, when ATM VP leased line service isused). The shaping mode parameter cannot be changed after VP level object hasbeen created except by deleting and creating the given object again. VP leveltraffic shaping is only applicable to VP connection termination points, not for VPlevel semipermanent cross-connections.

VP shaping requires additional hardware resources for scheduling compared tonon-shaped VPs. Therefore the supported number of shaped VPs is limited on aper network interface unit basis.

VP shaping uses the following hardware resources:

Policing

Shaping

Policing

Shaping

Shaping

Policing

Shaping

Policing

Trafficreceiver/generator

Trafficreceiver/generator

Networkelement x

Networkelement y

PublicNetwork



ATM Services

. basic scheduler (amount: one per logical ATM interface) that supports twoshaped VPs

. extra type 2 scheduler (amount: 13 per NIS1/NIS1P unit) that supports fiveshaped VPs

. extra type 1 scheduler (amount: 16 � # of configured logical ATMinterfaces) that supports three shaped VPs

The hardware resource allocation is based on the following principles:

1. One basic scheduler is allocated for each logical ATM interface. The firsttwo shaped VPs on that interface are allocated to the basic scheduler. Alsothe non-shaped VPs use the basic scheduler.

2. When all the shaped VP resources of the basic scheduler are used, the nextfree extra type 2 scheduler is allocated to the interface. The next fiveshaped VPs on that interface are allocated to it.

3. When all extra type 2 schedulers are allocated, a free extra type 1 scheduleris allocated to the interface. The next three shaped VPs on that interface areallocated to it.

4. An attempt to create a shaped VP is rejected when all the shaped VPs ofthe allocated schedulers are used, and there are no longer free schedulersavailable. Please note that some other logical ATM interface on the unitmight still have room for a new shaped VP due to the granularity ofscheduler resources used for shaping the VPs. This practical limitation dueto scheduler granularity might be reached before the theoretical maximumamount of shaped VPs are created, depending on how many ATMinterfaces are configured and how the shaped VPs are distributed betweenthe configured interfaces.

For information on the number of shaped VPs, see VP/VC link termination points.



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Traffic shaping



ATM Services

7 Buffer management

Buffer management is used for controlling network elements' internal bufferscovering reservation rules and control mechanisms. Buffer management selectswhich cells or frames are dropped when the network element is overloaded. Ingeneral, it is more preferable to drop the whole frame (for example, AAL5packet) instead of dropping individual cells from many upper layer packets.Another important purpose of buffer management is controlling the fairness ofresource allocation between individual connections and different traffic classes.

Buffer management schemes in network elements

Cell loss priority bit in ATM cell header can be used to generate different prioritycell flows within a virtual path connection or virtual channel connection.Selective cell discard buffer management method ensures that lower priority(CLP=1) ATM cells are dropped before higher priority CLP=0 cells in congestionsituation. When buffer occupancy reaches pre-configured threshold value, buffermanagement starts to discard incoming lower priority CLP=1 cells.

Early Packet Discard (EPD) and Partial Packet Discard (PPD) buffer managementmethods can be taken into use for VCs carrying AAL5 traffic. The EPD thresholdis evaluated before a new AAL5 frame is admitted to the buffer. If the bufferthreshold is exceeded, all cells from the AAL5 frame are discarded. PPD occurs ifa user cell is discarded because of policing violations, a cell loss priority (CLP1or CLP0+1) threshold violation or if no free buffer space is available. PPDdiscards all remaining user cells of the AAL5 frame except for the last cell of theframe.

EPD and PPD can be used to improve the throughput of upper layer protocolssuch as TCP/IP. If some cells which are part of a larger packet are dropped forsome reason, the remaining cells are not sent forward since that entire packet hasto be retransmitted. Enabling EPD/PPD is recommended for IP over ATM andsignalling VCCs.



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Buffer management

Explicit Forward Congestion Indication (EFCI) informs about congestion in theupstream direction. Any ATM network element can set the congestion indicationbit of payload type field in the ATM cell header to indicate that there iscongestion in the network. When the buffer occupancy level exceeds theconfigured threshold value, the network element starts to mark cells with thecongestion indication.



ATM Services

8 Scheduling

Traffic scheduling is needed to support service differentiation provided throughdifferent traffic classes. The main purpose of scheduling is to guarantee agreedQoS and traffic contract for each individual connection and virtual path. A virtualpath in this context means a group of connections under the same VP. Thescheduling functionality is implemented primarily at NIP1, NIS1, and MXU.

The traffic scheduler hardware in network interface units NIS1 and NIP1 takescare of utilising resources efficiently and allocates available resources fairlybetween different VC and VP level objects. Scheduling parameters are controlledby ATM level connection admission control software.

There are two types of schedulers in the scheduler hardware. The first sixteenschedulers (that is, subports from 0 to 15) are denoted as type 1 schedulers.Subports from 16 to 31 are denoted as type 2 schedulers. In addition to this, thereis one specific scheduler for ingress direction scheduling. Scheduling has beenimplemented as a hierarchical, three-level structure in the ingress direction and asa hierarchical, four-level structure in the egress direction. Figure General ingressand egress direction scheduler structure shows this structure.

GTS

EBS

SEL

UES

URS

SEL

SHAPING

Class 1

Class 3

GTS

EBS

CBR

Class 4

Class 0

SEL

INGRESS STRUCTURE

EGRESS STRUCTURE

To UTOPIAPort 0 to 31

To FabricPort 0 to 39

Sixteen Type 1Schedulers 0 to 15

Sixteen Type 2Schedulers 16 to 31

NONSHAPING

Class 2

Class 1

Class 3

Class 4

Class 0

Class 2

Class 1

Class 3

Class 4

Class 0

Class 2

SPS



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Scheduling

Terms:

EBS Excess Bandwidth Scheduler

GTS Guaranteed Traffic Scheduler

SEL Selector

SPS Subport Scheduler

UES Utopia Excess Scheduler

URS Utopia Rate Scheduler

Figure 6. General ingress and egress direction scheduler structure

All the scheduling points marked with gray color contain shaping capability,which is taken into account in the network interface units to egress direction forVP level shaping. VP level shaping is an optional feature defined by the operator.The used scheduling algorithms are:

. Asynchronous Shaped Virtual Clock scheduling in class 0 in type 1scheduler and ingress scheduler

. Starting Potential Fair Queueing scheduling in class 1 in type 1 schedulerand in ingress scheduler

. Weighted Round Robin in classes 3 and 4 in type 1 schedulers and iningress direction scheduler as well as in all classes in type 2 scheduler

. Synchronous Shaped Virtual Clock scheduling in GTS scheduler and SPSscheduler

. Weighed Round Robin scheduling in EBS scheduler

The model according to which the scheduler reservations for classes 0 to 4 areused depends on the unit, direction and the ATM traffic class in question.

NIS1 and NIP1 egress direction scheduling towards network

Figure NIS1 and NIP1 egress direction scheduling illustrates the schedulerreservations in the network interface units towards the network.



ATM Services

Figure 7. NIS1 and NIP1 egress direction scheduling

The scheduler class usage in egress direction in the network interface is tied to VPlevel shaping. Whenever a new shaped VPCtp has been created (or removed), onespecific scheduler class for the given VPCtp must be reserved (or released). Thisoperation will be automatically done by the system. All non-shaped CBR andUBR objects will share the scheduling resource in the basic type 1 schedulerreserved for each external interface.

In short, the rules of the scheduler hardware usage are the following:

Class 0

Class 1

Class 2

Class 3

Class 4

Unused

Unused

Unused

Internal use

Unused

EBS

Unused

Unused

CBR and UBR VC in shaped CBR VPCtp



Class 0

Class 1

Class 2

Class 3

Class 4

Unused

CBR VPXC, CBR VC in non-shapedCBR VPCtp, (CBR VC in UBR VPCtp)

UBR VPXC, UBR VC in UBR VPCtp



Class 0

Class 1

Class 2

Class 3

Class 4

GTS

EBS






Class 0

Class 1

Class 2

Class 3

Class 4

SEL

SEL

SPS inshapingmode

UES

URS

SEL

SPS inshapingmode

Network interface,Basic Type 1

Subport Scheduler

Network interface,Extra Type 2

Subport Scheduler

Unit comp. interface,Type 2

Subport Scheduler

To UTOPIAPort 0 to 31

Network interface,Extra Type 1

Subport Scheduler

GTS



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Scheduling

. Basic type 1 scheduler of has been mapped to an external interface whenthe interface has been created.

. Extra type 2 schedulers have been mapped to a given external interfaceafter there are no appropriate classes in the basic scheduler.

. When extra type 2 schedulers have run out, extra type 1 schedulers aretaken into use.

. All non-shaped real-time VC or VP level connections use class 1 from thebasic scheduler.

. All non-shaped non-real-time VC or VP level connections use class 2 fromthe basic scheduler.

Both schedulers and scheduler classes are dynamically reserved and releasedwhenever external objects are reserved or released. Scheduler reservations arecontrolled by connection admission control.

The same scheduler is used also in the multiplexer unit. Multiplexer unitscheduling is not described in this documentation since it is an internal designissue.



ATM Services

Related Topics

ATM services

Descriptions

ATM quality of service

Switching and multiplexing in ATM network


Descriptions

ATM services


Statistical multiplexing

Descriptions

ATM services


ATM layer service categories

Descriptions

ATM services



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Related Topics


Usage/Network Parameter Control (UPC/NPC)

Descriptions

ATM services


Traffic shaping

Descriptions

ATM services


Buffer management

Descriptions

ATM services

Scheduling

Descriptions

ATM services



ATM Services

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