Atm Basics

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3G ATM

ATM Basics

Training Document

6-65997Issue 3.0 en

© Nokia Networks Oy 1 (24)



ATM Basics

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2 (24) © Nokia Networks Oy 6-65997Issue 3.0 en



Contents

Contents

1 Objectives ...............................................................................................4

2 Network Transfer Mode .........................................................................5 2.1 Comparison between Synchronous and Asynchronous

Multiplexing...................................................................................5 2.2 Synchronous Transfer Mode....................................................................6 2.3 Packet Transfer Mode..............................................................................6 2.4 Asynchronous Transfer Mode (ATM) .......................................................7

3 ATM cell ................................................................................................10 3.1 Fields in the ATM cell header.................................................................12

4 ATM connection ...................................................................................13 4.1 ATM virtual connections.........................................................................13

5 ATM as a transport network................................................................15 5.1 Network elements involved in the transport of user plane

information..................................................................................16 5.2 Example of Nokia ATM cross-connect ...................................................18

6 Statistical multiplexing ........................................................................19

7 Review questions.................................................................................20

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ATM Basics

1 Objectives

After completing this module, the student should be able to:

• Identify different modes to implement Network Transfer

• Describe the main characteristic of an ATM network in terms of function

and capacity as opposed to more traditional methods.

• At an overview level, explain how the ATM connection is structured and

how routing is achieved in an ATM network.

• List the ATM interfaces in a 3G network.




Network Transfer Mode

2 Network Transfer Mode

2.1 Comparison between Synchronous andAsynchronous Multiplexing

Asynchronous multiplexing (which is used in ATM technology), is more

efficient than synchronous multiplexing technologies, such as Time Division

Multiplexing (TDM).

With TDM, each user is assigned to a time slot or time slots, and no other

station can send in that time slot or those time slots. If a station has a lot of data

to send, it can send only when its time slot comes up, even if all other time slotsare empty. If, however, a station has nothing to transmit when its time slot

comes up, the time slot is sent empty and is wasted. With asynchronous

multiplexing nature, 'bandwidth on demand' is offered, that is, the user traffic

can be sent at any available time slot according to the agreement between the

user and the network.

CACCC

A A

C C C C

A

CACAB

A A

BB B BB B B B

BB B BB B B B

C C C C

ABC

AsynchronousMultiplexing

SynchronousMultiplexing

Figure 1. Synchronous and Asynchronous Multiplexing

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ATM Basics

2.2 Synchronous Transfer Mode

Synchronous Transfer Mode is based on Time Division Multiplexing (TDM)

technologies.

PDH (Plesiochronous Digital Hierarchy) was introduced in the middle of the

70s and SDH (Synchronous Digital Hierarchy) in the middle of the 80s. These

technologies have been developed to transmit voice, but of course SDH

networks are also often used for data transmission.

Switched circuits allow data connections that can be initiated when needed and

terminated when the communication is complete. This works similar like a

normal telephone line does work for voice communication. Integrated Services

Digital Network (ISDN) is a good example of circuit switching. When a router

has data for a remote site, the switched circuit is initiated with the circuit

number of the remote network. In the case of ISDN circuits, the device actuallyplaces a call to the telephone number of the remote ISDN circuit. When the two

networks are connected and authenticated, they can transfer data. When the data

transmission is complete, the call can be terminated.

Figure 2. Synchronous Transfer Mode

2.3 Packet Transfer Mode

Packet switching is a WAN (Wide Area Network) technology in which users

share common carrier resources. Because this allows the carrier to make more

efficient use of its infrastructure, the cost to the customer is generally much

better than with point-to-point lines. In a packet switching setup, networks have

connections into the carrier's network, and many customers share the carrier's

network. The carrier can then create virtual circuits between customers' sites by

which packets of data are delivered from one to the other through the network.

The section of the carrier's network that is shared is often referred to as a cloud.





Some examples of packet-switching networks include: Frame Relay (see

below), Switched Multimegabit Data Services (SMDS), and X.25.

Figure 3. Packet Transfer Mode

Packet Transfer Mode operates at the physical and data link layers of the OSI

reference model.

Frame Relay is an example of a packet-switched technology. Packet-switched

networks enable end stations to dynamically share the network medium and the

available bandwidth. The maximum transmission rate is the physical interfacespeed of the end stations. Normally you have a guaranteed bandwidth which is

called Committed Information Rate (CIR).

Frame Relay reduces network overhead by implementing simple congestion-

notification mechanisms rather than explicit, per-virtual-circuit flow control.

Frame Relay is typically implemented on reliable network media, so data

integrity is not sacrificed because flow control can be left to higher-layer

protocols.

2.4 Asynchronous Transfer Mode (ATM)

The basic functioning of Asynchronous Transfer Mode (ATM) can be compared

with the working principle of a cable car. The cabins move constantly in up-

direction on one side and in down-direction on the other side. If a high number

of passengers have to be transported, almost every cabin will be used well, the

capacity of the cable car is highly used. If there are less passengers, some of the

cabins stay empty.

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Figure 5. Asynchronous Transfer Mode (ATM)

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ATM Basics

3 ATM cell

The user traffic is split and delivered in fixed length packets called ATM cells.

The size of the cell is 53 bytes, which is divided into a 5-byte header and a48-byte payload field. The ATM cell is relayed by a label at the header:

Virtual Channel Identifier (VCI) and Virtual Path Identifier (VPI).

Header5 bytes

Payload48 bytes

53 bytes

Figure 6. ATM cell structure

There are two formats of an ATM cell (depending on the type of the interface):

• ATM UNI (User-Network Interface) cell, that is used for

communication between ATM endpoints and ATM switches.

• ATM NNI (Network-Node Interface) cell, that is used for

communication between ATM switches.

For ATM interfaces in 3G network, User-Network Interface (UNI) refers to the

interface between terminal equipment and a network termination where accessprotocols apply. The interface between a RNC and a WCDMA BTS is seen as

an UNI interface.

Network-Node Interface (NNI) is the interface between two network nodes like

a RNC and an MGW.

Figure 7 shows the ATM interfaces between network elements in 3G network.




ATM cell

PSTNMGW MSCBSUE

A BIu-CSIubUu

UNI NNI

IP networkGGSN

Iu-PS

NNI

RNC

SGSN

RNCBS

BS

Iur

NNI

UNI

UNI

ATM is employed

Gn Gi

Figure 7. ATM interfaces in 3G network

There is a slight difference between the first byte of the UNI and NNI header.

The NNI header does not include the Generic Flow Control (GFC) field.

Additionally, the NNI header has a Virtual Path Identifier (VPI) field that

occupies the first 12 bits, allowing larger trunks between public ATM switches.

GFC Generic Flow ControlVPI Virtual Path IdentifierVCI Virtual Channel Identifier

PT Payload TypeCLP Cel l Loss Pr ior ityHEC Header Error Control

VCI

GFC VPI

VPI

VCI

VCI PT CLP

HEC

123457 68

VCI

VPI

VPI

VCI

VCI PT CLP

HEC

123457 68

User Network Interface (UNI) Network Node Interface (NNI)

Payload Payload

Header(5 bytes)

Payload(48 bytes)

Figure 8. Basic ATM cell format

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ATM Basics

3.1 Fields in the ATM cell header

Generic Flow Control (GFC)Provides local functions, such as identifying multiple stations that share a single

ATM interface. This field is typically not used and is set to its default value.

Virtual Path Identifier (VPI)

In conjunction with the VCI, identifies the next destination of a cell as it passes

through a series of ATM switches on the way to its destination.

Virtual Channel Identifier (VCI)

In conjunction with the VPI, identifies the next destination of a cell as it passes

through a series of ATM switches on the way to its destination.

Payload Type (PT)

Indicates in the first bit whether the cell contains user data or control data. If the

cell contains user data, the second bit indicates congestion, and the third bit

indicates whether the cell is the last in a series of cells that represent a single

AAL5 frame.

Cell Loss Priority (CLP)

Indicates whether the cell should be discarded if there is congestion in the

network. If the CLP bit equals 1, the cell should be discarded in preference tocells with the CLP bit equal to zero.

Header Error Control (HEC)

Calculates the checksum only on the header itself. Any cell that fails the header

error check is instantly discarded by the network.




ATM connection

4 ATM connection

ATM is a connection-oriented technique. The end-to-end route is defined

through the network in the beginning of the connection and that remains thesame throughout the connection. ATM cells are routed on the same route to

both directions. This guarantees that the cells arrive in the receiving end in the

same order that they are sent. Furthermore, cell delay variation is also

minimised.

4.1 ATM virtual connections

Virtual connections (VC) are used for providing connectivity between

communicating endpoints. There are two types of ATM connections:

• Virtual Channel Connection (VCC)

• Virtual Path Connection (VPC).

Each ATM cell contains a label in its header to explicitly identify the VC, to

which the cell belongs. This label consists of two parts: Virtual Channel

Identifier (VCI) and Virtual Path Identifier (VPI).

Virtual Channel Connection (VCC) is a logical connection in ATM.

Virtual Channel Identifier (VCI) identifies a particular VC link under a given

VPC. A specific value of a VCI is assigned each time a VC is switched in the

network. Hence, it has only local meaning.

Virtual Path Connection (VPC) is a logical grouping of VCCs having the

same endpoints. Thus, all the cells flowing in a single VPC are switched

together. Virtual paths are used for bundling a number of virtual channels into a

higher bandwidth stream routed through ATM switches. That is, cross-

connection and switching can be done on a higher level and not on individual

VCC level.

Virtual Path Identifier (VPI) identifies a group of VC links at a given

reference point that share the same VPC. A specific value of a VPI is assigned

each time a VP is switched in the network.

Transmission path is a bundle of VPs. The following figure shows the relationamong VCs, VPs and a transmission path.

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ATM Basics

VC

VC

VP

VP

Transmission path

Figure 9. Relation between a transmission path, VPs and VCs

Virtual paths help to reduce the control cost by grouping connections that share

common paths through the network into a single unit. Network management

actions can then be applied to a small number of groups of connections instead

of a large number of individual VCC connections.

Virtual Path Connections (VPCs) have many advantages:

Simplified network architecture

Network transport functions can be separated into those related to individual

logical connections (VCC) and those related to a group of logical connections

(VPC).

Increased network performance and reliability

The network deals with fewer, aggregated entities.

Segregation of traffic

A form of priority control can be implemented by segregating traffic types

requiring different quality of service (QoS).

Reduced processing and short connection setup time

Much of the work is done when the VPC is set up. By reserving capacity on a

VPC in anticipation of later call arrivals, new VCCs can be established by

executing simple control functions at the end points of the VPC; no call

processing is required at transit nodes. Thus, the addition of new VCCs to an

existing VPC involves minimal processing which decreases the connectionsetup delay.

Enhanced network services

The VPC is used internally in the network but is also visible to the end user.

Thus, the user may define closed user groups or closed networks of VC bundles.




ATM as a transport network

5 ATM as a transport network

Nowadays transmission protocols of many telecommunication networks are

based on pulse code modulation (PCM). The switching is also based on theswitching of 64 or 56 kbit/s PCM connections. ATM is selected to be the

transport technology for 3G network especially in UTRAN and also at the Iu

interface in Release 99.

ATM provides efficient support for transmission of bursty wideband services

and offers an integrated solution to voice (circuit mode as well as packet voice),

data, and video. It provides QoS guarantee and reliability. It utilises statistical

multiplexing to take advantage of the inherently bursty nature of applications.

Less bandwidth can be reserved than if bandwidth reservation would be based

on the peak rate of the connections. Achieved transmission cost savings are

considerable.

In addition, ATM is able to support the soft handover functionality, which

requires the capability of fast connection setup and teardown in Wideband

CDMA (WCDMA), which is the radio interface technology for 3G.

In the 3G network, the radio network controller (RNC) is assigned the task of

radio resource management and handover control. When the mobile terminal

communicates with one BS, a connection is established from the Serving RNC

(S-RNC) node to the BS, where the connection is terminated. When in SHO, the

mobile terminal communicates with several BSs, and correspondingly several

SHO legs are established from the S-RNC node to the BSs currently in the

active set. In order for SHO to work seamlessly for the user, the transmission

path should be established quickly (for instance, in the range of 100 ms or less).

Specifically, because of the necessary radio level synchronisation between theSHO legs, these connections are required to meet strict delay and jitter

requirements. This is a challenging task in the ATM network, since the

transmission capacity over the Iub and Iur interfaces is expensive, and

consequently it is required to achieve high Iub utilisation.

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ATM Basics

5.1 Network elements involved in the transport ofuser plane information

The following are the definitions of the network elements involved in the

transport of user plane information (from ITU-T I.311):

VP cross-connect is a network element, which connects VP links. It translates

VPI (not VCI) values and is directed by management plane functions − not by

control plane functions.

VC cross-connect is a network element, which connects VC links. It terminates

VPCs and translates VCI values and is directed by management plane functions

− not by control plane functions.

VP-VC cross-connect is a network element that acts both as a VP cross-connect and as a VC cross-connect. It is directed by management plane

functions − not by control plane functions.

VP switch is a network element that connects VP links. It translates VPI (not

VCI) values and is directed by control plane functions.

VC switch is a network element that connects VC links. It terminates VPCs and

translates VCI values and is directed by control plane functions.

VP-VC switch is a network element that acts both as a VP switch and as a VC

switch. It is directed by control plane functions.

Routing functions of virtual channels are done at a VC switch/cross-

connect. This routing involves translation of the VCI values of the incoming

VC links into the VCI values of the outgoing VC links.

Figure 10 and Figure 11 show examples of VP and VC switching.




ATM as a transport network

Figure 10. VC and VP switching

Figure 11. VP switching

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ATM Basics

5.2 Example of Nokia ATM cross-connect

Nokia ATM cross-connect (AXC) is an integrated transmission solution and

ATM cross-connect device for Nokia WCDMA base stations. Nokia AXC canalso function as a standalone network element. It interconnects different sectors

inside the BTS and connects the BTS to the radio network controller (RNC)

through the Iub interface. Moreover, it is capable of cross-connecting traffic

between other base stations and the RNC.

Nokia AXC serves as a virtual path (VP) and virtual channel (VC) cross-

connect device for permanent ATM connections.

Figure 12 shows the example of VP/VC cross connection table inside AXC.

VC2 / VP2

VC1 / VP1

RNC

ATMswitch

VC1 / VP1

BTS 1

AXC

VC3 / VP3VC3, VC4 / VP4

VC3, VC4, VC5, VC6 / VP7VC5 / VP5

VC6 / VP6

VC1/VP1 THROUGH-CONNECTED IN AXC2

VC/VP CROSS-CONNECTION TABLEVC3/VP4 <-> VC3/VP 7VC4/VP4 <-> VC4/VP 7VC5/VP5 <-> VC5/VP 7VC6/VP6 <-> VC6/VP 7

AXC / ATM switch

BTS 2

AXC

BTS 3

AXC

BTS 4

AXC

BTS 5

AXC

BTS 6

AXC

StandaloneAXC

Figure 12. ATM cross-connect in 3G network




Statistical multiplexing

6 Statistical multiplexing

Statistical multiplexing is one of the main benefits of ATM. Operators can

utilise statistical multiplexing to take advantage of the inherently bursty natureof applications. Users of ATM networks generate numbers of cells according to

the amount of information they want to transfer. The amount of network

resources required by the users changes as a function of time. When these

resources are shared among users, like in ATM, it is very unlikely that all users

send at their peak cell rate simultaneously. This means that the network operator

can either reduce the amount of resources required for a fixed load or it can

accommodate more load with the same amount of resources. This phenomenon

is called statistical multiplexing. The network resources are shared among users

with either VCCs or VPCs.

Figure 13 shows an example of statistical multiplexing. The picture on the left-

hand side shows required amount of bandwidth when the capacity of eachconnection is reserved according to the peak cell rate. The picture on the right

hand side shows the so-called statistical multiplexing gain, when principle of

statistical multiplexing is used in the bandwidth reservation.

Figure 13. Statistical multiplexing gain

When virtual paths are used, two levels of multiplexing exist: VC level and

VP level. 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.

If VPs are deterministically multiplexed, they do not share the bandwidth

reserved for them with the other VPs on the same link. The sum of the reserved

bandwidths of the VPs cannot exceed the bandwidth of the link. If VPs are

statistically multiplexed, they do share the bandwidth nominally reserved for

them with the other VPs on the same link. VPs do not have strictly reserved

bandwidths.

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ATM Basics

7 Review questions

Please take some time and use the material in this module as a reference to

answer the following questions.

1. Which of the following sentences is not true?

a. multiplexes user traffic by using asynchronous multiplexing.

b. Time Division Multiplexing (TDM) is also based on asynchronous

multiplexing.

c. With TDM, each user is assigned to a time slot or time slots, and

no other station can send in that time slot or those time slots.

d. ATM offers "bandwidth on demand", that is, the user traffic can be

sent at any available time slot according to the agreement between

the user and the network.

2. Why is the ATM selected to be the transport technology for 3G?

a. It

b.

d.

e. All of the above.

3. Match the correct definitions.

a. Transmission path

b. Virtual Channel Connection (VCC)

c. Virtual Path Connection (VPC)

is a logical connection in ATM.

is a logical grouping of VCCs that have the same endpoints.

It is used for bundling a number of virtual channels into a

higher bandwidth stream so that cross-connection andswitching can be done on a higher level and not on individual

VCC level.

is a bundle of VPs.

4. Fill in the following figure to present the structure of the ATM virtual

connection, as well as the relation between AAL2 connection, VP, VC,

and transmission path.




Review questions

5. The routing in an ATM network achieved

True ο False ο

6. Fill in the type of the ATM interfaces used at different 3G interfaces.

PSTNUE

A BIu-CSIubUu

IP network

Iu-PS

Iur

Gn Gi

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ATM Basics

Abbreviations

AAL ATM Adaptation Layer

ABR Available Bit Rate

ALCAP Access Link Control Application Part

ATM Asynchronous Transfer Mode

AXC ATM cross-connect

B-ISUP Broadband ISDN User Part

BS Base Station

CAC Call Admission Control

CBR Constant Bit Rate

CDV Cell Delay Variation

CDVT Cell Delay Variation Tolerance

CID Channel Identification

CLP Cell Loss Priority

CLR Cell Loss Ratio

CPS Common Part Sublayer

CS Convergence Sublayer

CTD Cell Transfer Delay

EPD Early Packet Discard

GFC Generic Flow Control

HEC Header Error Control

IMA Inverse Multiplexing for ATM

LAN Local Area Network

MBS Maximum Burst Size

MCR Minimum Cell Rate

MGW Media Gateway

MSC Mobile Switching Centre

MT Mobile Terminal

MTP3b Broadband Message Transfer Part lever 3

nrt-VBR non-real time Variable Bit Rate

NBAP Node B Application Protocol

NNI Network-Node Interface

NPC Network Parameter Control




Abbreviations

PCR Peak Cell Rate

PDH Plesiochronous Digital Hierarchy

PDU Protocol Data Unit

PMD Physical Medium Dependent sublayer

PPD Partial Packet Discard

PT Payload Type

PVC Permanent Virtual Circuit

QoS Quality of Service

RANAP Radio Access Network Application Part

RNC Radio Network Controller

RSNAP Radio Network System Application Part

rt-VBR Real time Variable Bit Rate

SAAL Signalling ATM Adaptation Layer

SAR Segmentation and Assembly Sublayer (SAR)

SCR Sustained Cell Rate

SCCP Signalling Connection Control Part

SDH Synchronous Digital Hierarchy

SHO Soft Handover

SMDS Switched Multimegabit Data Service

SONET Synchronised Optical NETwork

S-RNC Serving RNC

SS7 Signalling System Number 7

SSCF Service Specific Co-ordination Function

SSCOP Service Specific Connection Oriented Protocol

SSCS Service Specific Convergence Sublayer

STC Signalling Transport Converter

SVC Switched Virtual Circuit

TC Transmission Convergence Sublayer

TDM Time Division Multiplexing

UNI User-Network Interface

UPC Usage Parameter Control

UBR Unspecified Bit Rate

VBR Variable Bit Rate

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ATM Basics

VCC Virtual Channel Connection

VCCTP Virtual Channel Connection Terminating Point

VCI Virtual Channel Identifier

VCL Virtual Channel Link

VPC Virtual Path Connection

VPI Virtual Path Identifier

VPI Virtual Path Link

VCCTP Virtual Path Connection Terminating Point

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