Atm Functional Reference Model

8/3/2019 Atm Functional Reference Model

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ATM FUNCTIONAL REFERENCE MODEL

To describe an ATM network and the various network components (ATM mux, ATM cross

connect, ATM switch, ATM Access Devices, ATM Network Termination Unit), we will refer

to the ATM functional reference model shown below.

Figure 2: ATM reference modelThere are three main functional blocks:

The User Plane in charge of transporting the various users information (voice, data orvideo) to their destination, according to the subscribed traffic contracts.

It relies on the ATM layer for multiplexing and switching, with guaranteed Quality of

Services (QOS). For that purpose, the user information (data protocol or particular

media) is converted to ATM via the "User Protocol or Media Adaptation" layer and

the "ATM Adaptation Layer (AAL)".

The Control Plane for the connection set up and release according to subscribedtraffic contracts. Various types of connections are supported: symmetrical or

asymmetrical point to point, multipoint, multicast, unicast, etc.). It also uses the AAL,

ATM and physical layers. A dedicated layer is used to map the "control protocol"

onto ATM in order to ensure high reliability.

The Management Plane for ATM monitoring and configuration.The following presentation will be organized according to the ATM functional reference

model and we will proceed accordingly with:

the ATM layer and its new differentiating services (versus TDM or PM) ,


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the Physical layer with ATM over SONET/SDH and over LAN wiring the ATM adaptation layer the Control plane for connection management the user's high layer protocol or media mapping the Management Plane

ATM LAYER

The ATM layer provides the following services:

Cell transmission : generation, reception, validation Cell multiplexing/demultiplexing, cell relaying, cell copying Cell payload discrimination Support of multiple QOS classes Traffic management: usage control, traffic shaping, congestion notification Connection assignment and removal

Those services are supported through the ATM protocol data unit so called ATM cell.

ATM CELL STRUCTURE

The ATM cell structure is represented in the next table with its two variants "UNI "and

"NNI".

"UNI cells "are used at the User to Network Interface (UNI), "NNI cells" at the Network to

Network Interface (NNI) connecting two ATM networks.

Type ATM CELL Overhead

HEADER PAYLOAD

5 Bytes

11 %

UNI GFC VPI VCI PT CLPHECNNI VPI

4

bit8 bit 16 bit

3

bit1

bit8 bit 48 bytes

Review of the different cell header fields:

GFC: used at the UNI interface and originally intended to support simplemultiplexing implementations. No standardized use of this field exists. The current

standards define for this field a "0000" value coding.


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PT:used to discriminate payload types (user's or management data), to indicatecongestion status and to mark end of AAL5 framing (see AAL ), with the following

encoding:

PTcode

SIGNIFICANCE NOTES

000 User data cell - EFCI=0 - AAL5_EOF=0 EFCI=0: no congestion

001 User data cell - EFCI=0 - AAL5_EOF=1 AAL5_EOF=1 : end of AAL5 framing

010 User data cell - EFCI=1 - AAL5_EOF=0

011 User data cell - EFCI=1 - AAL5_EOF=1

100 OAM F5 segment associated cell OAM F5 is a maintenance flow (see

Management plane )

101 OAM F5 end-to-end associated cell

110 Resource Management Cell Resource management cell: used for

ABR flow control (see traffic

management )

111 For future use

CLP: indicates the priority of a cell. A cell with CLP=1 can be destroyed by thenetwork in case of congestion

HEC: error checking of the header to ensure proper processing of the received headerfields

VCI-VPI: User channel identified by a combination of a Virtual Path (VP) and aVirtual Channel (VC). Those two parameters characterize the user channel in terms of

origination and destination but also in terms of the subscribed class of traffic. They

also identify full or empty cells and also non-user's data as shown below.

VPI VCI PT CLP USAGE NOTES

0 0 000 1 Idle cell identification Recommended by ITU

0 0 *** 0 Unassigned cell Recommended by ATM Forum

0 0 100 1 OAM F3 - physical layer

See "Management plane"

ID 3 0x0 x Segment OAM F4 - Virtual Path

ID 4 0x0 x End-to-end OAM F4 - Virtual Path

ID ID 100 x Segment OAM F5 - Virtual Channel

ID ID 101 x End-to-end OAM F5 - Virtual Channel

ID 5 0xx c Point to Point Signaling channel Dedicated signaling channels.

VPI=0 commonly used between user and

local exchange

ID 2 0xx c Broadcast Signaling channel

ID 1 0xx c Meta-signaling channel

ID 3 110 x Resource management cell for VP See "traffic management"


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ID ID 110 x Resource management cell for VC

0 16 xxx 0 ILMI : Interim Local ManagementInterface

Management between user and network

ID 48 xxx 0 PNNI: Private Network-NetworkInterface

Dynamic routing protocol

The Virtual Paths and Channels are innovative concepts that position ATM as the ideal

technology to build Virtual Networks(VN) as Virtual LAN (VLAN), Virtual Enterprise

Network (VEN) or Virtual Private Network (VPN) capable of supporting multiple services &

media.

ATM VIRTUAL PATH & CHANNEL

With ATM several types of connections are possible :Virtual Paths equivalent to flexible

"digital lines", Virtual Channels that will carry the end-users communication applications. A

connection is not only characterized by its end-points : source and destination but also by

traffic service quality parameters (peak & average throughput, cell loss, transit delay).

The VCs are transported on VPs, themselves on Transmission Paths (TP) or Physical Links.

A VC or VP connection is made of VC or VP Links interconnected via multiplexers,

crossconnects or switches.

ATM NETWORK INTERFACES

To assure ubiquitous broadband ATM communications, standards for interoperability of

ATM products and ATM networks are defined (still an ongoing work) by the ITU for the

ATM public services and by the ATM forum for private ATM network. The ITU reference

connection model (identical to narrowband ISDN) is shown on the next figure as well as the


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ATM network interfaces.

UNI (User-to-Network Interface)provides for interconnection of end systems to anATM switch with precise definition of the ATM transmission and switching services

with the related exchanged signals (ATM transmission layer and ATM signaling).

At present the more unified interface is the UNI 3.1 a merge of ATM forum and ITU

specifications. New enhancements (signaling & routing, traffic management,

configuration) are part ofUNI 4.0 with still ongoing work.

To be noted: the existence of distinct private and public UNIs although with a very

few differences, the main one being the addressing plan (see "switching section").

ATM Forum works also on the definition of a Residential UNI

NNI (Network-to-Network Interface) is intended for interconnection of ATMswitches. If it is a private interconnection, the interface is the PNNI (Private NNI)

specified by the ATM forum. If it's public, the interface is the (Public) NNI and isdefined by the ITU.

The NNI is a more complicated interface, with an ATM layer similar to UNI

(extended VPI cell structure but with signaling, addressing and routing more

elaborate. Work is not really stabilized yet in that field.

B-ICI (Broadband Inter-Carrier Interface) connects ATM networks of two serviceproviders.

ILMI (Interim Local Management Interface): used to perform interfacemanagement between an end system and a private or public switch as well as between

switches. Through SNMP and MIBs (see "management section"), configuration and


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supervision can be done directly between ATM network elements. It's similar to the

Frame Relay LMI.

ATM PHYSICAL LAYER

The different functions of this layer are split into two sublayers as presented in the next table.

Sublayer Functions

TransmissionConvergence

TC

HEC generation and verification

Cell scrambling and descrambling

Cell delineation

Path signal indication

Time phasing-pointer processing

Multiplexing

Scrambling/descrambling

Transmission frame generation/recovery

Physical Media Dependent

(PMD)

Bit timing, line coding

Physical medium

PMD: The characteristics of the main Physical Media used for ATM are summarizedhereafter.

Physical

Carrier

Bit rate

(Mbps)

Media Line

Encoding

Distance Use

SDH-STM4 622 SM-1300um fiber NRZ unlimited* WAN

SDH-STM4 622 MM-1300um fiber NRZ 300 m LAN

SDH-STM1 155 SM-1300um fiber NRZ unlimited WANSDH-STM1 155 MM-1300um fiber NRZ 2 km LAN

SDH-STM1 155 UTP5/UTP3 NRZ/64CAP 100 m LAN

SDH-STM1 155 Plastic-1300um fiber NRZ 50 m LAN

TAXI (FDDI) 100 MM-1300um NRZ-4B5B 2 km LAN

PDH-E3/DS3 45/34 Coax-75ohms HDB3/B3ZS unlimited WAN

ATM25 25.6 UTP3 NRZI 100 m LAN

PDH-E1/DS1 2.048 TP/Coax-75ohms HDB3 unlimited WAN

DH- DS1 1.544 TP AMI/B8ZS unlimited WAN

AIMUX N* Same as PDH idem idem WAN


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E1/DS1 E1/DS1

* : "unlimited" distance because of the PDH & SDH WAN carrier networks

Acronyms:

SM: Single Mode (fiber) - MM: Multimode - UTP: Universal Twisted Pair

AIMUX: ATM Inverse Multiplexer

1. A broad range of transmission bit rates are possible for ATM from 1.544 to 622Mbps. To fill in the bit rate hole between E1/DS1 and E3/DS3, an Inverse

Multiplexing scheme is defined to transport an ATM "N*(E1 or DS1)" stream on N

parallel E1/DS1 physical links.

TC (Transmission convergence): responsible to insert and recover ATM cells in thebit stream of the Physical media. ATM cells mappings into SDH and PDH carriers are

standardized. A mapping example of ATM over STM1 (155 Mbps) is shown below.

Bytes SDH Line & Section Overhead (OH)**

(9 bytes)

Path

OHSDH Payload

1 A1 A1 A1 A2 A2 A2 C1 C1 C1 J1

2 B1 B1 B1 B3

3 C2 ATM ATM ATM

4 H1 H1* H1* H2 H2* H2* H3 H3 H3 G1 ATM ATM ATM

5 B2 B2 B2 K2 -- ATM ATM ATM

6 H4 ATM ATM ATM

7 -- ATM ATM ATM

8 -- ATM ATM ATM

9 Z2 Z2 Z2 -- ATM

**: fore more details, refer to the SONET/SDH tutorial

A1,A2,B1,C1:Section management Channels - B2: Line error check

H1,H2,H3: pointer to the STM-1 payload start

J1,B3,C2,G1: Path management channels - H4: pointer to the ATM sequence start

ATM SERVICE ADAPTATION LAYER

Previously we have seen how ATM could be mapped on transport networks as SDH. The

question is now how to map, on ATM, the user transmitted information flows carried on


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different media (data, voice, video) with dedicated communication protocols (IP, Frame

Relay, SNA, X25, ISDN, MPEG,etc.).

The ATM Adaptation Layer (AAL) is responsible for the conversion between user's data and

ATM cells. The AAL layer is divided into separate functional sublayers as shown on the next

figure.

Sublayer Significance Services

SSCS Service Specific

Convergence Sublayer

Protocol mapping and encapsulation

CPCS Common Part

Convergence Sublayer

Timing recovery for CBR & rt-VBR

Frame and channel delineation, Frame

error checking

SAR Segmentation

And Reassembly

Cell Segmentation & Reassembly, error

detection & correction, Multiplexing

There are so many user applications able to be transported over ATM that they cannot be

adapted one by one. Applications are grouped in service classes (related those of traffic

management seen previously) with a different adaptation for each class. As a result, four

AALs are currently defined.

AAL Type Service Class Attributes Applications

AAL1 CBR Constant Bit rate

Timing synchronization

Connection oriented

E1,DS1

N*64 Kbps

AAL2 rt-VBR Variable Bit rate

Timing synchronization

Connection oriented

Packetized Video, Audio

AAL3/4 VBR Variable Bit rate

Connection Oriented or

Connectionless

SMDS

AAL5 VBR, UBR,

ABR

Variable Bit rate

Connection Oriented or

Connectionless

Data and protocols (Frame Relay,

IP, X25), ATM signaling

AAL1 is optimized for CBR traffic, for Circuit Emulation AAL2 is intended for variable bit rate video or audio signals, as MPEG video. It is not

fully specified and is in competition with MPEG over AAL1 and AAL5.

AAL3/4 is the combination of AAL3 for connection oriented traffic and AAL4 forconnectionless. It's mainly used for SMDS

AAL5 is the most recent AAL and replaces AAL3/4 for all data protocols exceptSMDS. It may also supersede AAL5.


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ATM CONTROL PLANE

The user plane connections are setup and released by the control plane, by a signaling

exchange between the ATM end systems and the ATM intermediate systems (switches).

ATM is a Connection Oriented protocol. The transmission characteristics (QOS,

throughput, latency) are fixed during all the time the connection is active. Moreover, the

signaling information and the user data do not share the same channel path; ATM uses an

outband signaling scheme.

To identify the ATM network subscribers, there are two different standardized addressing

schemes : E164 specified by the ITU for public networks and already used in ISDN and

NSAP defined by the ATM Forum for private Networks. Multicast and anycasting (group

addressing) is supported in addition to single addressing.

ATM supports permanent and switched connections of various types:

Point to Point (symmetrical or asymmetrical) Point to Multipoint Multipoint to Point Multipoint-to-Multipoint

A large variety of switching services are or will be provided.

Basic Services Supplementary Services

. Point-to-Point connection set up

&release

. VPI/VCI selection & assignment

. Quality Of Service class request

. Traffic parameters request

. Subaddress support

. Identification of calling party

. Transit Network Selection

. Basic error handling

. User-to-user signaling

. Point-to-Multipoint

. Symmetric operation

. Multipoint-to-Point or

Multipoint

. Multiple connections setup

. Call Transfer

. Call Forwarding

. Call Offer

. Call "Do not Disturb"

. Multiple Subscriber NumberEtc.

To implement those services, ATM uses a special signaling protocol (although at the higher

level similar to ISDN) referred to the Q2931 (ITU standard) protocol , which is embedded in

the UNI 3.x and 4.0 specifications. To ensure reliable transmission of the signaling messages,

a particular AAL is specified for signaling, so-called SAAL. It is based on AAL5 and adds to

it a reliable transport layer. To convey the signaling information between adjacent ATM

devices, a dedicated ATM channel is used (VPI=0/VCI=5 usually)

To setup a route between the end users, when a setup message is received, the switches will

strive for finding the best route to reach the destination but also to fulfill the traffic contract

(service class, traffic parameters, QOS) requested by the user. For that purpose the ATMForum has defined for private ATM networks:


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a dynamic routing protocol , the PNNI (Private NNI), to exchange "networkreachability and network traffic conditions" information between switches

and an algorithm to find the best path, the GCAC (Generic Call Admission Control)A lot of standardization work to be done in order that all the connection services attached to

the Broadband ATM network be available ubiquitously

ATM MANAGEMENT PLANE

Within the ATM functional reference model (see fig 2), the management plane is in charge of

managing the different ATM layers of both the user and control planes. It must also

undertake management coordination across the layers and the different planes, all this in

order to ensure that everything works properly. It manages faults, performances,

configuration, accounting and security within the ATM network.

To accomplish those different tasks, a management model has been defined by the ATM

Forum on the basis of the TMN (Telecommunication Management Network) used in

public networks and standardized by the ITU.

The management services are structured in different layers: the Network Elements (NE)

management, the network management itself, the service management. Basic Management

entities (agents, managers) are accordingly defined with the interfaces interconnecting them.

For each interface, a management protocol is defined: SNMP for private networks, CMIP for

public networks), as well as the management information (MIB) processed in those entities

and related to the ATM network and services.

To simplify the configuration of ATM network devices, a special protocol, the ILMI has alsobeen defined by the ATM Forum, with its associated MIB.

To monitor in real time operational status and performance of the ATM connections (VC,

VP, Transmission Path), special maintenance flows (OAM flows) are specified. They are

also used to verify proper operation of the VCs and VPs through activation of loopbacks.

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Atm Functional Reference Model