Table of Contents INTRODUCTION  ATM Positioning  PHYSICAL LAYER  ATM FUNCTIONAL MODEL  ADAPTATION LAYER  AAL Details  ATM LAYER  Cell Structure  ATM VP-VC  ATM Virtual Connection  ATM Interfaces  CONTROL PLANE  Switching & Signaling    MANAGEMENT PLANE  ATM MIBs & ILMI

Back to Backbone Networks

 ATM results from combining TDM (Time Division Multiplexing) used in circuit networks with PM (packet Multiplexing) used in data networks. ATM takes advantage of:

• TDM, with its capacity to transport integrated services and media as required by the new information era where communications are based on multiple combined media (data, text, graphics, voice and video)
• PM, with its statistical multiplexing efficiency and communication flexibility.

Figure 1: TDM - PM - ATM multiplexing

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 model

There are three main functional blocks:

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

• The Control Plane for the connection set up and release according to subscribed traffic 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 ATM layer and its new differentiating services (versus TDM or PM) ,
• 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

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	CLP	HEC
NNI	VPI
	4 bit	8 bit	16 bit	3 bit	1 bit	8 bit	48 bytes

Review of the different cell header fields:

• GFC: used at the UNI interface and originally intended to support simple multiplexing implementations. No standardized use of this field exists. The current standards define for this field a "0000" value coding.
• PT: used to discriminate payload types (user's or management data), to indicate congestion status and to mark end of AAL5 framing (see AAL §), with the following encoding:

 

PT code	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 the network in case of congestion
• HEC: error checking of the header to ensure proper processing of the received header fields
• VCI-VPI: User channel identified by a combination of a Virtual Path (VP) and a Virtual 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.

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.

 

For details on VP, VC, QOS & traffic management, click on: ATM VIRTUAL SERVICE CONNECTION You will then return here

 

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 ATM network interfaces.

 

• UNI (User-to-Network Interface) provides for interconnection of end systems to an ATM 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 of UNI 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 ATM switches. 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 is defined 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 service providers.
• ILMI (Interim Local Management Interface): used to perform interface management between an end system and a private or public switch as well as between switches. Through SNMP and MIBs (see "management section"), configuration and 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
Transmission Convergence 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 summarized hereafter.

 

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	WAN
SDH-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*  E1/DS1	Same as PDH E1/DS1	idem	idem	WAN
* : "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 622 Mbps. 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 the bit 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  OH

SDH 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

Table of Contents

ATM SERVICE ADAPTATION LAYER

 

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 for connectionless. It's mainly used for SMDS
• AAL5 is the most recent AAL and replaces AAL3/4 for all data protocols except SMDS. It may also supersede AAL5.

 

For details on AALs, click on: AALs BOLTS & NUTS You will then return here

 

 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

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 ATM Forum has defined for private ATM networks:

• a dynamic routing protocol , the PNNI (Private NNI), to exchange "network reachability and network traffic conditions" information between switches
• and an algorithm to find the best path, the GCAC (Generic Call Admission Control)

 

For details on ATM switching & signaling, click on: ATM SWITCHING & SIGNALING You will then return here

Table of Contents

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 also been 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.

For details on ATM management, click on: ATM MIBs & ILMI You will then return here

Table of Contents