ATM Signalling

White Paper „ATM Signalling“ (Draft)

Table of Contents Introduction Signalling - Reasons and Standards User Network Interface (UNI) Private Network to Network Interface (P-NNI) Signalling System No. 7 Future Signalling Networks Siemens Newbridge Alliance Solution

White Paper „ATM Signalling“ Signalling“

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Introduction There will be two important developments that will shape telecommunications networks in the near future. One is the expected significant increase in the demand for bandwidth which will result in particular from the explosive growth in volumes of data. The other is that a carrier’s data and telephone networks, which are currently separate entities, will grow together to form an integrated broadband ISDN network. The new ATM technology will help to meet the demands imposed by these new developments. The growing together of the various data and telephone networks will place particularly high demands on the signalling in the individual network elements, especially at the core nodes. This means that future core nodes will need to be able to handle all widely-used signalling interfaces but will also need to accommodate interworking between the individual variants. This paper will present the signalling systems that are currently in use. It will also provide a glimpse into the complexity of future networks and the demands that this will place on signalling.

Signalling - Reasons and Standards The introduction of Switched Virtual Connections (SVC) in ATM networks brings with it numerous advantages over today’s Permanent Virtual Connections (PVC). An end customer with SVCs is in a position to request “Bandwith on Demand“. This means that customers only pay for the connection for the time and the bandwith that they are using. Since this will make the broadband services cheaper to use, a higher demand for such services is expected. In addition ATM networks will attract customers who for reasons of cost can neither afford nor require PVCs. Using ATM networks will be cheaper, the customer base will get larger and this will also benefit carriers. They will be able to be more flexible in the way that they operate the networks and their costs will be lower since less administrative intervention will be necessary. To allow SVCs, information for connection control and for using features must be transferred between exchanges. This is done using signalling. Messages for controlling the signalling network can also be sent (e.g. overload protection). Generally, a distinction is made between two different types of signalling. One type is subscriber signalling, the signalling between terminal and switch, meaning the User Network Interface (UNI). The other requirements is for signalling between the exchanges.

White Paper „ATM Signalling“

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This so-called interexchange signalling is referred to as the Network to Network Interface (NNI). Since a carrier’s customers also wish to communicate with another carrier’s customers, it is necessary for the networks of the various providers to be interoperable. To achieve this, international standard interfaces are employed. To provide manufacturer independence and make existing hardware more future proof, standardized UNI and NNI signalling is also needed within the networks. The most important current standardization bodies for ATM networks are the International Telecommunications Union (ITU) and the ATM Forum. Both organizations issue new or expanded recommendations for UNI and NNI standards, with the recommendations of the ATM Forum mostly being based on those of the ITU. The diagram below shows the current UNI and NNI signalling standards of the two organizations for ATM which will then be discussed in more detail in subsequent paragraphs.

ATM Forum

ITU-T

User Network Interface (UNI)

Latest Version: UNI 4.0 (based on ITU-T UNI)

DSS2: Q.2931 + Capability Sets (CS1 and CS2.1 approved)

Network to Network Interface (NNI)

P-NNI V1.0

[Inter Carrier Interface (ICI)]

B-ICI V2.0 (based on ITU-T)

SS7 (B-ISUP + MTP)

User Network Interface (UNI) The function of the UNI is to control the connection sections (setup/release of connections, activation of features etc.) between the Customer Premises Equipment (CPE) and the ATM switch. To do this, it uses the layer below it, the Signalling ATM Adaptation Layer (SAAL). The SAAL in its turn accesses the ATM layer and the physical layer. The UNI for its part receives requests from the terminal equipment. The following diagram contains the complete UNI protocol stack for a connection.


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UNI

UNI Protocol

SAAL

UNI

SAAL

Interworking B-ISUP

ATM Layer

ATM Layer

MTP 3b SAAL ATM Layer

Physical Layer

Physical Layer

Physical Layer

Network

UNI was specified by the ITU-T in Recommendation Q.2931. It is described in this document under the name DSS2 (Digital Subscriber Signalling System No. 2) and is a further development of the approved DSS1 protocol for narrowband ISDN. For the basic connection, specified in Q.2931, ITU is constantly standardizing new features, which are grouped together in what are referred to as Capability Sets (CS). There are currently Capability Sets 1 and 2.1. In contrast to ITU, the ATM Forum brings out completely new UNI versions. The version which is current at present is Version 4.0. It is based on ITU and also largely contains the features of the ITU Capability Sets, but Version 4.0 does have a few features which the ITU has not yet finally specified. The UNI protocol has proven its worth over the last few years in broadband applications. The specifications of ITU and the ATM Forum are largely identical since they have the same basis and existing and new features are included from either side.

Private Network to Network Interface (P-NNI) P-NNI is the protocol defined by the ATM Forum for interexchange signalling in private ATM networks. P-NNI Version 1.0 was approved in 1996 . It uses the SAAL in exactly the same way as the UNI protocol, and the SAAL in its turn accesses the ATM layer and the physical layer. The diagram below shows the protocol stack of the P-NNI.


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P-NNI Control UNI

P-NNI Protocol Control

P-NNI Control P-NNI Protocol

P-NNI Protocol Control

SAAL

SAAL

ATM Layer

ATM Layer

Physical Layer

Physical Layer

UNI

Network

The layout and structure of the P-NNI comes from the world of data processing. It consists of a signaling protocol and a routing protocol. The signalling protocol is based on UNI V4.0 from the ATM Forum with additional functions to support source routing, crankback and alternate routing. What sets P- NNI apart is the routing protocol. This uses what are known as „hello packets“ to identify and verify neighbouring switches after being put into service and to determine the status of the individual links between the nodes. The hierarchy of the network is constituted. The network configures itself automatically, by which, however, possibilities to influence the way in which the system is configured are taken away from the carrier. Its self-configuration is the great advantage of P-NNI. This does away with the need to create and maintain routing tables, such as those need for ITU’s SS7 protocol for example. But it is still necessary to plan in advance the network and hierarchy by assigning addresses to the individual peer groups, since the network topology derives from the previously allocated address structure. The diagram below shows the possible design structure of a P-NNI network.


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A

C B

Peer Group A B3 A1

Peer Group B

B1

A2

B2

A1.3

A1.1

A2.1

B1.1

A1.2 Peer Group A.1

A2.3 B1.3

A2.2

B3.1 B1.2

Peer Group A.2

Peer Group B.1

B2.1

B3.3

B2.3 B3.2 B2.2

Peer Group B.3

Peer Group B.2

Peer Group Leader

The P-NNI is particularly suitable for smaller networks in which the administration overhead is to be minimized. Functions such as billing, overload and security mechanisms and interworking with narrowband networks are not yet specified. Interworking to the ATM Forum Broadband Intercarrier Interface Protocol, B-ICI has also to be specified by the ATM Forum.

Signalling System No. 7 (SS7) The SS7 protocol was specified by ITU and, exactly like P-NNI, it is a protocol for interexchange signalling. It came originally from the world of public telephone switching where it proved very successful. Since then it has been implemented in all the world’s telephone networks. Its great flexibility makes it ideally suited to ATM networks as well. The SS7 comprises various user parts, all of which work together in accordance with the OSI model. The diagram below shows the user parts in the protocol stack which can currently be used in broadband applications and the text thereafter discusses their functions in brief.


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SS7 User TCAP B-ISUP SCCP MTP 3b SAAL ATM Layer Physical Layer

The MTP3b (Message Transfer Part) has two functions. The first, Signalling Network Management, manages the SS7 network and is responsible for execution of measures during changes to network availability (e.g. failure or putting into service/removal from service of signalling links, paths and points) and overload in the signalling network. This means that the transfer facilities in the signalling network are maintained without affecting signalling traffic. The second function is Signalling Message Handling. This is used to implement the actual message switching function for signalling messages within the network. The B-ISUP (Broadband ISDN User Part) includes the signalling function for controlling connections, handling services and management of connections within the B-ISDN. Connectionless services can also be supported in the broadband network using the SCCP (Signalling Connection Control Part) and the TCAP (Transaction Capabilities Application Part), i.e. TCAP/SCCP can be used to exchange information independently of connections. Applications for this part include the Intelligent Network or the OMAP (Q3 interface) for network management. B-ICI (Broadband Intercarrier Interface) has been specified by the ATM Forum as the intercarrier interface. It is based on SS7 and includes a subset of MTP and B-ISUP. This guarantees a transition from ITU networks to ATM Forum networks without any problems. The diagram below shows an ATM network using SS7 signalling with the interworking with other networks currently supported by SS7.


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SS7-Network (EWSX 36170) Gatewayto foreign narrow band network

B-ICI gateway to ATM Forum network

B-ISUP

N - I S U P

EWSX 36190

Core Net (B-ISUP)

EWSX 36190

EWSX 36190

B-ISUP

SS7-Network (EWSX 36170)

N-ISUP

Narrowband Network (EWSD)

gateway to Intelligent Network

There are numerous advantages to using SS7. It is covered by international standards, tried and tested in all the countries of the world and suitable for all communication services (i.e. data and voice). It can be used for national and international networks, even for gateways. SS7 in ATM networks allows interworking to narrowband networks and to IN, enabling the IN functionality to be extended to the broadband network. Connectionless Services are also supported (for Intelligent Network and Features for example). SS7 offers great availability, it is flexible and it fulfills the most exacting security requirements. The high level of acceptance and consequent widespread use of SS7 makes it highly futureproof. The signalling network is automatically controlled and monitored. Mechanisms for controlling overload are also provided automatically.

Future Signalling Networks All signalling protocols discussed in this paper will be used in the ATM networks of the future. They will therefore produce very complicated network structures for carriers which will be particularly attributable to the expected interworking required and to the networks to be linked. Very great demands will be placed on the ATM backbone switches in particular in respect of signalling.


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Because the availability and security demands placed on the carrier ATM backbone netoworks are so high, it can be assumed that SS7 or B-ICI signalling will be used there. However, in order to be able to link-in P-NNI networks, the SS7/P-NNI interworking not yet specified by the ATM Forum will also have to be available. The diagram below shows a network structure with the signalling discussed.

P-NNI-Network (EWSX 36170) Gatewayto foreign narrow band network

P-NNI

B-ICI gateway to ATM Forum network

N - I S U P

EWSX 36190

B-ISUP

Core Net (B-ISUP)

EWSX 36190

EWSX 36190

SS7-Network (EWSX 36170) gateway to Intelligent Network

N-ISUP

Narrowband Network (EWSD) Gateway to foreign P-NNI network

Siemens Newbridge Alliance Solution The UNI protocol provides subscriber signalling which is already largely in line with the requirements of the carriers. The ATM Forum and ITU standards are identical for the most part, so that the differences between the two standardization worlds are small The EWSXpress 36190 ATM Core Node and the EWSXpress 36170 ATM Edge Switch from the Siemens Newbridge Alliance support the UNI protocols of ITU and ATM Forum and thereby allow easy connection of Customer Premises Equipments (CPE). With interexchange signalling it is no longer possible to talk of almost identical standards between ITU and ATM Forum, since the ways in which P-NNI and SS7 function are based on fundamentally different approaches. Despite this both signalling variants will be legitimate in the ATM networks of the future.


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SS7 is a proven interexchange signalling system. It will be used particularly in public ATM carrier networks where high availability and security are required. Its widespread use in the narrowband world ensures interworking with the narrowband networks and intelligent networks. The EWSXpress 36190 Core Node is ideally suited to such networks. It incorporates the competence and experience which Siemens has accumulated in recent years as the world market leader of narrowband switches. However, as EWSXpress 36190 also has full P-NNI capabilities, it can be used to the full in private networks as well. In future interworking between the various signalling systems will place particular demands on the ATM core nodes. EWSXpress 36190 supports all interworking between the UNI and NNI signalling systems that it supports. In tandem with its high level of availability and the fact that it fulfills the security requirements of the narrowband world, EWSXpress 36190 already meets the demands that will be placed on core nodes in the future. P-NNI will find its main area of application in private data networks in the campus area, in which the administrative overhead for network operation is to be kept low. The EWSXpress 36170 from Newbridge is particularly suitable for these networks. Newbridge has incorporated its wealth of experience from data networks into this switch. However area of application for the EWSXpress 36170 will also extend to the edge segment of public carriers’ data networks. Since it supports also SS7 it will be possible to integrate it into these networks. The greatest advantage of the EWSXpress 36190 ATM Core Switch and EWSXpress 36170 Edge Switch products however lies in using them together in large networks, for which they have been especially optimized under the auspices of the Siemens/Newbridge Alliance. In this regard the two switches are also optimally matched in respect of signalling. In large public carrier networks both switches support SS7 in the core and edge segment in private networks they support P-NNI. Since the ability to interpret the P-NNI standard in different ways means that it cannot be ensured that P-NNI networks will be signalling-compatible with switches from different manufacturers, the signalling agreed between Siemens and Newbridge also becomes particularly significant in these networks. The alliance between Siemens and Newbridge thus takes account of the development tendencies in telecommunications mentioned at the start of this paper. With its EWSXpress 36190 and EWSXpress 36170 products, the alliance offers solutions which, even with regard to signalling, are perfectly matched, right through from the smallest data network to large public carrier networks.


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ATM Signalling

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