ATM BASICS - Chapter 5 ppt

79 Chapter 5 Using ATM to Connect Systems ATM is commonly used to connect systems. There exits also a number of pro- tocols and scenarios for interworking between ATM networks. This chapter introduces signaling flavors and routing concepts, which are deployed in contemporary ATM networks. In the first place, the chapter covers address- ing as well as general signaling issues. Then it presents signaling protocols that are used to setup connections across UNI and PNNI interfaces. Finally different methods of interworking between ATM networks are discussed. Connecting systems using ATM must involve the use of ATM addressing and signaling. Both elements are important parts on the ATM concept. Addressing actually takes place at two levels: at the ATM level using VPI/VCI identifiers and at the logical network level. All signaling protocols assign match VPI/VCI values to ATM address and physical ATM UNI and /or NNI ports. Every physical ATM UNI port must have at least one unique ATM address. Both VCCs and VPCs are unidirectional, so signaling proto- cols must specify traffic parameters separately for each direction of the con- nections. Note that in addition to signaling operation there is also a need for routing capabilities necessary to find the optimal route for a connection. 5.1 ATM Address Formats There are two different types of ATM addressing plans. First, ATM supports the use of NANP (North American Numbering Plan) that is based on E.164 ITU-T Recommendation. Such addressing format is called E.164-Native or NSAP E.164 format. Secondly, ATM supports the alternative addressing plan developed by ATM Forum and called ATM End System Addressing (AESA). The AESA addresses are used in private and enterprise addresses. AESA also includes support of E.164 address format (E.164 ATM) to ensure compatibility with classical public networks such as PSTN and ISDN. Each ATM end station requires a unique ATM address to uniquely identify other end stations. The ATM network also uses unique ATM addresses to locate the destination end switch. Finally ATM switches themselves use ATM addresses in PNNI networks to uniquely identify each switch. The for- mat of a private ATM address is totally different from addresses used in other technologies. The size of an AESA ATM address is equal to 20 bytes, which is 16 bytes more than IPv4 address and 4 bytes more that IPv6 address. The structure of an AESA address is given in the Fig. 5-1. ATM Basics 80 Fig. 5-1, General format of an ATM address There are three major parts of an ATM address used within a private ATM network: •The 13-bytes Network Prefix field, which is used to identify the net- work part of the ATM address. This field may have different formats depending on the type of the network and the position in the network. •The 6-bytes Endpoint System Identifier, which is a hardware part of the ATM address. This filed is also referred to as a MAC address, as it uniquely identifies ATM hardware. The 48-bits of ESI are assigned by the vendor of the ATM equipment. •The 1-byte Selector field that is a logical part of the ATM address. It is used to identify a logical function in the ATM device. For example, an ATM end station may support a number of terminal equipment to access an ATM network, and the Selector field may be used to address different devices. The field is not used in routing. Note that a particular ATM device may have more than one ATM address. It is possible due to the fact that different values of the Selector field can be hosen to create separate ATM addresses. Public and private ATM networks use different ATM address formats. Public ATM networks use E.164 addresses (i.e., telephone numbers). Private ATM network addresses are based on the Open System Interconnection (OSI) Network Service Access Point (NSAP) format. NSAP addresses are based on the concept of hierarchical addressing domains. The exact format of an ATM address is coded in the first byte of an ATM address. This byte is called the AFI (Address Format Indicator). The three different formats are given in the Fig.5-2. Chapter 5 81 The fields of the address formats are as follows: •The DCC (Data Country Code) specifies the country in which the address is registered. •The ICD (International Code Designator) provides unified address- ing for international organizations. The ICD code is issued centrally by the British Standards Institute (BSI). •The E.164 address specifies the numbering system with ISDN. In ATM, the international numbering format is used. These numbers may con- tain up to 15 figures, the length of the E.164 field is eight bytes. •The DFI (Domain-specific part Format Identifier) specifies the structure of the rest of the address field (the AA, RD, Area, ESI and SEL fields). ATM Basics 82 Fig. 5-2, Different formats of ATM addresses •The AA (Administrative Authority) field identifies national organi- zations such as the ATM network operator, ATM user or ATM device man- ufacturer. •The RD (Routing Domains) field specifies an address range that must be unique within E.164, DCC/DFI/AA or ICD/DFI/AA BULThe Area field specifies a unique range of addresses with a routing domain. As it can be easily derived from any format of the ATM address, their struc- ture follows a hierarchical model. This model is given in the Fig. 5-3. The network operator configures his network device with network prefixes in way that reflects the topology of his network. Chapter 5 83 Fig. 5-3, ATM addressing hierarchy. It has to be mentioned that some ATM devices in a public ATM network may support also E.164-Native format. This format defines an address as an ISDN number or as a telephone number specified by ITU-T. 5.1 Address Registration Address registration is an initial procedure, which is executed when an ATM end station is connected to an ATM switch. Therefore, users can move their terminal equipment from one location to another without the need for manual address configuration. The registration of end-station devices is carried out by Integrated Local Management Interface (ILMI), which operates on the object table within a MIB. When the ATM interface in the ATM end station is enabled, a cold start trap is transmitted out along reserved VPI 0, VCI 16. The ATM switch receives this start trap and replies with the network prefix associated with that ATM switch by the operator. The end station then adds its own MAC part of ATM address and Selector field to the prefix to form a full ATM address. This address is sent to the switch where it is registered. The end station must be able to accept multiple network prefixes, which enables a prefix to change on-line, if needed, allowing for expansion and interconnec- tion of previously separates networks. Also, an ATM switch must be able to accept multiple MAC addresses on the same physical port. The network must ignore and transparently handle the one-byte Selector field of the address on an end-to-end basis. The SEL field provides a Service Access Point within the end station for multiple functions. 5.2 UNI Protocol The User-Network Interface (UNI) is the point between the end-point ATM equipment and the first ATM switch. There have been several versions of the UNI specification, defined by the ATM Forum: UNI 2.0, UNI 3.0, UNI 3.1, UNI 4.0 and UNI 4.1. UNI 2.0 supports only PVCs, while the latter ATM Basics 84 three versions also support SVCs. Note that UNI version 3.1 and later ver- sions are not backwards compatible with UNI version 3.0. UNI signaling consists of three layer protocol stack and it largely based on the signaling protocols developed for narrowband ISDN (ITU-T Q.931) referred to as DSS 1 (Digital Subscriber Signaling No 1). The UNI protocol is aligned with the ITU-T Q.2931 signaling standard designed for broadband signaling (DSS 2) UNI signaling uses the predefined VPI 0, VCI 5 for transmission of signal- ing packets. The signaling is performed by means of the different signaling messages related to the call establishment; call clearing and operation of point-to-point connections. The UNI messages consist of a variety of basic building blocks containing the necessary information. These building blocks are known as information elements (IEs) and each element has a standard 4-byte header followed by the IE content. Information elements carry dif- ferent elements such as AAL parameters, the calling party number, traffic descriptor, the called party number and broadband bearer capability. The process of the SVC establishment is initiated by the Setup message issued by the ATM user device. The signaling message is transported to the nearest switch via pre-established VC (VPI 0, VCI 5). The Setup message contains the characteristics (AAL type, traffic descriptor, QoS parameters) and the destination ATM address. Then the switch should perform the CAC function. If the resources in the switch are sufficient to accept a new con- nection, the switch notifies the edge device that triggered the setup process with a Call Proceeding message. Next the ATM switch reserves resources, assigns VPI/VCI values and identifies the route to the destination. Then the process of the connection setup takes place internally within the network involving other signaling protocols. Once the device at the destination sig- nals it is ready, a Connect message is propagated through the network. Each node that receives this message issues a Connect Acknowledgment. Finally, the first ATM switch passes a Connect message to the source. Chapter 5 85 It is important to note that in ATM networks one can find both the instances of Private UNI as well Public UNI. Technically, both interfaces use the same set of equipment. Differences are essentially in the service capabilities and a range of supported features. Note that Public UNI can be also used as the interface between private and public ATM network (see the Fig. 5-6). ATM Basics 86 Fig. 5-4, The UNI setup process 5.3 PNNI Protocol Private Network-Network Interface (PNNI) was developed to facilitate the deployment of large and high performance ATM networks. The major intent was to design a protocol that would overcome limitations related to the use of the PNNI predecessor – IISP (Interim Inter-switch Signaling Protocol). IISP was developed to enable small, static routed private ATM network implementations. The greatest effort in PNNI development was to create the routing algorithms. While the routing algorithms were being perfected, IISP defined how to create static routing tables. IISP is not a scalable solu- tion, as the amount of configuration work grows exponentially with the number of switches and connections. The signaling specification in IISP is based on Q.2931, with the definition of some new Information Elements spe- cific to the private network. With IISP the signaling process is asymmetric. This means there are a clearly defined user side and a network side. Needless to say, this involves some configuration efforts from the operator. Being based on static routing tables, the switches cannot adapt to changing topology. If a link is down or a switch is down, alternatives will not be con- sidered unless manually entered in the routing tables. Once PNNI 1.0 (in 1997) was issued, the IISP solution to call set-up in the private network was considered to be far inferior. The decision was to base PNNI signaling UNI 4.0 access signaling specifi- cation and create a new PNNI routing component. However, some further modifications were necessary in order to avoid asymmetrical approach imposed by the UNI. Finally, the protocol procedures were modified and PNNI capable devices could operate in peer-to-peer model. Without this modification the contention related to VPI/VCI assignment operation would be a problematic issue as it was in case of the IISP. PNNI has been primar- ily issued for use in private networks. The successful establishment of a vir- tual circuit requires that an optimal route must be identified with regards to the customer requirements. Hence, the PNNI includes also a dynamic state routing protocol that implements some of the concepts presents in clas- sical routing protocols used in IP networks. Chapter 5 87 The PNNI routing protocol is able to collect and distribute link availability information on the facilities between switches within the PNNI network and choose the optimum route. The process of the route selection takes into account the type of ATM connection, bandwidth availability, and other QoS parameters. The PNNI routing protocol can serve very large private/enter- prises networks due to its hierarchical operation manner. On start-up, PNNI nodes send ‘hello’ packets on all interfaces to discover their neighbors. As a part of this process, neighboring nodes exchange their ID numbers. Please see the example given in Fig. 5-5. All nodes with matching numbers form a logical peer group (PG) using the matching digits as a group identi- fier. Nodes with at least one link terminating at a switch in another peer group are considered as Border Nodes. Nodes within a group exchange and relay information in the form of PNNI Topology State Packets (PTSPs) about link status including virtual bandwidth, availability and next hop address. A reliable transport mechanism is used to ensure that all nodes ultimately share the same database. ATM Basics 88 Fig. 5-5, PNNI operation [...]... of signaling messages at B-ICI is given in Fig 5- 7 91 ATM Basics Fig 5- 7 , BISUP/MTP-3 over ATM PRM Thus the B-ICI protocol consists of: •The Message Transfer Part 3b (MTP-3) signaling message handling protocol •The Broadband Integrated Services Digital Network User Part (BISUP) BISUP is a node-to-node signaling protocol that provides signaling capabilities for interconnected ATM networks BISUP allows... the ATM resources required to the service the link There must always be one node configured as the assigning node and the other node configured as the non-assigning node 5. 6 B-ICI Protocol The B-ICI protocol specifies signaling protocol that allows SVCs to be setup between different public networks The most recent ATM Forum specification for B-ICI signaling is B-ICI version 2.0 with Addendum 2.1 B-ICI... networks, or interconnecting PNNI and B-ISUP network Secondly, topology state information is not exchanged between switches of different operators The possible network scenarios involving AINI protocol is given in Fig 5- 6 Fig 5- 6 , ATM signaling protocols architecture 90 Chapter 5 The network operator gains the dynamic benefits of PNNI while retaining control of inter-domain routing since AINI uses manually... other PAR-capable devices active in the same ATM network Thanks to this concept the overlay routing network (e.g IP) can be established automatically on an ATM backbone 89 ATM Basics 5. 5 AINI Protocol ATM Inter-Network Interface (AINI) is based on the ATM Forum PNNI specification version 1.0, but uses static routing The current version of AINI 1.1 was released in September 2002 and is based on PNNI 1.1... information about non -ATM services to be distributed in an ATM network as part of the PNNI topology PAR was intended to simplify configuration and ongoing operation of IP level routing protocols when operated over PNNI environment According to PAR specification, non -ATM devices such as IP routers equipped with ATM interfaces can automatically learn about other PAR-capable devices active in the same ATM network... link MTP-2 defines networking (transport) functions common to all links in the end–to-end connection MTP 3 is a connectionless protocol designed to distribute messages to multiples MTP users and ensuring correct message sequencing In ATM networks the functions of MTP 1 and MTP 2 are performed by the physical layer, ATM layer and SAAL (CP of AAL 5, SSCF, SSCOP) The specific user part pertinent to B-ICI.. .Chapter 5 As part of this process the group members select a Peer Group Leader (PGL) based on a configured priority number, or by selecting the node with the lowest ATM address Group leaders establish logical connections with each other and exchange a summary of information about their... protocol that provides signaling capabilities for interconnected ATM networks BISUP allows for the carriage of signaling messages across B-ICI These messages provide for the establishment, supervision and release of virtual connections between inter-connected nodes B-ICI routing tables are static tables configured by the operator This means that the operator has full control of the network routing 92... point is that PNNI signaling does not support the transaction-based signaling so intelligent network services such as 800 numbers; mobility and others cannot be supported Over time, a number of enhancements to PNNI have been released, thus extending its capabilities They include the support of ABR traffic category as well as mobility of ATM network and particular switches The most important enhancement... any other group Note that all endstation addresses begin with the address sequence that matches the group to which they are attached The PNNI application is mainly restricted to the use within a private ATM network However, PNNI can be also used to connect private networks The limitations of this model are mainly related to the exchange of topology state information between private networks own by different . sig- naling protocol stack for transport of signaling messages at B-ICI is given in Fig. 5- 7 . Chapter 5 91 ATM Basics 92 Thus the B-ICI protocol consists of: •The Message Transfer Part 3b (MTP-3). the interface between private and public ATM network (see the Fig. 5- 6 ). ATM Basics 86 Fig. 5- 4 , The UNI setup process 5. 3 PNNI Protocol Private Network-Network Interface (PNNI) was developed. SEL fields). ATM Basics 82 Fig. 5- 2 , Different formats of ATM addresses •The AA (Administrative Authority) field identifies national organi- zations such as the ATM network operator, ATM user or ATM