*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-* -=[SD]=- Sepulchral Darkness -=[SD]=- presents : An Introduction to B-ISDN (BroadBand Integrated Services Digital Network) Brought to you by : ---=[AZTECH]=--- Sepulchral Darkness '95 All Rights Worth Shit *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-* BROADBAND ISDN by Frank Booty ----------------------------- The European Commission wants European Network Operators to have the infrastructure to allow value added suppliers to link into networks within the European Framework. It wants telecommunications networks to provide the backbone of economic growth and social well-being : It wants networks to be standardised for interlinking, and for them to be open with other services to be provided by value added re-sellers. The idea is to build a network from scratch with unlimited funds. But today's equipment and practises are a mix of ancient and modern. The transition is from one network dominated by voice (The 60's Network) to one providing a spectrum of services including data, voice and vision. This requires technology and development to be integrated into an an existing base in such a way that it doesnt cause it to fall over. And it has to be agreed with other developers. Most countrie's operators are moving from existing telecommunications networks to the sophisticated networks of the future. But there has to be someone to make sure there is no disruption, to provide administrative services and to make sure there is a link to other countries. Signalling and Protocols must be harmonised at standards meetings too. What we do in Europe has to be agreed in Europe. But note that for example STC is now part of the Northern Telecom. Multinationals are exerting their influence on a global basis; take the case of a Bell operating company which has applied to become a member of a European Technical Standards Institute. There are other US concerns which are acting likewise. Also take the case of services which are available through networks, and map the bandwidth to support these services against the circuit occupancy or utilisation. Networks, whether ISDN, ATM (Asynchronous Transfer Mode) or SDH (Synchronous Digital Hierachy) can take a variety of services (voice, video, image, etc...) We need Networks that can take a variety of signals, ie;multi services networks. There is a debate in the telecommunications and computer world about what is most suited for their needs. What is the right type of network to have? In data Networks there are protocols and checking of re-transmission. Where there is a mixture with a real time element in it (Not a DP requirement) there is high integrity of information and a lower requirements for real time. Wideband service, carried over the broadband (defined as a point to point service, not including ditributive services not including services such as TV) Network and accessed via the ISDN narrowband Network, provides a high class service at affordable cost for domestic users and an adequate service for small and most medium buisness sites, at least throughout the 1990's. With voice, you can't wait but expect fluency. With Video, you expect the pictures to be linked together. The issue is to bring both requirements together under one protocol - ie; ATM which is now widely recognised as the technology for the implementation of broadband networks. This will accomodate a variety of servicessome of which are predominantly packet based and some vision and image based. There are two compatible switching technologies for ATM and those are MANs and ATM star switches. A low risk start to the broadband network is acheived with the introduction of MANs linked by simple bridges, providing a service from nodes on dedicated optical customer access conections to larger buisness sites. By offering only a connection-less datagram service at this stage, much of the complexity of network integration is avoided. This is the approach already adopted by AT+T, Bellcore and some organisations with the switched Multi Megabit Data Service (SMDS). As the popularity of the service grows, both in geographic and traffic terms, the number of MANs which must be interconnected increases. Where there are more than four fully interconnected MANs, the bridges should be replaced by routers and the switch in the router would be a small ATM star switch. Here, connection mode services should be added to the datagram service. With further growth, the direct interconnection of routers improves efficiency and represents a major step towards the ultimate target broadband network using large ATM star switches and (possibly) MANs as local concentrators. A circuit switched network with 64k-bits granularity will, however, continue to provide the core services for the majority of users for the forseeable future. The growth of the broadband network is shown in panels A to E. Figure 2a: ~~~~~~~~~~ Transport +------------------+ Network | MAN | | Manager | +------------------+ | +---+ +---------------| |---------------+ / +---+ \ Customer / \ Access +---+ +---+ Connection CAC -----| | IEEE 802.6 Man | |----- (CAC) +---+ +---+ or \ / Remote \ +---+ / Multiplexer +---------------| |---------------+ Access +---+ | CAC Stages of growth of the Broadband Network:A,Stage 1. A single MAN is embedded in the transport network. A datagram service is provided to a few large buisness sites on dedicated optical customer access circuits. Ports are directly addressed. Port address memories are maintained by a manager. Figure 2b: ~~~~~~~~~~ +-------------------------------------+ / \ +---+ +---+ CAC -----| | | |----- CAC +---+ Man +---+ \ / \ +---+ +---+ / | |-------------------------| | +---+ +---+ +------------+ +------------+ | Datagram | <- Bridges -> | Datagram | | Server | | | Server | +------------+ | +------------+ +---+ | +---+ +---| |---+ V +---| |---+ +-+-+ +---+ +-+-++------------++-+-+ +---+ +-+-+ CAC ---| | Man | || Datagram || | Man | |--- CAC +-+-+ +---+ +-+-+| Server |+-+-+ +---+ +-+-+ +---| |---+ +------------+ +---| |---+ +---+ +---+ | | CAC CAC Transport Network Stage 2: Further MANs are added to the network, linked by point to point bridges incorporating datagram servers. The limit is about four MANs before the structure becomes unwieldy. Figure 2c: ~~~~~~~~~~ Transport CAC CAC Network | | +---+ +---+ +-| |---------| |-+ / +---+ +---+ \ | Man | | | \ +---+ / CAC +--------| |---------+ CAC | +---+ | +---+ | +---+ +-----| |-----+ +-----------+ +-----| |-----+ | +---+ | | Router | | +---+ | | | +-----------+ | | +---+ +---+ | | | +---+ +---+ CAC -| | Man | |------+ | +------| | Man | |- CAC +---+ +---+ +-----------+ +---+ +---+ | | | Datagram | | | | +---+ | | Server | | +---+ | +-----| |-----+ +-----------+ +-----| |-----+ +---+ +---+ | | CAC CAC Stage 3 : Groups of bridges are replaced by routers which use small ATM Switches, typically with 16 to 32 ports, providing full rate connectivity between MANs and to a datagram server. The majority of the traffic will be on virtual circuits. Figure 2d: ~~~~~~~~~~ Transport CAC CAC Network | | +---+ +---+ +-| |---------| |-+ / +---+ +---+ \ | Man | | | \ +---+ / CAC +--------| |---------+ CAC | +---+ | +---+ | +---+ +-----| |-----+ +-----------+ +-----| |-----+ | +---+ | | Router | | +---+ | | | +-----------+ | | +---+ +---+ | | | | +---+ +---+ CAC -| | Man | |------+ | | +------| | Man | |- CAC +---+ +---+ | | +---+ +---+ | | | | | | | +---+ | | | | +---+ | +-----| |-----+ | | +-----| |-----+ +---+ | | +---+ | | | | CAC | | CAC | | +----------+ | | +----------+ | Datagram |--------+ | +--------| Datagram | | Server | | | | Server | +----------+ | | +----------+ CAC | | CAC | | | | +---+ | | +---+ +-----| |-----+ | | +-----| |-----+ | +---+ | | | | +---+ | | | | | | | +---+ +---+ | | +---+ +---+ CAC -| | Man | |------+ | | +----| | Man | |- CAC +---+ +---+ | | | | +---+ +---+ | | +-----------+ | | | +---+ | | Router | | +---+ | +-----| |-----+ +-----------+ +-----| |-----+ +---+ | +---+ | | | CAC +---+ CAC +----| |----+ / +---+ \ | Man | \ +---+ +---+ / +-| |--| |-+ +---+ +---+ | | CAC CAC Stage 4: Isolated routers are now inter-connected by direct links, removing the need to route transit traffic through intermediate MANs, simplifying the structure of the network and setting up the target network architecture. Figure 2e ~~~~~~~~~ Transport CAC CAC Network | | +---+ +---+ +-| |---------| |-+ / +---+ +---+ \ | Man | | | \ +---+ / CAC +--------| |---------+ CAC | +---+ | +---+ | +---+ +-----| |-----+ +-----------+ +-----| |-----+ | +---+ | | Router | | +---+ | | | +-----------+ | | +---+ +---+ | || | | +---+ +---+ CAC -| | Man | |-----+ || | +------| | Man | |- CAC +---+ +---+ || | +---+ +---+ | | || | | | | +---+ | || | | +---+ | +-----| |-----+ || | +-----| |-----+ +---+ || | +---+ | || | | CAC || | CAC || | || | +----------+ || +--------| Datagram | || | Server | || +----------+ || +-++-+ | | +-------------+ | | +-------------+ CAC -----| ATM Switch +|------+ +------| ATM Switch +|--------- CAC CAC -----| Server |------------------| Server |--------- CAC +-------------+ +-------------+ Stage 5:Routers are now linked to large ATM star switches with directly connected subscribers. The ATM network will continue to grow and the MANs will be relegated to the status of local concentrators. Some large buisness sites will gain access to the broadband network on direct optical lines using SDH STM-1 links. Initial studies suggest that there is no cost advantage in providing a physical path at less than the 155/150M-bit/s rate (assuming mono fibre in all public network applications). Other possibilities, serving a group of customers, include SDH add/drop multiplexers providing a contracted portion of the STM-1 bandwidth for narrowband and broadband services to each customer, or a MAN providing ATM mode and isochronous mode service over a single fault tolerant medium. Operating companies in Europe and the US are now investing heavily in an ISDN infrastructure. ISDN primary rate access can provide a 1,920kbit/s channel in the US. This is 200 or 150 times better than a 9.6kbits modem. To acheive superior performance for a packet mode service, the ratio between the upstream bearer rate and the peak user bandwidth should be greater than 8:1 to give good statistical smoothing. With a 2Mbit/s source, the implied upstream bearer rate should be greater than 16Mbit/s which the broadband network with 150Mbit/s bearers is well able to provide. New network requirements such as intelligent networks, PCNs and distributed control will put severe demands on the signalling network such that the present 64Kbit/s network will be inadequate. All signalling is therefore expected to migrate to broadband in time, thus D channel signalling will be adapted to ATM near to the local access. The packet switched Public Data Network (PSPDN) is expected to be replaced by eventually by ATM mode wideband packet services enabled by the broadband network. While Figure 2 gives an indication of the growth of the broadband network, a typical local network architecture providing for access to wideband services is shown in Figure 3. Figure 3 ~~~~~~~~ Typical local Network architecture showing direct access and wideband access to the broadband network. Customer +-----------+ +------+ Premises ---| Broadband |--------------------------------------| | Equipment | NT | | | +-----------+ | | n x 64Kbit/s | | o D Channel Packets Z / \ +---+ | | o Multi Slot Bearers +-------+ \ +------| B |-----| | o User to user signal | + +---+ | | | | +---+ | Y | +---------+ E + +---+------------| | | | | X |---+ / +-| F | | A | C |-----| | +---------+ \ / +---+------+ +--| | | | D | | | +---+ | | | | | | | | +----+ | | | | X21| | | | +----+ | | | | | | V V +------+ PSPDN A: HDLC Packets E: Concentrator Switch B: Rate Adaption F: Centralised D Channel Handler C: HDLC/ATM Gateway X: Subscriber Equipment D: D Channel Y: Broadband Network A Datagram Service may well be considered to be useful by other users. The datagram service is provided for by servers usually located on each MAN or at each star switch. The calling subscriber and succesive servers in the path are linked by reserved virtual paths. Each server will receive the complete datagram, examine the destination address held in the information field of the first ATM cell, and route to the next server in the relevant direction or to the destination subscriber. For the Network configuration of 2A, with a single MAN and providing only datagram service, each MAN node may provide address translation, kept up to date by a MAN manager. For figure 2B, the servers are provided in the bridges while for C, connection mode service is assumed for the majority of the traffic and a limited number of servers connected on router ports provide adequate capacity; similarly for configurations in D and E. In a typical local access arrangement, the rate adaption unit would have a permanently assigned VP to the nearest datagram server in the broadband network. Access to the service could also be via the D-channel and an HDLC/ATM adaption unit. This value added service could be particularly attractive to domestic customers where a datagram may replace a letter in many cases. The provision of an ATM based broadband overlay network may be economically justified by enabling enhanced services access over the narrow band ISDN Network. Simultaneously, direct access broadband services may be provided to the relatively few large buisness users for which the higher access costs are viable. Transmission Networks based on the Synchronous Digital Hierarchy (SDH) offer a number of significant advantages over existing networks, particularly in terms of the ability to manage the capacity. This managability is crucial as it will enable a faster provision and re arrangement of services, and a much more rapid restoration in the event of facility failure. It has been noted that there is a wide acceptance of the layered nature of tranport networks, with a client/server realtionship between layers. Consider a 2 Mbit/s based plesiochronous (nearly synchronous) Digital Hierarchy (PDH): The 34 Mbit/s server Network supports the 8Mbit/s client network, and the 8 Mbit/s server network supports the 2 Mbit/s client network. The signal structure of the server network generally comprises a payload (into which one or more signals from the client network are multiplexed), and a server network 'overhead'. This overhead ensures the integrity of the server network. So it's possible to distinguish between the bit rates of the server network and those of the client network,eg:The pleisochronously multiplexed hierarchical bit rate 34,368 Kbit/s - the server network bit rate is 34,368 Kbit/s . but the client bit rate is 16 x 2,048 kbit/s (ie:32,768 Kbits) Likewise, SDH based Networks exhibit a server/client relationship between their layers, eg: higher order Virtual Container (VC) networks serve to support lower order VC client Networks. Further, these VC Networks can serve to support client signals from the existing PDH. Agreements in the CCITT have resulted in a significant simplification of the SDH multiplexing structure which offers the potential of widespread networks based on a common set of VCs. So any SDH deployment strategy should recognise the emergence of VC Networks in the longer term and the eventual demise of existing PDH bit rates. Although the multiplexing structure reccomended by the CCITT provides for the support of most PDH bit rates, including simultaneous mixes of different bit rates, there are likely to be advantages by limiting the range of PDH bit rates to be supported. So the strategy should be optimised for the support of the most important client networks of the PDH, whilst taking into account the likely requirements of future client networks such as ATM or digitally encoded TV. This means it is necessary to identify the most important client networks. Figure 4 indicates the Broad services categories which either exist or are expected to exist or are expected to emerge in the future, and the corresponding network requirements in terms of bit rates. Figure 4 ~~~~~~~~ Service Categories and network requirements SERVICE CATEGORIES NETWORK REQUIREMENTS ------------------ -------------------- Voice 2M bit/s paths Low speed data 2M bit/s paths High speed data n x 2 M bit/s paths (eg: 4,6,8,10 up to 30/40 M bit/s) Multi Service (Very High speed data or video) =140 M bit/s paths (probably using ATM) Broadcast TV Currently 140M bit/s, but moving to lower bit rates(eg: = 34 or 45M bit/s) High Definition TV (HDTV) Currently = 600M bit/s, but moving to lower rates (maybe = 140M bit/s) What emerges is that, currently 2M bit/s is a key bit rate and will undoubtedly remain so. 140M bit/s is also an important rate with rather modest quantity requirements now, but with the likelihood of significant increase especially with the advent of ATM. It is also the PDH server Network layer at which protection switching is commonly provided. Commercially important services are expected to emerge between 2 and 140M bit/s, with requirements which will not neccesarily be related to existing PDH bit rates. For example, The LAN interconnect and MAN requiements with bit rates of perhaps 10 to 100M bit/s. Introductory deployment strategies should be optimised around the rates of 2 and 140M bit/s for the most important client networks. Such strategies should not, however, jeopardise the emergence of other client networks. The support of signal at around 140M bit/s can be provided by SDH based networks, since the signals can be mapped directly into the VC and all SDH based Networks will offer VC Networking. It is the support of lower bit rate signals which requires more detailed consideration - thus, consider two approaches: Approach 1 ~~~~~~~~~~ This recognises the importance of the 2M bit/s client network. The SDH network is thus mainly organised to offer management of the paths of the 2M bit/s network and the interfaces to the SDH island are at 2M bit/s. The early introduction of SDH based facilities can thus be shown in Figure 5, in which the 2M bit/s signals are mapped onto VCs and routed across the SDH island. Figure 5 ~~~~~~~~ Early introduction stage optimised on key rate. +----------------+ | | 2M bit/s +---+ +---+ 2M bit/s Leased | I | | I | Leased Lines \ | N | | N | / Lines \ | T | SDH | T | / +---------| E | Island | E |---------+ +--------+ 2M bit/s | R | | R | 2M bit/s +--------+ | PSTN/ | +---------| F | | F |---------+ | PSTN/ | | ISDN |/ | A | | A | \ | ISDN | | switch | | C | | C | | switch | +--------+ | E | | E | +--------+ +---+ +---+ | | +----------------+ A Variant of this approach (Figure 6) is where the interface to the SDH island combines pleisochronous multiplexing and SDH tributary mapping functions in a back to back arrangement, sometimes referred to as transmultiplexing. Figure 6 ~~~~~~~~ Early introduction with a transmultiplexer function. +---------------------------------------+ | | | SDH +---+ | Island | I | | +--------+ | N | 2M bit/s +--------+ | | PSTN/ | | T | | PSTN/ | | | ISDN |- - - - - - - - - - - - -| E |----------| ISDN | | | Switch | | R | | Switch | | +--------+ | F | +--------+ | | A | | | C | | | E | | +---+ | | +---------------------------------------+ As in the case of figure 5, the 2M bit/s signals are supported in VCs in the SDH island. This approach could prove a cost effective solution, noting that plesiochronous skip multiplexing can be realised with a fairly modest chip set, and that physical interfaces at 2M bit/s are not required. It should be noted that in this latter case the SDH island could be just a cross connect equipment at a single site, where the desired initial benefit is the automated management of the network as a whole. Approach 1 has a high degree of future proofing: as plesiochronous islands diminish and the SDH islands increase in size, the same routing principles in the SDH island can be used thus offering a smooth evolution to al VC based SDH based transmission interfaces (such as the PSTN/ISDN switches shown in Figure 7). Figure 7 ~~~~~~~~ Emergence of SDH based switch ports +--------+ 2M bit/s | | 2M bit/s Leased +-------+ +---+ Leased Lines +----------------+ |P X I| | I | Lines \ | SDH | |C M N| | N | \ | island | |H U T| | T | / +-------| (includes | |R X E| | E |---+ 2M bit/s| plesiochronous |---|O I R| | R | 2M bit/s +-------| multiplexing) | ^ |N N F| | F |---+ / +----------------+ | |O G A| | A | \ +--------+ | |U C| | C | +--------+ | PSTN/ | | |S E| | E | | PSTN/ | | ISDN | | +-------+ +---+ | ISDN | | Switch | | | | | Switch | +--------+ | +--------+ +--------+ Pleisochronously multiplexed signal (eg: 34M bit/s) However geographically small or widespread the SDH island, it is important from the outset to regard the SDH island as providing VC networking rather than networking of PDH bit rates. Approach 2 ~~~~~~~~~~ This is an alternative for the SDH island to support Plesiochronously multiplexed server network layers (ie: 8,34 and 140M bit/s) as well as 2M bit/s in the case of the 2M bit/s hierarchy as shown in Figure 8. Figure 8 ~~~~~~~~ Early introduction supporting all pleisochronously multiplexed rates. 2M bit/s +-----------+ 2M bit/s Leased | | Leased Lines \ +--+ +--+ +--+ +--+ / Lines \ | |---| | SDH | |---| | / +-----|PM|---|IF| island |IF|---|PM|-----+ | |---| | | |---| | +-----| |---| | | |---| |-----+ / | | ^ | | | | ^ | | \ / +--+ | +--+ +--+ | +--+ \ +-------+ | | | | +-------+ | PSTN/ | | +-----------+ | | PSTN/ | | ISDN | +--- 2,8,34,140 ---+ | ISDN | | Switch| M bit/s | Switch| +-------+ +-------+ IF = Interface PM = Plesiochronous Multiplexing Elements of existing transmission networks with this approach are replaced or added to by SDH based facilities on a functionally like-for-like basis but with the potential for management. Pleisochronous multiplexing/demultiplexing, by definition, cannot occur in the SDH island. This approach has the advantage that the planning and routing arrangements developed for the existing network can be used in the SDH based arrangements. But there are some drawbacks: since the 8,34 and 140M bit/s signals are server network layers containing pleisochronously multiplexed 2M bit/s tributaries, pleisochronous multiplexer (outside the SDH island) will continue to be required (Figure 8);and migration to an all SDH based network will require new routing arrangements to be implemented. It is important to distinguish between case where a PDH network layer is a server network, as discussed under approach 2 above, and those where it is a client network only (such is the case when these rates are offered as service rates) and they will therefore be required to be supported transparently in the SDH island. In such cases the client PDH rate will be mapped into an appropriately sized server VC. Although Approach 1 demands a more rigourous examination of client network requirements prior to deployment, it is considered that it has a greater degree of future proofing than Approach 2. For this reason, network operators in Europe envisage adopting the principles of Approach 1. Moreover these principles are reflected in an emerging CCITT draft reccomendation on SDH network aspects. BT considers that in the long term ATM should enable the decoupling of service bit rates for transport rates, and thereby offer the flexibility to meet the uncertain service needs of the future. Within transmission networks, ATM signals will be mapped into the various transport rates that are available. For example, with SDH, ATM signals will generally be mapped into the VC for transmission between ATM switches and terminals. However, in the pre-ATM enviroment it will be possible for SDH based networks to support a range of intermediate rates, eg; television encoded into 34 or 45M-bit/s. With agreement in the CCITT on the standards of the ATM cell and the broadband ISDN user/network interface (CCITT Reccomendations I.361 and I.432), there is the possibility of an early introduction of a LAN interconnect service, and, subsequently, as ATM switches emerge in the network, with full broadband ISDN services. It is considered within BT that this concept could prove an attractive national and, importantly, international service. This demonstrates that an introductory deployment strategy optimised for the most important existing client networks (ie: 2 and 140M-Bit/s) not only safeguards existing service categories but it also provides a high degree of future proofing for the uncertain demands of future service categories and offers a migratory route to broadband ISDN capability. In Europe our networks are well advanced. The US however needs high quality data communications facilities. The Europeans see that sort of need as not so relevant at the moment. They do not want a data only service - They want an ATM solution that allows a variety of services. In summary, The US wants seperate networks, In Europe we want one pipe for the lot. As regards standards, commercial realism has crept in within the last year or two. Standards are important to a companys bottom line. There is no lock-in to a single supplier, and theres no lock-in to a dominant supplier. Companies want purchasing power to get the best deal. It is a customer driven requirement. Therefore standards are important to the bottom line. Chief executives must realise the importance of standards. -=[SD]=- -=[SD]=- -=[SD]=- -=[SD]=- -=[SD]=- -=[SD]=- The article then goes blabbing on about management structures, bleh, bleh, for another four paragraphs before dying in a rather undignified manner. ---=[AZTECH]=---