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Operating and managing a commercial worldwide network
case study
O p e r a t i n g and managing a commercial worldwide network The demands on international business communication networks are high: Terry Reed describes the GElS network, and discusses how it meets that challenge
GE Information ServicesLtd., Shortlands, Hammersmith, London14/68BX, UK. Teh 44 81 741 9900
groups in order to perform common transactions. These currently include order processing within manufacturing companies and between manufacturers, suppliers and Large international data net- puters, and must be able to distributors, online cash-balworks need to be reliable interconnect with public ance reporting and funds and flexible. The factors data networks (PDN). In the transfer instructions between and their involved in achieving this operation and management corporations are demonstrated by describ- of a commercial worldwide banks, and international ing how GE Information data network, these chal- financial reporting. The netServices Ltd (GELS)has con- lenges have to be met work is also used to provide simultaneous one of the world's most figured its global telepro- through upgrades to ensure 24 hours extensive international ecessing network. The design criteria used are explained, a day availability. This paper mail and EDI services. Local access points are and the network architecture describes how GElS has conand internal protocol are figured its global telepro- provided in 750 cities coverdescribed. The method cessing network to meet the ing over 35 countries. This employed to manage and needs of a new generation of coverage is extended to over control the network is out- more demanding and com- 80 countries by using GElS gateways to the many lined, and an example is puter literate end users. national public PDNs. To given showing how the difsatisfy a 10,000 strong multiferent components work Technical to business national client base, it is together. applications The GElS network, estab- necessary to provide reliabillished in the early 1970s, ity, fast response, error free initially connected asyn- transmission, data security, The new challenge for data chronous terminals world- data integrity and local supnetworking professionals, as wide to a host in the US on a port, in addition to wide for their colleagues in other traditional timesharing basis. geographical coverage (see areas, is to integrate new Its purpose was to allow Table 1). technology and services with technically oriented users to existing operations. Demand develop and run programs Design criteria is enormous, with more and and databases for scientific, In designing the network, more users requiring trans- technical and business com- certain key objectives were set to provide a reliable and parent network access for puting applications. their computers. Until all of The network has evolved responsive commercial serthe ISO's tele- over the last 20 years to vice. These might equally the latest apply to any data network, communications standards incorporate are accepted and imple- communications technology whether it serves the needs mented, the need for the (including satellites, X.25 of one company or many: IBM SNA/SNI, protocol-independent trans- gateways, parent network will grow. APPN and other develop- • fast response (less that 1 second delay across the Indeed, it also has a long ments), to support a number network; term future, since it is unre- of synchronous terminal proalistic to assume that compa- tocols, and to access three • rated throughput to enduser devices; nies would scrap large GElS computer centres, investments in hardware two in the US and one in the • minimum cost in hardware, modems, lines, etc; simply to replace them with Netherlands. In addition, universally standard equip- users' hosts can be linked to • capacity to extend at any level in the network to the GElS hosts and multiple ment. accommodate growth In this environment, the global applications. simply by plugging in The pattern of use has also data network must be capaadditional capacity with ble, through the use of the changed, with a trend away minimum interruption to latest technologies, of from technical applications the users; achieving greater throughput to multi-location business from its communications systems linking geographi- • extensive network management facilities to give spread functional lines connected to host com- cally 0140-3664/91/009563-07 © 1991 Butterworth-Heinemann Ltd
vol 14 no 9 november 1991
563
case study via Germany to Amsterdam Network covers: and one via the UK to • over 35 countries (up to 80 including access Amsterdam. From the UK via PDNs) three diverse routes are used; • on five continents UK-France-Amsterdam; UK• across23 time zones Belgium-Amsterdam; and • in 750 cities Over 10,000 companies use the network worldwide UK-Amsterdam (direct). Network availability = 99.8% All software enhance800,000 km of circuits, 7,500 access ports ments are downline loaded Satellite (2 trans-Pacific, 2 trans-Atlantic), fibre optic and from a central point to mainmicrowave transmission tain current software level Over 1000 data communication processorsemployed compatibility throughout the Three interconnected supercentres (host processors, network, and all changes are IBM/Honeywell) controlled and tested by a 400M characters currently transmitted during peak hour quality assurance group. (2000M capacity) 4000 concurrent usersduring peak hours There are currently 22 3500M characters transmitted per day manned remote network 875000 user sessions per week management centres worldInterconnection to public packet data networks and the wide, with 200 people Reliability, availability and telex network working split shifts involved serviceability Support for terminals running at speedsfrom 50 baud to in network monitoring, fault The resilience of the net64 Kbit/s work is high, providing an diagnosis and correction Support for 2780, 3780, 3270, TTY, SNI, APPC, APPN overall availability of and maintenance activities. and X.25 protocols 99.8%. This is achieved by Comprehensive network Operational features include: incorporating several levels management ensures relia• diversifiedtransmission links • distributedcontrol of redundancy through a bility of the network. This • automaticcircuit recovery combination of mesh, star includes proactive monitor• onlinenetwork monitor and tree topology structures, ing with the gathering and • downline load software distribution together with multiple inter- display of information on • reconfigurationflexibility connected intelligent both degraded and failed switching processors. These components and facilities on Table 1. Network are located at three separate a real-time basis. The statis- device, and then reconfigure statistics geographical locations, tics, which are monitored the network and paths to know as switching centres, constantly, provide opera- balance the load over the that are manned 24 hours a tions personnel with the devices without disrupting day, 365 days a year. Each data needed to identify and the service. In summary, the pair of switching processors isolate problems quickly -- network provides distributed is only 50% loaded, so that from the access nodes to the control, automatic reconfigif one processor fails com- systems level. The network uration and automatic cirpletely the warm standby management software also cuit and network processor processor can automatically produces a number of recovery. pick up the total network reports on system performload for both processors. ance, including information Terminals, protocols and Redundancy is also pro- on system throughput, data integrity vided in the distributed device downtime and avail- The network supports a access nodes and in the ability, nodal bandwidth uti- number of communication multiplication of the com- lization, and other service protocols, allowing a wide munication links, which are activity data. variety of devices to be conmade through 800,000 km The network provides for nected. The communof landlines, under-sea alternative paths and diverse ications protocols supported cables, fibre optics and routing. Each network com- are detailed in Table 1. satellite links. ponent is expected to As well as handling a Diversity of leased lines is handle peak loads. When a wide variety of 'dumb' teralso very important -- even device is overloaded, the minals, the network supports internationally. For example, network automatical ly intelligent devices, including from our access centre in reroutes traffic to the alter- the IBM PC and other microCopengagen, multiple 64 native device. At regular computers. Microcomputer kbit/s links are diversely intervals network analysts software is available to prorouted over two routes, one examine the load on each vide intelligent integration high visibility of operational conditions, errors and failures, in order to quickly diagnose and correct any problems; • high level of automation in network operation -particularly network recovery; • insensitivity to network topology (satellites, microwave links, transient noise, etc.; • ability to optimize network resources automatically.
564
computer communications
case study with hosts via the network. Once data enters a network access node, it is wrapped in a packet and given a sequence number, together with routing information, and sent through the network over the least congested route, determined at the time the packet is assembled, and merged with the other packets travelling through the network. Error detection and retransmission are performed automatically, and individual users' packets, received over diverse routes, are resequenced and presented to the host in the correct order by using the packet sequence numbers. For asynchronous terminals with no error-correcting facilities, it is possible to deploy error-correcting devices between the customers' premises and the network access points to eliminate errors. In addition, other standard error-coding protocols are used, like XModem, X.PC, V.42bis, MNP V5, etc. For synchronous traffic, normal cyclic redundancy check and block retransmission protocols are implemented m the network access points.
Network architecture
Concepts GELS' three-level architecture employs hierarchical packet switching, and is optimized for cost performance: • remote user access nodes, known as mini-remote concentrators (MRC) or remote devices (RD), are connected in a tree structure to central nodes, known as central concentrators (CC). Additionally, the traffic from a number of MRCs/RDs may be
vol 14 no 9 november 1991
multiplexed through remote device multiplexers (RDM) before being passed on to a CC over high bandwidth links; CCs are connected in a star topology to switchers (SW). Each CC will be connected to two SWs; SWs are interconnected in a mesh. The network devices run different software depending on the function within the network. An additional network device, known as an intelligent multiplexer (IMUX) can be installed between the user's premises and an MRC/RD, enabling a number of user terminals to be multiplexed down one synchronous error-corrected data stream to a single MRC/RD port. Redundant paths are provided between all network nodes. Figure 1 shows the resulting network topology. Remote access nodes A single RD or MRC provides: • a direct interface between the user's terminals and the network; • an indirect interface between the user's terminals and the network via an intelligent multiplexer; • support for 32 asynchronous and 12 synchronous terminal users; • programmability to meet different users' requirements; • the capability to perform speed/protocol conversion, handling the emulation of various terminal communication protocols, i.e. it acts as a terminal PAD (packet assembler/disassembler).
The RDs support access for terminals running at speeds ranging from 110 bit/s to 64 kbit/s, and a range of communication protocols (see Table 1). CC nodes CCs provide support for RDs (source CCs) and hosts (object CCs), and each CC by way of special broadcast control messages, knows which host processor is connected to every other CC, together with the utilization of every CC. Information between CCs is updated periodically, during steady state and when host processors come online. Switching nodes SWs provide dynamic, best path routing on a per packet basis between CCs, and warm standby SWs are provided at each switching centre site. By using multiple switching centres certain benefits are realized: • substantial growth (additional connectivity), e.g. up to 192 CCs can be accommodated; • a network transmission capability of up to 2000 million characters/hour; • increased reliability -alternate paths through the central network; • greater flexibility and configurability in the network topology. Internal network protocol A packet switching mechanism is used to deliver data over the least congested routes. The remote devices (MRCs, network/host gateways, Remote Network Nodes and host PADs) convert external communications protocols to the internal network protocol. The structure of the packets is straightforward:
565
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I • each packet contains a message from one user; • packets are composed of a 9 byte header, including a sequence number and routing information (destination/source), followed by up to 1008 bytes of user data. Data integrity is maintained by employing packet acknowledgements (ACK) techniques; the ACK is contained solely in the first 6 byte header. Negative acknowledgements (NAK) are not used -if a transmitting device does not receive an ACK within a specified timeout period, the packet is retransmitted (over
566
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an alternative path if necessary). Acknowledgements of all good data blocks must be sent, but to ensure minimum network delay and maximum throughput of data, 'transmit-ahead' techniques are used. These allow the transmission of a predetermined number of data blocks before requiring a positive acknowledgement. One acknowledgement then implicitly acknowledges all previously sent data blocks. Satellite
There is a significant amount of satellite transmission in the network, although primary routes are always land
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lines and undersea fibre optics. Earth stations and satellite capacity are leased from authorized bodies, including Intelsat and PTTs. There are multiple wideband circuits using four satellites and four earth stations on both sides of the Atlantic to provide the data paths between the USA and Europe. Two of the satellites are provided for backup. Across the Pacific a combination of satellite and undersea cables are used to transmit data to and from the Far East, with one satellite over the Indian Ocean and one over the Pacific Ocean. Undersea cables are used, in this case, to provide
Figure I. Network topology. SW: switcher; CC: central concentrator; RD: remote device; RDM: RD multiplexer (remote device cluster (RDC) = 2 x RDMs); IMUX: intelligent multiplexer (concentrates multiple asynchronous terminals). It should be noted that RDs, RDMs and IMUXs are connected in a tree structure
computer communications
case study The NGW is connected to the PDN over an X.25 synchronous link, having multiple logical channels accessed over corresponding switched virtual circuits. The X.25 packets arriving at Gateways and PADs The network architecture the NGW over these multidescribed so far deals pri- ple logical channels from marily with hoe terminal the PDN are stripped of the users can access GElS hosts packet headers and conby connecting their termi- verted to the internal packet nals directly to GELS' remote network protocol. The access nodes either by NGW puts X.25 headers on data arriving from the interdialup or leased line. However, there are other nal network before presentaroutes into the network con- tion to the PTT PDN. Call sistent with its three-level acceptance is handled by architecture. These exten- the NGWon receipt of a sions provide facilities for Call Request packet origiusers to connect with the nating from the PDN(or network via other X.25- user) PAD. based packet data networks, In addition, X.25 virtual and also allow their termi- circuits are available, by nals (or hosts) to connect which native X.25 terminals with either GElS or non- can connect to the network. GElS hosts, where this is This enables the X.25 protolegally permitted by the col to be carried transparPTTs. ently across the network (i.e. X.25 - GElS - X.25), with the Gateways: Network capability to perform X.25 Gateways (N GW) and call-out from the RNN, as Remote Network and Nodes well as call-in. In effect, in network (RNN) are provided between PDNs and the network (see architecture terms, the Figure 2). Support is avail- NGW/RNN replaces the able for asynchronous RD/MRC in those cases originated terminals con- where access is required via nected to the DN via an a PDN rather than directly X.28 PAD. The PDN (or pri- to an MRC. In effect, any vate user)PAD parameters country with a PDN can are controlled by the NGW access the network from that PDN. using the X.29 protocol.
diversity, because there is not enough satellite capacity over the Pacific to provide all the channels that are needed.
Figure 2. Network gateway configuration. NGW: network gateway; CC: central concentrator
GElS network
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vol 14 no 9 november 1991
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Terminal PADs (X.25): PADs are sited at a user's premises to allow a number of asynchronous terminals to be multiplexed over a single high-speed X.25 synchronous link providing a similar facility to that of the IMUX. Alternatively, qualified customer PADs can be used. The X.25 data-streams will arrive at the GElS network gateways in the same form as if they had come from a PDN PAD. Host PADs: provide the capability to interface both GElS and non-GElS hosts to the network, performing conversion from the GElS internal network protocol (and vice versa) to the protocol recognized by the receiving host (e.g. IBM 3270, asynchronous). The host PAD can be located at the client's site near to his host, and it would be connected to an object CC over a leased line. Alternatively, an asynchronous host PAD could be located with an object CCand connected over a single asynchronous leased line to a client's onsite intelligent multiplexer. Remote Network Node The RNN is an interface device that connects the network (via an object CC) to a non-GElS host using the X.25 synchronous protocol, and having multiple logical channels. The client's host or communications controller is responsible for stripping off the X.25 packet headers and presenting the data to the host systems; this is in contrast to the function of the host PAD (see Figure 3). Currently, an asynchronous terminal datastream entering the network via an MRC or an NGW can
567
case study be routed to a host gateway and presented to the client's host as an X.25 packet.
Networkmanagement andcontrol An NM and control software package called NMON is used across the network. It: • gathers performance and status messages from devices and processors in the network; • provides real-time display for processors and network devices; • provides historical tracking of device performance; • provides essential data for the operation, maintenance, diagnostics, planning and management of the network and central systems. It is an effective tool for maintaining reliability and availability. The real-time displays show: • exception status (gives up and down status changes for all monitored devices and interconnected links; • central concentrator performance (indicators include the number of users, characters per second processed, buffer utilization and retransmission rates); • interface processor performance (host PADs/ gateways, etc.) (performance indicators include the number of users, processor utilization percentage and number of characters per second.
used in each node, memory dumps of tables in nodes, and many other debugging capabilities. In sum, the NM and control systems can: • gather event information; • gather statistics; • record and change configuration data; • receive notification of an impending failure; • perform load measurementJbalancing.
Illustration
To illustrate how the various
components work together to enable information to be transmitted, we examine what happens when a user logs onto the network. A dialup user would dial his nearest access point: for example, a user in Stuttgart would make a local call to the MRC in the same city. The MRC will then ask the user for his user number and password. This data and all subsequent data will be combined with many other data streams being handled by that device, and multiplexed through the RDM
Figure 3. Difference between host PADs and host gateways; the RD/MRC acts as a terminal PAD, terminal protocol is converted by RD/MRC to GElS packet protocol
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In addition to NMON, a remote access control (RAC) program is used to debug an operational node (e.g. hung ports), and it also gives statistics on the performance of network nodes, which version of software is being 568
computer communications
case study located in the nearest of the 22 worldwide network service centres. This NSC is linked via several dedicated highspeed circuits to the nearest supercentre, which in this case is in Amstelveen, a suberb of Amsterdam. Once linked to the supercentre, the user's data is now in the network transmission operation centre, located in the same building as the host processors but in a separate area. Next, the user's data is examined by the CC which looks at the user's number and determines whether his files are being processed at the centre. If so, and assuming the user number is valid, the CC will pass the user onto the least loaded processor of the local computer cluster, where the password is checked and, if valid, pro-
vol 14 no 9 november 1991
cessing will begin. A virtual circuit has now been established and will remain on the same route unless a failure occurs. If this happens, an alternative route will be established. Let us assume that the CC has discovered that the user's files are in another centre, such as Cleveland (Ohio, USA). The CC then sets a path through the network switcher, also located in the network transmission operation centre (NTOS), and sends the data to the least loaded CC at Cleveland NTOC, which in turn passes the data to the least loaded processor in the Cleveland supercentre.
International integration Clearly, the data network will have a key part to play in the evolving computing
marketplace. Networks that sustain very high levels of availability, and which can offer national and international coverage can play an important role in bridging the equipment incompatibility gap. They can also offer companies a way of integrating business operations internationally.
Future developments GElS is upgrading its backbone network by replacing its current 'switchers' with Fast Packet Switchers and developing frame relay interfaces at the CC and RNN level. It is also planned to install fast packet switchers at each of its 22 remote network management centres, and to provide multiple TI/EI 1.544/2.048 Mbit/s links between them.
569
O p e r a t i n g and managing a commercial worldwide network The demands on international business communication networks are high: Terry Reed describes the GElS network, and discusses how it meets that challenge
GE Information ServicesLtd., Shortlands, Hammersmith, London14/68BX, UK. Teh 44 81 741 9900
groups in order to perform common transactions. These currently include order processing within manufacturing companies and between manufacturers, suppliers and Large international data net- puters, and must be able to distributors, online cash-balworks need to be reliable interconnect with public ance reporting and funds and flexible. The factors data networks (PDN). In the transfer instructions between and their involved in achieving this operation and management corporations are demonstrated by describ- of a commercial worldwide banks, and international ing how GE Information data network, these chal- financial reporting. The netServices Ltd (GELS)has con- lenges have to be met work is also used to provide simultaneous one of the world's most figured its global telepro- through upgrades to ensure 24 hours extensive international ecessing network. The design criteria used are explained, a day availability. This paper mail and EDI services. Local access points are and the network architecture describes how GElS has conand internal protocol are figured its global telepro- provided in 750 cities coverdescribed. The method cessing network to meet the ing over 35 countries. This employed to manage and needs of a new generation of coverage is extended to over control the network is out- more demanding and com- 80 countries by using GElS gateways to the many lined, and an example is puter literate end users. national public PDNs. To given showing how the difsatisfy a 10,000 strong multiferent components work Technical to business national client base, it is together. applications The GElS network, estab- necessary to provide reliabillished in the early 1970s, ity, fast response, error free initially connected asyn- transmission, data security, The new challenge for data chronous terminals world- data integrity and local supnetworking professionals, as wide to a host in the US on a port, in addition to wide for their colleagues in other traditional timesharing basis. geographical coverage (see areas, is to integrate new Its purpose was to allow Table 1). technology and services with technically oriented users to existing operations. Demand develop and run programs Design criteria is enormous, with more and and databases for scientific, In designing the network, more users requiring trans- technical and business com- certain key objectives were set to provide a reliable and parent network access for puting applications. their computers. Until all of The network has evolved responsive commercial serthe ISO's tele- over the last 20 years to vice. These might equally the latest apply to any data network, communications standards incorporate are accepted and imple- communications technology whether it serves the needs mented, the need for the (including satellites, X.25 of one company or many: IBM SNA/SNI, protocol-independent trans- gateways, parent network will grow. APPN and other develop- • fast response (less that 1 second delay across the Indeed, it also has a long ments), to support a number network; term future, since it is unre- of synchronous terminal proalistic to assume that compa- tocols, and to access three • rated throughput to enduser devices; nies would scrap large GElS computer centres, investments in hardware two in the US and one in the • minimum cost in hardware, modems, lines, etc; simply to replace them with Netherlands. In addition, universally standard equip- users' hosts can be linked to • capacity to extend at any level in the network to the GElS hosts and multiple ment. accommodate growth In this environment, the global applications. simply by plugging in The pattern of use has also data network must be capaadditional capacity with ble, through the use of the changed, with a trend away minimum interruption to latest technologies, of from technical applications the users; achieving greater throughput to multi-location business from its communications systems linking geographi- • extensive network management facilities to give spread functional lines connected to host com- cally 0140-3664/91/009563-07 © 1991 Butterworth-Heinemann Ltd
vol 14 no 9 november 1991
563
case study via Germany to Amsterdam Network covers: and one via the UK to • over 35 countries (up to 80 including access Amsterdam. From the UK via PDNs) three diverse routes are used; • on five continents UK-France-Amsterdam; UK• across23 time zones Belgium-Amsterdam; and • in 750 cities Over 10,000 companies use the network worldwide UK-Amsterdam (direct). Network availability = 99.8% All software enhance800,000 km of circuits, 7,500 access ports ments are downline loaded Satellite (2 trans-Pacific, 2 trans-Atlantic), fibre optic and from a central point to mainmicrowave transmission tain current software level Over 1000 data communication processorsemployed compatibility throughout the Three interconnected supercentres (host processors, network, and all changes are IBM/Honeywell) controlled and tested by a 400M characters currently transmitted during peak hour quality assurance group. (2000M capacity) 4000 concurrent usersduring peak hours There are currently 22 3500M characters transmitted per day manned remote network 875000 user sessions per week management centres worldInterconnection to public packet data networks and the wide, with 200 people Reliability, availability and telex network working split shifts involved serviceability Support for terminals running at speedsfrom 50 baud to in network monitoring, fault The resilience of the net64 Kbit/s work is high, providing an diagnosis and correction Support for 2780, 3780, 3270, TTY, SNI, APPC, APPN overall availability of and maintenance activities. and X.25 protocols 99.8%. This is achieved by Comprehensive network Operational features include: incorporating several levels management ensures relia• diversifiedtransmission links • distributedcontrol of redundancy through a bility of the network. This • automaticcircuit recovery combination of mesh, star includes proactive monitor• onlinenetwork monitor and tree topology structures, ing with the gathering and • downline load software distribution together with multiple inter- display of information on • reconfigurationflexibility connected intelligent both degraded and failed switching processors. These components and facilities on Table 1. Network are located at three separate a real-time basis. The statis- device, and then reconfigure statistics geographical locations, tics, which are monitored the network and paths to know as switching centres, constantly, provide opera- balance the load over the that are manned 24 hours a tions personnel with the devices without disrupting day, 365 days a year. Each data needed to identify and the service. In summary, the pair of switching processors isolate problems quickly -- network provides distributed is only 50% loaded, so that from the access nodes to the control, automatic reconfigif one processor fails com- systems level. The network uration and automatic cirpletely the warm standby management software also cuit and network processor processor can automatically produces a number of recovery. pick up the total network reports on system performload for both processors. ance, including information Terminals, protocols and Redundancy is also pro- on system throughput, data integrity vided in the distributed device downtime and avail- The network supports a access nodes and in the ability, nodal bandwidth uti- number of communication multiplication of the com- lization, and other service protocols, allowing a wide munication links, which are activity data. variety of devices to be conmade through 800,000 km The network provides for nected. The communof landlines, under-sea alternative paths and diverse ications protocols supported cables, fibre optics and routing. Each network com- are detailed in Table 1. satellite links. ponent is expected to As well as handling a Diversity of leased lines is handle peak loads. When a wide variety of 'dumb' teralso very important -- even device is overloaded, the minals, the network supports internationally. For example, network automatical ly intelligent devices, including from our access centre in reroutes traffic to the alter- the IBM PC and other microCopengagen, multiple 64 native device. At regular computers. Microcomputer kbit/s links are diversely intervals network analysts software is available to prorouted over two routes, one examine the load on each vide intelligent integration high visibility of operational conditions, errors and failures, in order to quickly diagnose and correct any problems; • high level of automation in network operation -particularly network recovery; • insensitivity to network topology (satellites, microwave links, transient noise, etc.; • ability to optimize network resources automatically.
564
computer communications
case study with hosts via the network. Once data enters a network access node, it is wrapped in a packet and given a sequence number, together with routing information, and sent through the network over the least congested route, determined at the time the packet is assembled, and merged with the other packets travelling through the network. Error detection and retransmission are performed automatically, and individual users' packets, received over diverse routes, are resequenced and presented to the host in the correct order by using the packet sequence numbers. For asynchronous terminals with no error-correcting facilities, it is possible to deploy error-correcting devices between the customers' premises and the network access points to eliminate errors. In addition, other standard error-coding protocols are used, like XModem, X.PC, V.42bis, MNP V5, etc. For synchronous traffic, normal cyclic redundancy check and block retransmission protocols are implemented m the network access points.
Network architecture
Concepts GELS' three-level architecture employs hierarchical packet switching, and is optimized for cost performance: • remote user access nodes, known as mini-remote concentrators (MRC) or remote devices (RD), are connected in a tree structure to central nodes, known as central concentrators (CC). Additionally, the traffic from a number of MRCs/RDs may be
vol 14 no 9 november 1991
multiplexed through remote device multiplexers (RDM) before being passed on to a CC over high bandwidth links; CCs are connected in a star topology to switchers (SW). Each CC will be connected to two SWs; SWs are interconnected in a mesh. The network devices run different software depending on the function within the network. An additional network device, known as an intelligent multiplexer (IMUX) can be installed between the user's premises and an MRC/RD, enabling a number of user terminals to be multiplexed down one synchronous error-corrected data stream to a single MRC/RD port. Redundant paths are provided between all network nodes. Figure 1 shows the resulting network topology. Remote access nodes A single RD or MRC provides: • a direct interface between the user's terminals and the network; • an indirect interface between the user's terminals and the network via an intelligent multiplexer; • support for 32 asynchronous and 12 synchronous terminal users; • programmability to meet different users' requirements; • the capability to perform speed/protocol conversion, handling the emulation of various terminal communication protocols, i.e. it acts as a terminal PAD (packet assembler/disassembler).
The RDs support access for terminals running at speeds ranging from 110 bit/s to 64 kbit/s, and a range of communication protocols (see Table 1). CC nodes CCs provide support for RDs (source CCs) and hosts (object CCs), and each CC by way of special broadcast control messages, knows which host processor is connected to every other CC, together with the utilization of every CC. Information between CCs is updated periodically, during steady state and when host processors come online. Switching nodes SWs provide dynamic, best path routing on a per packet basis between CCs, and warm standby SWs are provided at each switching centre site. By using multiple switching centres certain benefits are realized: • substantial growth (additional connectivity), e.g. up to 192 CCs can be accommodated; • a network transmission capability of up to 2000 million characters/hour; • increased reliability -alternate paths through the central network; • greater flexibility and configurability in the network topology. Internal network protocol A packet switching mechanism is used to deliver data over the least congested routes. The remote devices (MRCs, network/host gateways, Remote Network Nodes and host PADs) convert external communications protocols to the internal network protocol. The structure of the packets is straightforward:
565
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I • each packet contains a message from one user; • packets are composed of a 9 byte header, including a sequence number and routing information (destination/source), followed by up to 1008 bytes of user data. Data integrity is maintained by employing packet acknowledgements (ACK) techniques; the ACK is contained solely in the first 6 byte header. Negative acknowledgements (NAK) are not used -if a transmitting device does not receive an ACK within a specified timeout period, the packet is retransmitted (over
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an alternative path if necessary). Acknowledgements of all good data blocks must be sent, but to ensure minimum network delay and maximum throughput of data, 'transmit-ahead' techniques are used. These allow the transmission of a predetermined number of data blocks before requiring a positive acknowledgement. One acknowledgement then implicitly acknowledges all previously sent data blocks. Satellite
There is a significant amount of satellite transmission in the network, although primary routes are always land
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lines and undersea fibre optics. Earth stations and satellite capacity are leased from authorized bodies, including Intelsat and PTTs. There are multiple wideband circuits using four satellites and four earth stations on both sides of the Atlantic to provide the data paths between the USA and Europe. Two of the satellites are provided for backup. Across the Pacific a combination of satellite and undersea cables are used to transmit data to and from the Far East, with one satellite over the Indian Ocean and one over the Pacific Ocean. Undersea cables are used, in this case, to provide
Figure I. Network topology. SW: switcher; CC: central concentrator; RD: remote device; RDM: RD multiplexer (remote device cluster (RDC) = 2 x RDMs); IMUX: intelligent multiplexer (concentrates multiple asynchronous terminals). It should be noted that RDs, RDMs and IMUXs are connected in a tree structure
computer communications
case study The NGW is connected to the PDN over an X.25 synchronous link, having multiple logical channels accessed over corresponding switched virtual circuits. The X.25 packets arriving at Gateways and PADs The network architecture the NGW over these multidescribed so far deals pri- ple logical channels from marily with hoe terminal the PDN are stripped of the users can access GElS hosts packet headers and conby connecting their termi- verted to the internal packet nals directly to GELS' remote network protocol. The access nodes either by NGW puts X.25 headers on data arriving from the interdialup or leased line. However, there are other nal network before presentaroutes into the network con- tion to the PTT PDN. Call sistent with its three-level acceptance is handled by architecture. These exten- the NGWon receipt of a sions provide facilities for Call Request packet origiusers to connect with the nating from the PDN(or network via other X.25- user) PAD. based packet data networks, In addition, X.25 virtual and also allow their termi- circuits are available, by nals (or hosts) to connect which native X.25 terminals with either GElS or non- can connect to the network. GElS hosts, where this is This enables the X.25 protolegally permitted by the col to be carried transparPTTs. ently across the network (i.e. X.25 - GElS - X.25), with the Gateways: Network capability to perform X.25 Gateways (N GW) and call-out from the RNN, as Remote Network and Nodes well as call-in. In effect, in network (RNN) are provided between PDNs and the network (see architecture terms, the Figure 2). Support is avail- NGW/RNN replaces the able for asynchronous RD/MRC in those cases originated terminals con- where access is required via nected to the DN via an a PDN rather than directly X.28 PAD. The PDN (or pri- to an MRC. In effect, any vate user)PAD parameters country with a PDN can are controlled by the NGW access the network from that PDN. using the X.29 protocol.
diversity, because there is not enough satellite capacity over the Pacific to provide all the channels that are needed.
Figure 2. Network gateway configuration. NGW: network gateway; CC: central concentrator
GElS network
X.28
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packet data
network
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User site
PTT network
X.3 PAD
asynch.
Terminal I-
Terminal PADs (X.25): PADs are sited at a user's premises to allow a number of asynchronous terminals to be multiplexed over a single high-speed X.25 synchronous link providing a similar facility to that of the IMUX. Alternatively, qualified customer PADs can be used. The X.25 data-streams will arrive at the GElS network gateways in the same form as if they had come from a PDN PAD. Host PADs: provide the capability to interface both GElS and non-GElS hosts to the network, performing conversion from the GElS internal network protocol (and vice versa) to the protocol recognized by the receiving host (e.g. IBM 3270, asynchronous). The host PAD can be located at the client's site near to his host, and it would be connected to an object CC over a leased line. Alternatively, an asynchronous host PAD could be located with an object CCand connected over a single asynchronous leased line to a client's onsite intelligent multiplexer. Remote Network Node The RNN is an interface device that connects the network (via an object CC) to a non-GElS host using the X.25 synchronous protocol, and having multiple logical channels. The client's host or communications controller is responsible for stripping off the X.25 packet headers and presenting the data to the host systems; this is in contrast to the function of the host PAD (see Figure 3). Currently, an asynchronous terminal datastream entering the network via an MRC or an NGW can
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case study be routed to a host gateway and presented to the client's host as an X.25 packet.
Networkmanagement andcontrol An NM and control software package called NMON is used across the network. It: • gathers performance and status messages from devices and processors in the network; • provides real-time display for processors and network devices; • provides historical tracking of device performance; • provides essential data for the operation, maintenance, diagnostics, planning and management of the network and central systems. It is an effective tool for maintaining reliability and availability. The real-time displays show: • exception status (gives up and down status changes for all monitored devices and interconnected links; • central concentrator performance (indicators include the number of users, characters per second processed, buffer utilization and retransmission rates); • interface processor performance (host PADs/ gateways, etc.) (performance indicators include the number of users, processor utilization percentage and number of characters per second.
used in each node, memory dumps of tables in nodes, and many other debugging capabilities. In sum, the NM and control systems can: • gather event information; • gather statistics; • record and change configuration data; • receive notification of an impending failure; • perform load measurementJbalancing.
Illustration
To illustrate how the various
components work together to enable information to be transmitted, we examine what happens when a user logs onto the network. A dialup user would dial his nearest access point: for example, a user in Stuttgart would make a local call to the MRC in the same city. The MRC will then ask the user for his user number and password. This data and all subsequent data will be combined with many other data streams being handled by that device, and multiplexed through the RDM
Figure 3. Difference between host PADs and host gateways; the RD/MRC acts as a terminal PAD, terminal protocol is converted by RD/MRC to GElS packet protocol
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In addition to NMON, a remote access control (RAC) program is used to debug an operational node (e.g. hung ports), and it also gives statistics on the performance of network nodes, which version of software is being 568
computer communications
case study located in the nearest of the 22 worldwide network service centres. This NSC is linked via several dedicated highspeed circuits to the nearest supercentre, which in this case is in Amstelveen, a suberb of Amsterdam. Once linked to the supercentre, the user's data is now in the network transmission operation centre, located in the same building as the host processors but in a separate area. Next, the user's data is examined by the CC which looks at the user's number and determines whether his files are being processed at the centre. If so, and assuming the user number is valid, the CC will pass the user onto the least loaded processor of the local computer cluster, where the password is checked and, if valid, pro-
vol 14 no 9 november 1991
cessing will begin. A virtual circuit has now been established and will remain on the same route unless a failure occurs. If this happens, an alternative route will be established. Let us assume that the CC has discovered that the user's files are in another centre, such as Cleveland (Ohio, USA). The CC then sets a path through the network switcher, also located in the network transmission operation centre (NTOS), and sends the data to the least loaded CC at Cleveland NTOC, which in turn passes the data to the least loaded processor in the Cleveland supercentre.
International integration Clearly, the data network will have a key part to play in the evolving computing
marketplace. Networks that sustain very high levels of availability, and which can offer national and international coverage can play an important role in bridging the equipment incompatibility gap. They can also offer companies a way of integrating business operations internationally.
Future developments GElS is upgrading its backbone network by replacing its current 'switchers' with Fast Packet Switchers and developing frame relay interfaces at the CC and RNN level. It is also planned to install fast packet switchers at each of its 22 remote network management centres, and to provide multiple TI/EI 1.544/2.048 Mbit/s links between them.
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