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Operating and managing a commercial worldwide network
Operating and managing a commercial worldwide network
International networks playa major part in business communications, but the demands placed on them are high. Terry Reed describes the configuration of one such network, and discusses how it meets that challenge. Large international data networks need to be reliable and flexible. The factors involved in achieving this are demonstrated by describing how Geisco has configured its global teleprocessing network. The design criteria used are explained, and the network architecture and internal protocol are described. The method employed to manage and control the network is outlined, and an example is given showing how the different components work together. Keywords: computer networks, telecommunication links, teleprocessing, case study
Geisco Ltd, 2 Manor Gate Road, Kingstonupon-Thames, Surrey KT1 1LN, UK
The new challenge for data networking professionals, as for their colleagues in other areas, is to integrate new technology and services with existing operations. Demand is enormous, with more and more users requiring transparent network access for their minicomputers, microcomputers and word processors. Until all ISO's telecommunications standards are implemented and accepted, the need for the protocol-independent transparent network will grow. Indeed, it also has a long-term future, since it is unrealistic to assume that companies would scrap investments in hardware simply to replace them with universally standard equipment. In this environment, the data network must be capable, through the use of the latest technologies, of achieving greater throughput from its communications lines connected to host computers and must be able to interconnect with public data networks (PDNs). In the operation and management of a commercial worldwide data network, these challenges have to be met through simultaneous upgrades to ensure 24 h a day availability. This paper describes how Geisco has configured its global teleprocessing network to meet the needs of a new generation of more demanding and computer literate end users.
Technical to business applications The Geisco network, established in the early 1970s, initially connected asynchronous terminals worldwide to a host in the USA on a traditional timesharing basis. Its purpose was to allow technically oriented users to develop and run programs and databases for scientific, technical and business computing applications. The network has evolved over the last 15 years to incorporate the latest communications technology (including satellites, X.25 gateways and other developments), to support a number of synchronous terminal protocols and to access three large Geisco computer centres, two in the USA and one in the Netherlands. In addition, users' hosts can be linked to the Geisco hosts where it is logical for an application to be shared. The pattern of usage has changed too, with a trend away from technical applications to multilocation business systems linking geographically spread functional groups in order to perform common transactions. These currently include order processing within manufacturing companies, and between manufacturers, suppliers and distributors, online cashbalance reporting and funds transfer instructions between corporations and their banks and
0140-3664/85/030141-07 $03.00 © 1985 Butterworth & Co (Publishers) Ltd vol 8 no 3 june 1985
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international financial reporting. The network is also used to provide one of the world's most extensive international electronic mail services. Local access points are provided in 750 cities covering over 30 countries. This coverage is extended to over 50 countries by using Geisco gateways to the many national public PDNs. To satisfy a 6 000 strong client base, it is necessary to provide reliability, fast response, error free transmission, data security, data integrity and local support in addition to wide geographical coverage (see Table 1).
Design criteria In designing the network, certain key objectives were set in order to provide a reliable and responsive commercial service. These might equally apply to any data network, whether it serves the needs of one company or many: • fast response (less than I s delay across the network), • rated throughput to end-user devices, • minimum cost in hardware, modems, lines, etc., • capacity to expand at any level in the network to accommodate growth simply by plugging in additional capacity with minimum interruption to the users, • extensive network management facilities for giving 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.
Reliability, availability and serviceability The resilience of the network is high, providing an overall
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Table 1. Network statistics Network covers: • over 30 countries (up to 50 including access via PDNs) • on five continents • across 23 time zones • in 750 cities Over 6 000 companies network worldwide
use the
Access from 90% of the Free World via local telephone call Network availability -- 99.8% 800 000 km of circuits Satellite and mission
microwave
trans-
• two satellites transPacific • three satellites transatlantic 22 manned network service centres 7 500 access ports Over 1 000 data communication processors deployed Three interconnected supercentres (50 host processors, IBM/ Honeywell) 400M characters currently transmitted during peak hour (2000M capacity) 4 000 concurrent users during peak hours 3 500M characters transmitted per day 875 000 user sessions per week Interconnection to public packet data networks and the telex network Support for terminals running at speeds from 50 baud to 19.2 kbit/s Support for 2780, 3780, 3270, TTY and X.25 protocols Operational features: • • • • •
diversified transmission links distributed control automatic circuit recovery online network monitor downline load software distribution • reconfiguration flexibility
availability of 99.8%. This is achieved by incorporating several levels of redundancy through a combination of mesh, star and tree topology structures together with multiple interconnected intelligent switching processors. These are located at three separate geographical locations, known as switching centres, that are manned 24 h/day, 365 days/year. Each pair of switching processors is only 50% loaded, so that if one processor fails completely, the warm standby processor can automatically pick up the total network load for both processors. Redundancy is also provided in the distributed access nodes and in the multiplication of the communication links, which are made through 800 000 km of landlines and five satellite links. All software enhancements are downline loaded from a central point to maintain current software level compatibility throughout the entire network, and all changes are controlled and tested by a quality assurance group. Around the world, there are currently 22 manned network service centres, with nearly 200 people working split shifts, involved in network monitoring, fault diagnosis and correction and maintenance activities. Comprehensive network management ensures reliability of the network. This includes proactive monitoring with the gathering and display of information on both degraded and failed components and facilities on a realtime basis. The statistics, which are monitored constantly, provide operations personnel with the data needed to identify and isolate problems quickly -- from the access nodes to the systems level. The network management software also produces a number of reports on system performance, including information on system throughput, device downtime, device availability, nodal bandwidth, utilization and other service activity data. The network provides for alternative paths and diverse routing. Each network component is expected to handle peak loads. When a device is overloaded, the network
computer communications
automatically reroutes traffic to the alternative device. At regular, frequent intervals, network analysts examine the load on each device in the network and then reconfigure the network and the paths in order to balance the load over the devices without disrupting the service. In summary, the network provides distributed control, automatic reconfiguration and automatic circuit and network processor recovery.
Terminal protocols and data integrity The network supports a number of communications protocols, allowing a wide variety of devices to be connected. The communications protocols supported include: 3270-BSC, 3780, 2780, Telex, asynchronous TTY and X.25. IBM's 3270/SDLC is to be supported during 1985. As well as handling a wide variety of 'dumb' terminals, the network supports intelligent devices including the IBM Personal Computer and other microcomputers. Microcomputer software is available to provide intelligent integration 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 customer's premises and the network access points to eliminate line errors. For synchronous traffic, normal cyclic redundany check and block retrans-
vol 8 no 3 june 1985
mission protocols are implemented in the network access points.
Network architecture General concept Geisco three-level architecture employs hierarchical packet switching and is optimized for cost performance. • Remote user access nodes, known as mini remote concentrators (MRCs) or remote devices (RDs), are connected in a tree structure to central nodes, known as central concentrators (CCs). Additionally, the traffic from a number of MRCs/RDs may be multiplexed through remote device multiplexers (RDMs) before being passed onwards to a CC over highbandwidth links. • Central concentrators (CCs) are connected in a star topology to switchers (SWs). Fach CC will be connected to two SWs. • Switchers are interconnected in a mesh. The network devices run different software depending on their 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 users'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 assem bly/disassem bly). The RDs support access for terminals running at speeds ranging from 110 bit/s to 19.2 kbit/s and communication protocols such as IBM 3270-BSC, 3780, 2780, Telex, asynchronous TTY and X.25.
Central concentrator nodes Central concentrators (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 ultilization of every CC. Information between CCs is updated periodically, during steady state and when host processors come online.
Switching nodes Switchers provide dynamic, best path routing on a per packet basis between CCs, and warm standby switchers are provided at each switching centre site. By using multiple switching centres, the following benefits are realized: • substantial growth (additional connectivity), for example, up to 192 CCs can be accommodated, • a network transmission capability of up to 2 O00M char/h, • increased reliability-- alternate paths through the central network, • greater flexibility and configurability in the network topology.
Internal network protocol General concept A packet switching mechanism is used for delivering data over the least congested routes. The remote devices (MRCs, network gateways,
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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.
host gateways and host PADs) convert external communications protocols to the internal network protocol. Basically, the structure of the packets is straightforward: • each packet contains a message from one user, • packets are composed of a 9 byte header, including a sequence number and routing inormation (destination and source), followed by up to 1 008 byte of user data.
Satellites
There is a significant amount of satellite transmission in the network, and most of the intercontinental traffic is transmitted this way. Earth stations and satellite capacity are leased from authorized bodies, including Intelsat and PTTs. In recent years, satellite transmission has been preferred to undersea cables because:
Data integrity is maintained by employing packet acknowledgement (ACK) techniques; the ACK is cotained solely in the first 6 byte of the header. Negative acknowledgements (NAKs) are not used. If a transmitting device does not receive an ACK within a specified timeout period, the packet is retransmitted (over an alternative path if necessary). Acknowledgements of all good data blocks must be sent, but, to ensure minimum network delay
.
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.
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.
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• it has provided outstanding quality of service, • it provides higher reliability than undersea cables -- satellitecaused network interrupts have been rare,
.
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• bandwidth capacity is extremely large, • it is the lowest cost option available for intercontinental communication. Today, all circuits between the USA and Europe are satellite circuits, as are the circuits between the USA and Saudi Arabia, Puerto Rico and Venezuela. Across the Pacific, all circuits to Hong Kong and Korea and more than half the circuits to Japan, Australia and Singapore are via satellite. There are wideband circuits using three satellites and three earth stations on both sides of the Atlantic to provide the data paths between the USA and Europe (approximately 200kbit/s bandwidth). 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
Ohio I- --centre I
Switcher
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I Switcher
-I I,
1
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Figure 7. Network topology; SW ----switcher, CC = central concentrator, RD ----remote device, RDM = RD multiplexer (remote device cluster ( R D C ) = 2 ×RDMs), IMUX=intelligent multiplexer (concentrates multiple asynchronous terminals) (note" RDs, RDMs and IMUXs are connected in a tree structure)
144
computer communications
Pacific Ocean. Undersea cables are used, in this case, to provide diversity, because there is not enough satellite capacity over the Pacific to provide all the channels that are needed. Gateways
and PADs
General
The network architecture described so far deals primarily with how terminal users can access Geisco hosts by connecting their terminals directly to Geisco's remote access nodes either by dialup or leased line. There are, however, other routes into the network consistent with its three-level architecture. These extensions provide facilities for users to connect with the network via other X.25-based packet data networks and also allow their terminals (or hosts) to connect with either Geisco or nonGeisco hosts, where this is legally permitted by the PTTs. Network gateways
Network gateways (NGWs) are provided between PDNs and the network (see Figure 2). Current support is for asynchronous originated terminals connected to the PDN via an X.28 PAD. The PDN (or private user) PAD parameters are controlled by the NGW using the X.29 protocol. 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 the NGW over these multiple logical channels from the PDN are stripped of the packet headers and converted to the internal packet network protocol. The NGW puts X.25 headers on data arriving from the internal network before presentation to the PTT PDN. Call acceptance is handled by the NGW on receipt of a Call Request packet originating from the PDN (or user) PAD.
vol 8 no 3 june 1985
An X.25 virtual circuit is being developed, by which native X.25 terminals will be able to connect to the network. This will enable the X.25 protocol to be carried transparently across the network (i.e. X.25 -- Geisco-- X.25), with the capability to perform X.25 call-out from the NGW as well as call-in. In effect, in network-architecture terms, the NGW replaces the RD/ MRC in those cases where access is required via a PDN rather than directly to an MRC. Currently, network gateways are deployed in Canada (Datapac), France (Transpac), the FRG (Datex-P), Australia (Austpac) and the UK (PSS), while Datapak (Denmark) and DN-I (Holland) are under test at the moment. In effect, any other country with a PDN can access the network from that PDN, providing that country has an X.75 connection (either direct or indirect via two or more countries) to one of those countries listed above.
network gateways in the same form as if they had come from a PDN PAD.
Host P A D s
Host PADs provide the capability to interface both Geisco and nonGeisco hosts to the network, performing conversion from Geisco's internal network protocol (and vice versa) to the protocol recognized by the receiving host (e.g. IBM 3270, asynchronous). The host PAD is not required between Geisco's Honeywell hosts and the network, but is required between the network and Geisco's IBM hosts or clients' hosts. The host PAD can be located on the client's site near to his host and would be connected to an object CC over a leased line. Alternatively, an asynchronous host PAD could be located with an object CC and connected over a single asynchronous leased line to a client's onsite intelligent multiplexer.
Terminal pads (X.25) Host g a t e w a y
PADs are sited on users' 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 described earlier. Alternatively, qualified customer PADs can be used. The X.25 datastreams will arrive at the Geisco
Geisco network
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The host gateway is an interface device that connects the network (via an object CC) to a nonGeisco host using the X.25 synchronous protocol and having multiple logical channels. The client's host or communications controller is responsible for
PTT network
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Figure 2. P D N - - a network gateway configuration; gateway, CC = central concentrator
NGW =network
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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 asynhcronous terminal datastream entering the network via an MRC or an NCW can be routed to a host gateway and presented to the client's host as X.25 packets.
Network management and control A powerful network monitoring and control software package called NMON is used across the network. It does the following: • gathers performance and status messages from devices and processors in the network, • pr.ovides real-time displays 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 realtime displays show the following: • 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 processed). 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 performance of network nodes, which version of software is being used in each node, memory dumps of tables in
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nodes and many other debugging capabilities. In summary, network management and control systems can: • gather event information, • gather statistics, • record and change configuration data, • receive notification of an impending failure, • perform load measurement/ balancing.
Illustration To illustrate how the various components work together to enable information to be transmitted, examine what happens when a user logs on to 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 datastreams being handled by that device and multiplexed through the RDM located in the nearest of the 22 worldwide network service centres. This NSC is linked via several dedicated high-speed circuits to the nearest supercentre, which in this case is in Amstelveen, a suburb of Amsterdam. Incidentally, if the user happened to be located in the same city as the supercentre, then the RC would link him directly. 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 central concen-
ASYNC HOST PAD
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HOST GATEWAY
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Async terminals
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 Geisco packet protocol
computer communications
trator, which looks at his user number and determines whether his files are being processed at the centre. If so, and assurningthe user number is valid, the CC will pass the user on to the least loaded processor of the local computer cluster, where the password is checked and, if valid, processing 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 (USA). The CC then sets a path through the network switcher, also located in the network transmission operation centre (NTOC), and sends the data to the least loaded CC at the 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.
iii= Whatever happened to the paperless office? The office of the future is a vision sought by many manufacturers, yet successful users are rare. A special issue of Data Processing, the international journal for senior computing professionals, examines these questions and takes a look at how the office is finally being automated For further details and an order form, contact
Data Processing, Butterworth Scientific Ltd, PO Box 63, Westbury House, Bury St, Guildford, Surrey GU2 5BH. Tel: (0483) 31261. Telex: 859556 SCITEC G
data process vol 8 no 3 june 1985
147
International networks playa major part in business communications, but the demands placed on them are high. Terry Reed describes the configuration of one such network, and discusses how it meets that challenge. Large international data networks need to be reliable and flexible. The factors involved in achieving this are demonstrated by describing how Geisco has configured its global teleprocessing network. The design criteria used are explained, and the network architecture and internal protocol are described. The method employed to manage and control the network is outlined, and an example is given showing how the different components work together. Keywords: computer networks, telecommunication links, teleprocessing, case study
Geisco Ltd, 2 Manor Gate Road, Kingstonupon-Thames, Surrey KT1 1LN, UK
The new challenge for data networking professionals, as for their colleagues in other areas, is to integrate new technology and services with existing operations. Demand is enormous, with more and more users requiring transparent network access for their minicomputers, microcomputers and word processors. Until all ISO's telecommunications standards are implemented and accepted, the need for the protocol-independent transparent network will grow. Indeed, it also has a long-term future, since it is unrealistic to assume that companies would scrap investments in hardware simply to replace them with universally standard equipment. In this environment, the data network must be capable, through the use of the latest technologies, of achieving greater throughput from its communications lines connected to host computers and must be able to interconnect with public data networks (PDNs). In the operation and management of a commercial worldwide data network, these challenges have to be met through simultaneous upgrades to ensure 24 h a day availability. This paper describes how Geisco has configured its global teleprocessing network to meet the needs of a new generation of more demanding and computer literate end users.
Technical to business applications The Geisco network, established in the early 1970s, initially connected asynchronous terminals worldwide to a host in the USA on a traditional timesharing basis. Its purpose was to allow technically oriented users to develop and run programs and databases for scientific, technical and business computing applications. The network has evolved over the last 15 years to incorporate the latest communications technology (including satellites, X.25 gateways and other developments), to support a number of synchronous terminal protocols and to access three large Geisco computer centres, two in the USA and one in the Netherlands. In addition, users' hosts can be linked to the Geisco hosts where it is logical for an application to be shared. The pattern of usage has changed too, with a trend away from technical applications to multilocation business systems linking geographically spread functional groups in order to perform common transactions. These currently include order processing within manufacturing companies, and between manufacturers, suppliers and distributors, online cashbalance reporting and funds transfer instructions between corporations and their banks and
0140-3664/85/030141-07 $03.00 © 1985 Butterworth & Co (Publishers) Ltd vol 8 no 3 june 1985
141
international financial reporting. The network is also used to provide one of the world's most extensive international electronic mail services. Local access points are provided in 750 cities covering over 30 countries. This coverage is extended to over 50 countries by using Geisco gateways to the many national public PDNs. To satisfy a 6 000 strong client base, it is necessary to provide reliability, fast response, error free transmission, data security, data integrity and local support in addition to wide geographical coverage (see Table 1).
Design criteria In designing the network, certain key objectives were set in order to provide a reliable and responsive commercial service. These might equally apply to any data network, whether it serves the needs of one company or many: • fast response (less than I s delay across the network), • rated throughput to end-user devices, • minimum cost in hardware, modems, lines, etc., • capacity to expand at any level in the network to accommodate growth simply by plugging in additional capacity with minimum interruption to the users, • extensive network management facilities for giving 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.
Reliability, availability and serviceability The resilience of the network is high, providing an overall
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Table 1. Network statistics Network covers: • over 30 countries (up to 50 including access via PDNs) • on five continents • across 23 time zones • in 750 cities Over 6 000 companies network worldwide
use the
Access from 90% of the Free World via local telephone call Network availability -- 99.8% 800 000 km of circuits Satellite and mission
microwave
trans-
• two satellites transPacific • three satellites transatlantic 22 manned network service centres 7 500 access ports Over 1 000 data communication processors deployed Three interconnected supercentres (50 host processors, IBM/ Honeywell) 400M characters currently transmitted during peak hour (2000M capacity) 4 000 concurrent users during peak hours 3 500M characters transmitted per day 875 000 user sessions per week Interconnection to public packet data networks and the telex network Support for terminals running at speeds from 50 baud to 19.2 kbit/s Support for 2780, 3780, 3270, TTY and X.25 protocols Operational features: • • • • •
diversified transmission links distributed control automatic circuit recovery online network monitor downline load software distribution • reconfiguration flexibility
availability of 99.8%. This is achieved by incorporating several levels of redundancy through a combination of mesh, star and tree topology structures together with multiple interconnected intelligent switching processors. These are located at three separate geographical locations, known as switching centres, that are manned 24 h/day, 365 days/year. Each pair of switching processors is only 50% loaded, so that if one processor fails completely, the warm standby processor can automatically pick up the total network load for both processors. Redundancy is also provided in the distributed access nodes and in the multiplication of the communication links, which are made through 800 000 km of landlines and five satellite links. All software enhancements are downline loaded from a central point to maintain current software level compatibility throughout the entire network, and all changes are controlled and tested by a quality assurance group. Around the world, there are currently 22 manned network service centres, with nearly 200 people working split shifts, involved in network monitoring, fault diagnosis and correction and maintenance activities. Comprehensive network management ensures reliability of the network. This includes proactive monitoring with the gathering and display of information on both degraded and failed components and facilities on a realtime basis. The statistics, which are monitored constantly, provide operations personnel with the data needed to identify and isolate problems quickly -- from the access nodes to the systems level. The network management software also produces a number of reports on system performance, including information on system throughput, device downtime, device availability, nodal bandwidth, utilization and other service activity data. The network provides for alternative paths and diverse routing. Each network component is expected to handle peak loads. When a device is overloaded, the network
computer communications
automatically reroutes traffic to the alternative device. At regular, frequent intervals, network analysts examine the load on each device in the network and then reconfigure the network and the paths in order to balance the load over the devices without disrupting the service. In summary, the network provides distributed control, automatic reconfiguration and automatic circuit and network processor recovery.
Terminal protocols and data integrity The network supports a number of communications protocols, allowing a wide variety of devices to be connected. The communications protocols supported include: 3270-BSC, 3780, 2780, Telex, asynchronous TTY and X.25. IBM's 3270/SDLC is to be supported during 1985. As well as handling a wide variety of 'dumb' terminals, the network supports intelligent devices including the IBM Personal Computer and other microcomputers. Microcomputer software is available to provide intelligent integration 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 customer's premises and the network access points to eliminate line errors. For synchronous traffic, normal cyclic redundany check and block retrans-
vol 8 no 3 june 1985
mission protocols are implemented in the network access points.
Network architecture General concept Geisco three-level architecture employs hierarchical packet switching and is optimized for cost performance. • Remote user access nodes, known as mini remote concentrators (MRCs) or remote devices (RDs), are connected in a tree structure to central nodes, known as central concentrators (CCs). Additionally, the traffic from a number of MRCs/RDs may be multiplexed through remote device multiplexers (RDMs) before being passed onwards to a CC over highbandwidth links. • Central concentrators (CCs) are connected in a star topology to switchers (SWs). Fach CC will be connected to two SWs. • Switchers are interconnected in a mesh. The network devices run different software depending on their 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 users'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 assem bly/disassem bly). The RDs support access for terminals running at speeds ranging from 110 bit/s to 19.2 kbit/s and communication protocols such as IBM 3270-BSC, 3780, 2780, Telex, asynchronous TTY and X.25.
Central concentrator nodes Central concentrators (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 ultilization of every CC. Information between CCs is updated periodically, during steady state and when host processors come online.
Switching nodes Switchers provide dynamic, best path routing on a per packet basis between CCs, and warm standby switchers are provided at each switching centre site. By using multiple switching centres, the following benefits are realized: • substantial growth (additional connectivity), for example, up to 192 CCs can be accommodated, • a network transmission capability of up to 2 O00M char/h, • increased reliability-- alternate paths through the central network, • greater flexibility and configurability in the network topology.
Internal network protocol General concept A packet switching mechanism is used for delivering data over the least congested routes. The remote devices (MRCs, network gateways,
143
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.
host gateways and host PADs) convert external communications protocols to the internal network protocol. Basically, the structure of the packets is straightforward: • each packet contains a message from one user, • packets are composed of a 9 byte header, including a sequence number and routing inormation (destination and source), followed by up to 1 008 byte of user data.
Satellites
There is a significant amount of satellite transmission in the network, and most of the intercontinental traffic is transmitted this way. Earth stations and satellite capacity are leased from authorized bodies, including Intelsat and PTTs. In recent years, satellite transmission has been preferred to undersea cables because:
Data integrity is maintained by employing packet acknowledgement (ACK) techniques; the ACK is cotained solely in the first 6 byte of the header. Negative acknowledgements (NAKs) are not used. If a transmitting device does not receive an ACK within a specified timeout period, the packet is retransmitted (over an alternative path if necessary). Acknowledgements of all good data blocks must be sent, but, to ensure minimum network delay
.
I I
.
.
I
.
.
• it has provided outstanding quality of service, • it provides higher reliability than undersea cables -- satellitecaused network interrupts have been rare,
.
Maryland centre - I
-
• bandwidth capacity is extremely large, • it is the lowest cost option available for intercontinental communication. Today, all circuits between the USA and Europe are satellite circuits, as are the circuits between the USA and Saudi Arabia, Puerto Rico and Venezuela. Across the Pacific, all circuits to Hong Kong and Korea and more than half the circuits to Japan, Australia and Singapore are via satellite. There are wideband circuits using three satellites and three earth stations on both sides of the Atlantic to provide the data paths between the USA and Europe (approximately 200kbit/s bandwidth). 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
Ohio I- --centre I
Switcher
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I Switcher
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Figure 7. Network topology; SW ----switcher, CC = central concentrator, RD ----remote device, RDM = RD multiplexer (remote device cluster ( R D C ) = 2 ×RDMs), IMUX=intelligent multiplexer (concentrates multiple asynchronous terminals) (note" RDs, RDMs and IMUXs are connected in a tree structure)
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computer communications
Pacific Ocean. Undersea cables are used, in this case, to provide diversity, because there is not enough satellite capacity over the Pacific to provide all the channels that are needed. Gateways
and PADs
General
The network architecture described so far deals primarily with how terminal users can access Geisco hosts by connecting their terminals directly to Geisco's remote access nodes either by dialup or leased line. There are, however, other routes into the network consistent with its three-level architecture. These extensions provide facilities for users to connect with the network via other X.25-based packet data networks and also allow their terminals (or hosts) to connect with either Geisco or nonGeisco hosts, where this is legally permitted by the PTTs. Network gateways
Network gateways (NGWs) are provided between PDNs and the network (see Figure 2). Current support is for asynchronous originated terminals connected to the PDN via an X.28 PAD. The PDN (or private user) PAD parameters are controlled by the NGW using the X.29 protocol. 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 the NGW over these multiple logical channels from the PDN are stripped of the packet headers and converted to the internal packet network protocol. The NGW puts X.25 headers on data arriving from the internal network before presentation to the PTT PDN. Call acceptance is handled by the NGW on receipt of a Call Request packet originating from the PDN (or user) PAD.
vol 8 no 3 june 1985
An X.25 virtual circuit is being developed, by which native X.25 terminals will be able to connect to the network. This will enable the X.25 protocol to be carried transparently across the network (i.e. X.25 -- Geisco-- X.25), with the capability to perform X.25 call-out from the NGW as well as call-in. In effect, in network-architecture terms, the NGW replaces the RD/ MRC in those cases where access is required via a PDN rather than directly to an MRC. Currently, network gateways are deployed in Canada (Datapac), France (Transpac), the FRG (Datex-P), Australia (Austpac) and the UK (PSS), while Datapak (Denmark) and DN-I (Holland) are under test at the moment. In effect, any other country with a PDN can access the network from that PDN, providing that country has an X.75 connection (either direct or indirect via two or more countries) to one of those countries listed above.
network gateways in the same form as if they had come from a PDN PAD.
Host P A D s
Host PADs provide the capability to interface both Geisco and nonGeisco hosts to the network, performing conversion from Geisco's internal network protocol (and vice versa) to the protocol recognized by the receiving host (e.g. IBM 3270, asynchronous). The host PAD is not required between Geisco's Honeywell hosts and the network, but is required between the network and Geisco's IBM hosts or clients' hosts. The host PAD can be located on the client's site near to his host and would be connected to an object CC over a leased line. Alternatively, an asynchronous host PAD could be located with an object CC and connected over a single asynchronous leased line to a client's onsite intelligent multiplexer.
Terminal pads (X.25) Host g a t e w a y
PADs are sited on users' 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 described earlier. Alternatively, qualified customer PADs can be used. The X.25 datastreams will arrive at the Geisco
Geisco network
I I I I I I
I I I I I
The host gateway is an interface device that connects the network (via an object CC) to a nonGeisco host using the X.25 synchronous protocol and having multiple logical channels. The client's host or communications controller is responsible for
PTT network
....
I Public II X.3 J packet l datanetwork I PAD J
X.25
I
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Figure 2. P D N - - a network gateway configuration; gateway, CC = central concentrator
NGW =network
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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 asynhcronous terminal datastream entering the network via an MRC or an NCW can be routed to a host gateway and presented to the client's host as X.25 packets.
Network management and control A powerful network monitoring and control software package called NMON is used across the network. It does the following: • gathers performance and status messages from devices and processors in the network, • pr.ovides real-time displays 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 realtime displays show the following: • 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 processed). 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 performance of network nodes, which version of software is being used in each node, memory dumps of tables in
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nodes and many other debugging capabilities. In summary, network management and control systems can: • gather event information, • gather statistics, • record and change configuration data, • receive notification of an impending failure, • perform load measurement/ balancing.
Illustration To illustrate how the various components work together to enable information to be transmitted, examine what happens when a user logs on to 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 datastreams being handled by that device and multiplexed through the RDM located in the nearest of the 22 worldwide network service centres. This NSC is linked via several dedicated high-speed circuits to the nearest supercentre, which in this case is in Amstelveen, a suburb of Amsterdam. Incidentally, if the user happened to be located in the same city as the supercentre, then the RC would link him directly. 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 central concen-
ASYNC HOST PAD
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3270
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Async terminals
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 Geisco packet protocol
computer communications
trator, which looks at his user number and determines whether his files are being processed at the centre. If so, and assurningthe user number is valid, the CC will pass the user on to the least loaded processor of the local computer cluster, where the password is checked and, if valid, processing 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 (USA). The CC then sets a path through the network switcher, also located in the network transmission operation centre (NTOC), and sends the data to the least loaded CC at the 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.
iii= Whatever happened to the paperless office? The office of the future is a vision sought by many manufacturers, yet successful users are rare. A special issue of Data Processing, the international journal for senior computing professionals, examines these questions and takes a look at how the office is finally being automated For further details and an order form, contact
Data Processing, Butterworth Scientific Ltd, PO Box 63, Westbury House, Bury St, Guildford, Surrey GU2 5BH. Tel: (0483) 31261. Telex: 859556 SCITEC G
data process vol 8 no 3 june 1985
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