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gbn-shieldThe Granta Backbone Network (GBN) is a network of underground ducts and cables providing an advanced communications infrastructure for the University and Colleges of Cambridge, capable of lasting well into the 21st century. The £3.6M cost of installing it was shared between the University and Colleges, with contributions from the Cambridge Foundation and some non-University institutions which have direct GBN connections.

The project, completed in March 1992, involved the excavation of some 30 kilometres of trenching linking 80 University and College sites stretching from Girton College in the north-west of the city through the central and western parts of Cambridge to Addenbrooke's Hospital in the south-east. A small number of non-University sites are directly connected to the GBN; these sites have close contacts with the University. They include the Cambridge campus of Anglia Ruskin University and the MRC Cognition & Brain Sciences Unit. Other research council units located on University sites are served from the GBN cabinet located on that site, for example, the MRC Laboratory of Molecular Biology is served from the University Clinical School.

Each route section contains three or more 96 mm diameter plastic ducts carrying fibre-optic and copper cables capable of meeting currently foreseen needs and with ample room for installing more cables as required. Where GBN routes run internally through buildings, the cables are carried in trunking or on 300mm and 100mm tray. The main fibre-optic cable is a specially constructed 48-fibre cable. Particular sites, expected to generate a large volume of traffic, have additional 16-fibre cables installed directly between University Information Services and the site concerned. Since 1992, an extra 2 kilometres of duct and 5 kilometres of cable have been installed to connect more University and College sites to the GBN. Additional fibre-optic cables have also been installed on heavily used routes to expand capacity. There have been a number of University and College building programmes necessitating the re-routeing of ducts and cables to avoid the footprint of new buildings.

A termination cabinet at each site provides the point of access between the GBN and local site networks. The fibre-optic and copper cables are installed site to site; splicing of fibres at individual cabinets allows circuits to be added or removed as the need arises and gives a high degree of flexibility in implementing different types of network topology. Connections into the GBN are made only at these cabinets; it is not possible to connect by breaking into ducts or routeing chambers between nodes. Circuits, made up of sections of fibre spliced together, can be rented to network service providers or to individual institutions.

End-user services are not provided directly by the GBN itself but by existing service providers, for example, University Information Services for data network services, the University Telephone Network for voice services. Current uses of the GBN infrastructure include private links for multi-site Departments and Colleges; communications research by the Computer Laboratory and Department of Engineering; security services such as alarms and CCTV surveillance; and for the various requirements of the Cambridge University Data Network (CUDN) - electronic mail, file transfer, remote computer access, access to national and international networks and so on.

The network is managed by the GBN Management Committee which comprises both University and College representatives. The day-to-day administration of the network is undertaken on behalf of the Management Committee by University Information Services.1

Formally, the GBN is owned by the University on behalf of all the participating bodies. As a result, where GBN plant crosses non-University land there is a wayleave agreement2 between the individual landowner concerned and the University. Amongst other terms and conditions in the wayleave agreement between the University and individual Colleges, a College's repositioning of any element of the network within College land should be carried out by the University with costs for doing so borne by the College. The University is also given the right to enter on College land on reasonable notice except in the case of emergency for the purpose of repairing, maintaining, renewing and relaying the parts of the network.

The onus is on the landowner, whether University, a College or other, to ensure that the GBN infrastructure is not damaged, either by day-to-day activities or by building works. Therefore, when an activity is likely to conflict with the GBN, the landowner must contact the UIS to negotiate a resolution of the conflict; Information Services will liaise with EMBS on relevant civil engineering matters. Landowners contemplating sale or lease of land which is subject to a Granta wayleave must draw the attention of the purchaser to the wayleave and its terms. If the sale or lease is to a third party (outside the University and Colleges), the landowner must consult Information Services first, and it may be necessary to arrange for diversion of the Granta ducts and cables, at the landowner's expense, before the sale or lease.

1. All matters concerning the GBN, be it requests for dark-fibre circuits, extending the GBN to serve other buildings or new building works conflicting with the GBN, should be directed in the first instance to the Granta Backbone Network Manager, .

2. Colleges will have signed copies of their executed wayleave agreements. At the completion of the original project maps were sent to each College indicating where GBN plant crosses land owned by that College. Copies of the terms and conditions in the wayleave and replacement maps may be obtained from the Granta Backbone Network Manager.

GBN Statistics


1986 Initial studies
1987 University and Colleges Joint Working Party
1990 Tender awarded to STC (now Pirelli Cables Ltd), implementation started
1992 Project completed



Total £3.6 million
University 49%
Colleges 39%
Cambridge Foundation 10%
Others 2%


  1992 2003  
Nodes 80 90  
Total route length 30 km 32 km  
Highway crossings 65 65  


Fibre optic 1992:56 km (16-fibre and 48-fibre cables)
  2003:68 km (8-, 16-, 24- and 48-fibre cables)


The 48-fibre cable contains:
16 62.5/125 micrometre, dual-window (850 and 1300 nm) fibres
8 50/125 micrometre, dual-window (850 and 1300 nm) fibres
24 8/125 micrometre, dual-window (1310 and 1550 nm) fibres

Of the original 16-fibre cables, one contains only 50/125 micrometre, dual-window (850 and 1300nm) fibres; the others contain only 62.5/125 micrometre, dual-window (850 and 1300nm) fibres. The additional 8-, 16- and 24-fibre cables contain either 62.5/125 micrometre or 8/125 micrometre fibres.

Last updated: 17 October 2016

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