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`First edition 1997
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`© 1997 Nick Burd
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`Printed in Great Britain by Cambridge University Press
`ISBN 0 412 49730 1
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`U interface standards
`
`123
`
`terms ofthe opelraation and performance ofthe ISDN U transmission systems used
`-
`rica and uroP°-
`.
`in fimzmerica,
`the NT1 is defined as customer premises equipment which is
`purchased and maintained by the user. The U interface may therefore be a user-
`network interface, and as such required standardization at an early stage inthe
`deployment of ISDN to ensure equipment compatibility. The result is that ANSI
`has led the way in the standardization of the U interface through the ANSI T1.601
`Standard [6] which defines the use of a 2BlQ transmission system.
`In Europe, the NT1 belongs to the network operator, and is installed at the time
`of subscription and subsequently maintained by the network operator. The user
`therefore never has direct access to the U interface, and instead connects to the
`ISDN through the S/T user—network interface.
`European ISDNs use both the 2BlQ or 4B3T line code. Through ETSI, a
`technical recommendation ETR 080 [7] has been defined which covers the
`licensed use of both of
`these. The ETSI document exists only as
`a
`recommendation which may be adopted by European network operators, and is
`not an enforced standard in order to respect specific requirements that may exist
`in the European national networks which fall outside this recommendation. For
`example, the test loops and conditions which are defined for conformance testing
`of the U transceiver in different countries may be different in order to use
`reference loops which more closely represent the population of subscriber loops
`that exist within a network than do the test loops and conditions defined in the
`ETSI recommendation. (A further ETSI standard, prETS 300 297 [8], has also
`been generated for the digital section which is equivalent to G.960.)
`Three main differences between the ETSI and ANSI U transmission systems
`are their performance testing specifications, power supply configurations, and test
`and maintenance functions.
`
`4-51 Performance testing
`
`The U transmission ‘standards and recommendations include test arrangements
`and procedures which are intended to be used to qualify the performance of LT
`and NT1 equipment for use in ISDN BRA subscriber loops. According to the
`requirements of
`the networks
`represented by the relevant
`standard or
`r§C0mmendation, a number of test subscriber loop cables are defined together with
`Sllnulated noise sources whose signals are injected onto the cable to which the
`equipment under test is connected. After activation, a pseudo-random binary
`Sequence (PRBS) test signal is used to simulate user information in the 2B+D
`Channels and the bit error rate (BER) is measured by comparing the transmitted
`and received data streams.
`loops which are
`The ANSI T1.601 standard specifies 12 mandatory test
`representative of the subscriber loops found in North America. A further three
`
`
`
`

`
`
`
`124 The U interface
`
`ases which are not so frequently found.
`optional test loops cover more demanding c
`ths of different types of cable with a
`The test loops are made up of different leng
`The test loops may also include up to
`total length of up to 18000 feet (5.5 km).
`f the ANSI loops is shown in Fig.
`three bridge taps. As an example, loop 14 0
`4.1221.
`
`500’
`
`LT
`
`22 AWG
`
`
`24 AWG
`
`
`24 AWG
`
`AWG=American Wire Gauge
`
`(8)
`
`
`
`BER tester
`
`
`
`PRBS
`receive
`
`noise source
`
`
`
`gainl
`attenuation
`
`
`
`‘ransmn
`
`transceiver
`
`‘
`
`receiver
`
`l
`
`
`
`test loop cable
`
`
`
`
`Fig. 4.12 The U transmission performance test system. (a) An example test 1
`(ANSI loop 14); (b). a typical test system.