Primary Rate ISDN extension over fibre with the FRM220-E1/T1

The FRM220-E1/T1 can be used whenever there's a need to transport E1 or T1 services over fibre optic cabling. The G.703 interface on the FRM220-E1/T1 can be ordered with either a pair of BNC connectors (75Ohm variant) or with an RJ45 interface (120Ohm variant).  In the UK the FRM220-E1R is commonly used to extend British Telecom's Primary rate ISDN offering known as ISDN30. This is usually presented as an RJ45 interface on their NTE.

The following diagram shows how the FRM220-E1R can be configured to extend an ISDN30 primary rate service. The FRM220-E1/T1 can be configured via onboard DIP switches when deployed as a standalone unit. When the FRM220-E1/T1 is used in conjunction with the network management card (FRM220-NMC) then all configuration can be done via HTTP, Telnet or SNMP and the DIP positions are disregarded.

FRM220-E1/T1 - Primary rate ISDN (ISDN30) extension over fibre
FRM220-E1R - Primary rate ISDN (ISDN30) extension over fibre

Primary rate ISDN is a channelised service carried, in Europe, on an 2.048Mbps E1 service. E1 services are synchronous in nature and therefore are locked to a clock. In this extension scenario the clock is always external to the FRM220-E1R's and delivered as part of the primary rate ISDN from the carrier.

The configuration of each FRM220-E1R must take this into account.  The FR220-E1R closest to the 'network' or carriers ISDN presentation is configured to derive its clock from incoming information that will arrive on its E1 RJ45 interface (SW#1, DIP7 Off, DIP8 On). The FRM220-E1R closest to the customer equipment is configured to derive its clock from incoming information that will arrive on its fibre optic interface (SW#1, DIP7 Off, DIP8 Off).

By following the direction of the clock, from carrier to end equipment, it's possible to see that each FRM220-E1R has been configured to extract clocking information from its respective interface that lies closest to the prevailing clock direction. The remaining DIPs on both FRM220-E1R's can be configured in the 'off' position.