BT Highway Tutorial

BT launched the Home Highway and Business Highway services, collectively known as BT Highway, in the UK in September 1998. They ceased to be available “for new supply” in September 2005 and April 2006 respectively, and were completely withdrawn in March 2008. Marconi and Ericsson supplied the exchange end of the link and Myacom supplied the Network Termination Equipment (NTE9, pictured right) for the user end. I wrote much of the software in the NTE9 when working at my previous employer and the original supplier, Telspec.

ETSI gave the service the generic name Narrowband Multiservice Delivery System (NMDS), described in European specification EN 301 141-1. The UK-specific requirements are detailed in NICC specification SSPE/SPEC001-1.

Features

PSTN ports

The important feature of Home Highway was that it provided two PSTN ports that behaved like normal analogue lines, almost as if two analogue exchange line cards had been incorporated inside the NTE9. Each port had its own number (port 1 usually got the same number as your original POTS line and the digital ports had a third number), and all the normal “select services” (such as 1471, charge advice, caller display etc.) were available. Voice quality was improved. Line reversals and disconnection pulses were implemented to enable answering machines to detect the beginning and end of a call. Each port could ring a REN of 4. In contrast, analogue ports on an ISDN terminal adapter or PBX typically lacked most of these features.

Electrical characteristics of the analogue ports can be found in SIN 351 and technical characteristics of supplementary services in SIN 354.

ISDN ports

A Home Highway line also provided two ISDN ports (wired in parallel) for fast, reliable data calls. Connection was much quicker, typically 2s instead of 20s or more with a modem, and big downloads could use both B channels to give 128kbit/s (or even more with compression). Users paid for two phone calls, but the faster speed meant calls were typically shorter. It was not possible to use the PSTN ports if both B channels were already in use. A caller would get “engaged” tone, but Call Minder could still be used to answer calls (for a fee). Home Highway was cheaper than an ISDN line, but more expensive than two PSTN lines.

Most computers need a terminal adapter to connect to the ISDN ports – these cost around £30 for an internal card or £100 for an external adapter. A standard (115kbit/s) serial port will not keep up with a 128kbit/s ISDN connection that uses both B-channels, so a USB external adapter or an internal card is preferable. A standard networking cable with RJ45 plugs at each end can be used. The cheap Billion PCI card works fine (except for hibernation mode) and allegedly gives the lowest “pings” for online gaming.

USB port

Home Highway lines installed after 1st November 2001 had a USB port on the underside of the NTE9. PCs running Windows 95 or MAC OS 9.0 or later could connect to this port with a standard USB cable up to 5m long. Driver software had to be installed on the PC. This saved the expense and inconvenience of installing a separate terminal adapter, providing users only wanted to connect one computer that was compatible with the driver software and within 5m of the NTE9.

Business Highway

A Business Highway line cost the same as a normal ISDN line. It was physically identical to a Home Highway line, the difference being that supplementary services were made available on the ISDN ports as well as the PSTN ports (a marketing decision). The most important of these are Multiple Subscriber Numbering (MSN), which allows additional numbers to be allocated to the ISDN ports, and Caller ID.

Installation

Wiring

The NTE9 was normally installed near the user’s computer, rather than near the existing “master socket”. The cable to the NTE9 also carried wires from PSTN port 1 back down to the master socket, where it connected to all existing extension wiring. Existing phones would therefore work as normal.

ISDN extension sockets for another computer could be installed at extra cost.

Customer extension wiring was connected to the green insulation displacement connector on the back of the master socket faceplate so that pins 2, 3, 4 and 5 on the extension sockets connect to the slots with the same number on the IDC connector. The colours in the following table were normally used.

The connecting cable from the NTE9 connected to the blue IDC connector. This belonged to the part of the installation that was BT’s responsibility. For reference, the connections were:

Connections in the PSTN sockets, digital sockets and USB socket were as standard.

Switches

The normal positions of the S-bus configuration switches under the NTE9 are “S” (for short bus configuration) and “IN” (for termination resistors connected in). Details of when to use the other settings are here.

A switch accessible through a hole in the back of the unit allows operation on lines (e.g. from “TPON” systems) that don’t supply a higher ISDN voltage.

Power

The NTE9 is usually locally powered and has to be located near a power point. If local power fails, PSTN port 2 is disabled. PSTN port 1 continues to operate normally using power drawn from the line, except that incoming calls ring with a modified cadence (shorter bursts, but the same voltage). The ISDN ports continue to work. The POWER indicator is illuminated whenever local power is present.

No “phantom” PS1 power is available from the ISDN ports (unlike on ISDN2e lines), because the line power is all used by PSTN port 1 if local power fails. For line-powered ISDN terminals that really need this “phantom” power, a separate power supply could be purchased.

Bypass

For a short period during installation, while the line is still connected to the analogue line card at the exchange, a special “bypass” relay inside the NTE9 connects the line to PSTN port 1. 

Indications

As soon as the higher ISDN line voltage is detected during installation, the STATUS indicator starts to flash. A few seconds later (when the NTE9 has synchronised with the signal from the exchange) the STATUS indicator will illuminate continuously and all ports will be operational. (On units with a USB connector, the STATUS indicator will blink off periodically if there is no USB connection.)

Layer 1

U interface

Electrically, a Home Highway line was almost identical to a normal ISDN line, both from the exchange to the NTE9 (the “U” interface) and from the NTE9 to users’ terminals (the “S” interface). It connected to a normal digital line card at the exchange using the existing twisted pair of copper wires, was fed with approximately 96V DC and used 2B1Q line code. The extra voltage was needed to provide power (roughly 1 watt) for the NTE9 if local power failed. The data was organised into two full-duplex 64kbit/s B-channels and one 16kbit/s D-channel (2B+D), plus some “overhead” bits used for line monitoring. Specification ETR 080 describes this interface in detail.

The main differences were that layer 1 was permanently activated (normal ISDN lines can deactivate if no calls are in progress, to save power) and the “SAI” bit was always true, since there was effectively an internal “terminal adapter” permanently connected to the S interface.

S interface

The two data ports (with blue shutters, wired in parallel) appear as a normal “S-bus”. Up to eight ISDN terminals can be connected, at distances up to 800m, with some restrictions. Two switches on the bottom face of the unit can be used to switch termination resistors in or out, and to optimise the bus timing for long or short connections. The normal settings are “IN” and “SHORT”, but on most short connections they will make no difference. See ITU specification I.430 for more information.

Layer 2

PSTN ports

D-channel messages intended for PSTN port 1 or port 2 always use TEI 126 or 125 (in the second address byte of every message) respectively. They are treated as “fixed” TEIs, which means that no ID request or ID assign messages are needed. The exchange simply sends a “SABME” layer 2 establishment message to each port as soon as layer 1 is activated, to establish the connection. The NTE9 applies feed to the PSTN ports at this time.

ISDN ports

D-channel messages intended for the ISDN ports work as usual, and are passed transparently without modification. TEIs 126 and 125 are never allocated to the ISDN ports on Home Highway lines.

See ETS 300 125 for more information.

Layer 3

The layer 3 messages for the PSTN ports use a completely different set of messages from normal ISDN lines, based on “V5.1” rather than “Q.931” signalling. See Chapter 13 of EN 300 324-1 for more information. The exchange tells the PSTN ports to “apply reverse feed” or “start ring cadence 1” rather than “here is a voice call”, and the ports tell the exchange about events such as “on-hook detected”, rather than “disconnect the call”.

Every SIGNAL message normally contains a sequence number, used to ensure that no messages were missed. Since an error-corrected layer 2 containing its own sequence numbers is used to carry every layer 3 message, the layer 3 sequence numbers are really superfluous.

Outgoing call

A typical call connected between two analogue phones via a pair of NTE9 units would be signalled as described below.

When a user (the “A-party”) goes off-hook to initiate an outgoing call, the NTE9 sends an ESTABLISH message to the local exchange (LE), indicating “off-hook” and “no B-channel”. The LE responds with an ESTABLISH ACK indicating the B channel to be used. The NTE9 connects the CODEC to the B channel and the user will hear dialtone.

If tone dialling is used, the tones are passed through the B channel and decoded by the exchange. The NTE9 does not contain DTMF decoders and does nothing. If loop-disconnect dialling is used, the NTE9 detects this and informs the LE using SIGNAL messages over the D-channel. SIGNAL ACK messages are periodically returned to acknowledge them.

When the remote “B-party” answers, the exchange connects the call and starts charging. This has no effect on the NTE9 at the originating end.

Incoming call

The local exchange sends an ESTABLISH message to the access network (AN, in this case the NTE9), indicating the B channel to be used for the connection. The NTE9 connects the CODEC to the indicated B channel and responds with an ESTABLISH ACK. The LE then sends a SIGNAL message indicating “steady reversed feed”. The NTE9 reverses the polarity of the feed to the PSTN port, which “wakes up” any caller display devices on the PSTN port. Caller display information may then be exchanged over the B channel.

When the caller display exchange is complete, the LE sends a SIGNAL “apply normal ring cadence”. The NTE9 starts ringing the port. If the user answers, the NTE9 sends an “off hook” SIGNAL message to the LE, which connects the call and starts charging for it. The NTE9 also autonomously restores normal “forward” feed to the PSTN port. Both sides send SIGNAL ACK messages to acknowledge all SIGNAL messages received so far.

Call cleardown

If the “A-party” caller hangs up first at the end of the call, the LE sends a SIGNAL “pulsed reduced battery” to the “B-party” NTE9. The NTE9 disconnects feed to the PSTN port for 100 ms (reduced feed is not implemented), to cause any answering machine to stop recording. The B channel is still connected at this stage so the user can hear any tones or announcements.

When the “B-party” user hangs up (whether before or after the A-party) the NTE9 sends an “on-hook” SIGNAL to the LE. The LE will respond with a SIGNAL “pulsed no battery” and the NTE9 again disconnects feed to the PSTN port for 100 ms. When this is complete, the NTE9 sends a SIGNAL “acknowledge” message and the LE responds with a DISCONNECT message indicating “no B-channel”.

The NTE9 then disconnects the CODEC and responds with a DISCONNECT COMPLETE message, and the cleardown is complete.