GB2094106A - Transmission system - Google Patents

Abstract

In a transmission system of the two-wire time separation type, data bursts are sent alternately in the GO and RETURN directions. Each burst ends with a suffix, usually 1 bit for balancing purposes, which does not convey other intelligence. Hence the transmission of the next burst overlaps the reception of this suffix: this improves the margin available between GO and RETURN Transmission.

Description

SPECIFICATION Transmission system The present invention relates to data transmission systems, and especially to such systems which are of the two-wire time separation type.

In such a system a twisted pair, which is usually referred to as being of the two-wire type, the line is used alternately for transmissions in the two directions. Thus in a system in which one terminal is regarded as a master and the other terminal as a slave, the line alternately carries data bursts in the master-slave and slave-master directions.

An object of the invention is to improve the line utilisation.

According to the present invention there is provided a data transmission system in which a single transmission channel interconnecting a first terminal and a second terminal is used to convey data bursts between the terminals in both directions, the channel being used to convey bursts alternately between the first terminal and the second terminal and between the second terminal and the first terminal, in which the format of the bursts conveyed is such that each burst has a suffix formed by one or more bits used for balancing purposes and thus not conveying information, in which when a burst has been sent in one direction the next burst to be sent in the other direction commences to be sent while the said suffice is being received so that the transmission in said other direction overlaps the reception of said suffix in said one direction.

The application of the present invention to a data transmission system will now be described with reference to the accompanying timing diagram, which shows the timing for data bursts at the slave terminal.

PCM characters are usually transmitted at 8000 characters per sec., i.e. a period of 1 25my, each companded PCM character being 8 bits long. With the addition of signalling and synchronisation, the total character becomes 10 bits which in a single character assembly must be transmitted in both directions with a margin for propagation delay.

With the parameters quoted the information rate is 80K bit/sec, i.e. 8K characters each of 10 bits. If this rate is increased three-fold and sent as a burst the information would occupy 125 3 = 412/3 used, leaving 831/3 lisec for a burst to be received in the reverse direction, with a margin for propagation delay which is needed to avoid overlap at the master. Naturally if such an overlap occurs the system fails as information would be lost. The margin could be increased by increasing the transmission rate, but this leads to greater distortion and attenuation of the signal.

The more efficient transmission encoding techniques, e.g. AMI (Alternate Mark Inversion) and Miller code modified to eliminate DC components, are not simply a reversal of polarity between 0 and 1. To ensure that there is accurate and adequate timing within a burst of data bits it is desirable to start the burst with a 1 bit; normally no information is conveyed by this bit in AMI and Miller code. However, the polarity of this first bit is relevant to the maintenance of balanced transmission over any even number of bits, e.g. for AMI. Thus if this first bit is used for timing extraction as well as for maintaining balance, then it cannot be used for data information. This would mean that the data burst for one PCM "word" would become 11 bits instead of 10 bits, and thus reduce the available margin.

However, in the arrangement to be described, the 11th bit is used only for balance purposes, which allows information to be conveyed by the first bit. This information could be conveyed as an absolute polarity or differential polarity change between the first bits of successive bursts. Since no data information is conveyed in the balance bit at the end of the burst, the start of the transmit burst at both the slave and the master can overlap the balance bit without any loss of information.

The drawing shows this overlap at the slave terminal. In this Figure the balance bits are shown shaded, and the following relations apply: T1 + T2 = Total propagation delay T1 + T2 + T3 = Total margin = T - 2T4 where T4 is the main burst length, T3 = remaining margin at the master; if T3 were reduced to zero, there would also be overlap thereat.

In the above description we have assumed that the data are conveyed in single character assembly form but the invention is also usable with multiple carrier assemblies.

1. A data transmission system in which a single transmission channel interconnecting a first terminal and a second terminal is used to convey data bursts between the terminals in both directions, the channel being used to convey bursts alternately between the first terminal and the second terminal and between the second terminal and the first terminal, in which the format of the bursts conveyed is such that each burst has a suffix formed by one or more bits used for balancing purposes and thus not conveying information, in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Transmission system The present invention relates to data transmission systems, and especially to such systems which are of the two-wire time separation type. In such a system a twisted pair, which is usually referred to as being of the two-wire type, the line is used alternately for transmissions in the two directions. Thus in a system in which one terminal is regarded as a master and the other terminal as a slave, the line alternately carries data bursts in the master-slave and slave-master directions. An object of the invention is to improve the line utilisation. According to the present invention there is provided a data transmission system in which a single transmission channel interconnecting a first terminal and a second terminal is used to convey data bursts between the terminals in both directions, the channel being used to convey bursts alternately between the first terminal and the second terminal and between the second terminal and the first terminal, in which the format of the bursts conveyed is such that each burst has a suffix formed by one or more bits used for balancing purposes and thus not conveying information, in which when a burst has been sent in one direction the next burst to be sent in the other direction commences to be sent while the said suffice is being received so that the transmission in said other direction overlaps the reception of said suffix in said one direction. The application of the present invention to a data transmission system will now be described with reference to the accompanying timing diagram, which shows the timing for data bursts at the slave terminal. PCM characters are usually transmitted at 8000 characters per sec., i.e. a period of 1 25my, each companded PCM character being 8 bits long. With the addition of signalling and synchronisation, the total character becomes 10 bits which in a single character assembly must be transmitted in both directions with a margin for propagation delay. With the parameters quoted the information rate is 80K bit/sec, i.e. 8K characters each of 10 bits. If this rate is increased three-fold and sent as a burst the information would occupy 125 3 = 412/3 used, leaving 831/3 lisec for a burst to be received in the reverse direction, with a margin for propagation delay which is needed to avoid overlap at the master. Naturally if such an overlap occurs the system fails as information would be lost. The margin could be increased by increasing the transmission rate, but this leads to greater distortion and attenuation of the signal. The more efficient transmission encoding techniques, e.g. AMI (Alternate Mark Inversion) and Miller code modified to eliminate DC components, are not simply a reversal of polarity between 0 and 1. To ensure that there is accurate and adequate timing within a burst of data bits it is desirable to start the burst with a 1 bit; normally no information is conveyed by this bit in AMI and Miller code. However, the polarity of this first bit is relevant to the maintenance of balanced transmission over any even number of bits, e.g. for AMI. Thus if this first bit is used for timing extraction as well as for maintaining balance, then it cannot be used for data information. This would mean that the data burst for one PCM "word" would become 11 bits instead of 10 bits, and thus reduce the available margin. However, in the arrangement to be described, the 11th bit is used only for balance purposes, which allows information to be conveyed by the first bit. This information could be conveyed as an absolute polarity or differential polarity change between the first bits of successive bursts. Since no data information is conveyed in the balance bit at the end of the burst, the start of the transmit burst at both the slave and the master can overlap the balance bit without any loss of information. The drawing shows this overlap at the slave terminal. In this Figure the balance bits are shown shaded, and the following relations apply: T1 + T2 = Total propagation delay T1 + T2 + T3 = Total margin = T - 2T4 where T4 is the main burst length, T3 = remaining margin at the master; if T3 were reduced to zero, there would also be overlap thereat. In the above description we have assumed that the data are conveyed in single character assembly form but the invention is also usable with multiple carrier assemblies. CLAIMS

1. A data transmission system in which a single transmission channel interconnecting a first terminal and a second terminal is used to convey data bursts between the terminals in both directions, the channel being used to convey bursts alternately between the first terminal and the second terminal and between the second terminal and the first terminal, in which the format of the bursts conveyed is such that each burst has a suffix formed by one or more bits used for balancing purposes and thus not conveying information, in which when a burst has been sent in one direction the burst to be sent in the other direction commences to be sent while the said suffix is being received so that the transmission in said other direction overlaps the reception of said suffix in said one direction.

2. A data transmission system substantially as described with reference to the accompanying drawing.