TRANSMISSION CIRCUITS

In telecommunication, data transmission circuit is the transmission media and the intervening equipment used for the transfer of data between data terminal equipments.

DIGITAL SIGNAL 0

The DS 0 rate was introduced to carry a single digitized voice call. For a typical phone call, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse-code modulation for each of the 8000 samples per second. This resulted in a data rate of 64 kbit/s.

Because of its fundamental role in carrying a single phone call, the DS0 rate forms the basis for the digital multiplex transmission hierarchy in telecommunications systems used in North America. To limit the number of wires required between two involved in exchanging voice calls, a system was built in which multiple DS0s are multiplexed together on higher capacity circuits. In this system, twenty-four (24) DS0s are multiplexed into a DS1 signal. Twenty-eight (28) DS1s are multiplexed into a DS3. When carried over copper wire, this is the well-known T-carrier system, with T1 and T3 corresponding to DS1 and DS3, respectively.

T1 (DIGITAL SIGNAL-1)

is a T-carrier signaling scheme devised by Bell Labs.^[1] DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside of North America, Japan, and South Korea. Technically, DS1 is the logical bit pattern used over a physical T1 line; however, the terms "DS1" and "T1" are often used interchangeably.

For the DS-1, also called T1, Time Division Multiplexing (TDM) is used to transport multiple channels over one line. Clocking of the serial transmission needs to occur at one end of the link or the other, sometimes you will see the clocking options as internal i.e. provided by the local device, or line meaning that the clock is provided by the remote device. Two-pairs are used in a T1 link. The T1 link can operate in full-duplex mode where one pair transmits and the other pair receives. 24 channels are available for transmission and these are grouped together to form a Frame i.e. the 24 time slots (8 bits each) plus one framing bit form one T1 frame (193 bits, the 193rd bit being the synchronisation/framing bit). For 8000 samples a second, a T1 frame must be transmitted every 125 usecs, we can therefore calculate the T1 line rate as 193 x 8000 = 1.544 Mbps (A DS0 line rate is 8 bits x 8000 = 64 Kbps).

DIGITAL SIGNAL 3

is a digital signal level 3 T-carrier. It may also be referred to as a T3 line.

• The data rate for this type of signal is 44.736 Mbit/s.
• This level of carrier can transport 28 DS1 level signals within its payload.
• This level of carrier can transport 672 DS0 level channels within its payload.

The level of transport or circuit is mostly used between telephony carriers, both wired and wireless.

E1

An E1 link operates over two separate sets of wires, usually twisted pair cable. A nominal 3 Volt peak signal is encoded with pulses using a method that avoids long periods without polarity changes. The line data rate is 2.048 Mbit/s (full duplex, i.e. 2.048 Mbit/s downstream and 2.048 Mbit/s upstream) which is split into 32 timeslots, each being allocated 8 bits in turn. Thus each timeslot sends and receives an 8-bit sample 8000 times per second (8 x 8000 x 32 = 2,048,000). This is ideal for voice telephone calls where the voice is sampled into an 8 bit number at that data rate and reconstructed at the other end. The timeslots are numbered from 0 to 31.

E1 circuits are very common in most telephone exchanges and are used to connect to medium and large companies, to remote exchanges and in many cases between exchanges. E3 lines are used between exchanges, operators and/or countries, and have a transmission speed of 34.368 Mbit/s.