T1 & E1

T1 E1 A T1 carrier is a dedicated phone connection supporting data rates of 1.544Mbits per second. A T-1 line actually consists of 24 individual channels, each of which supports 64Kbits per second. Each 64Kbit/second channel can be configured to carry voice or data traffic. Most telephone companies allow you to buy just some of these individual channels, known as fractional T-1 access. T-1 lines are a popular leased line option for businesses connecting to the Internet and for Internet Service Providers (ISPs) connecting to the Internet backbone. The Internet backbone itself consists of faster T-3 connections. T-1 lines are sometimes referred to as DS1 lines. When someone says they are running T1, they may be saying several different things: They may mean that they have a network that is passing data at 1.544 Mbps; they may mean that they have a network that conforms to the T1 electrical interface specification (DSX-1), or that they have a network that passes data that conforms to one of the several framing formats (D4, ESF, etc.). More likely than not, they mean all three but their concentration may be on only one of these item Similar to the North American T-1, E1 is the European format for digital transmission. E1 carries signals at 2 Mbps (32 channels at 64Kbps, with 2 channels reserved for signaling and controlling), versus the T1, which carries signals at 1.544 Mbps (24 channels at 64Kbps). E1 and T1 lines may be interconnected for international use. Extend T1 over CAT 5

T1 The most common legacy of this whole system is the line rate designations. A "T1" now seems to mean any data circuit that runs at the original 1.544 Mbit/s line rate. Originally the T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of up to 44.736 Mbit/s. Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground. To accommodate loading coils, cable vault manholes were physically 6600 feet apart, and so the optimum rate was chosen empirically--the capacity was increased until the failure rate was unacceptable, then reduced. Companding allowed acceptable audio performance with only seven bits per sample. A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest frequency at which voice communications occurs is at 4000 Hz, the required digital sampling rate is 8000 Hz (see Nyquist rate). Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8000 frames per second to maintain those 24 simultaneous voice channels. Because each frame of a T1 is 193 bits in length (24

channels X 8 bits per channel + 1 control bit = 193 bits), 8000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8000 X 193 = 1544000).

Notes Note 1: The designators for T-carrier in the North American digital hierarchy correspond to the designators for the digital signal (DS) level hierarchy. Note 2: T-carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission. Note 3: Historically, if an "F" precedes the "T", optical fiber cables are utilized at the same rates. Note 4: The North American and Japanese hierarchies are based on multiplexing 24 voice-frequency channels and multiples thereof, whereas the European hierarchy is based on multiplexing 32 voice-frequency channels and multiples thereof. See table below. T-Carrier Systems

North American

Japanese

European (CEPT)

Level zero (Channel data rate)

64 kbit/s (DS0)

64 kbit/s

64 kbit/s

First level

1.544 Mbit/s (DS1) 2.048 Mbit/s (32 1.544 Mbit/s (24 user (24 user channels) user channels) channels) (T1) (E1)

(Intermediate level, US. hierarchy only)

3.152 Mbit/s (DS1C) (48 Ch.)

Second level

6.312 Mbit/s (96 6.312 Mbit/s (DS2) 8.448 Mbit/s (128 Ch.), or 7.786 Mbit/s (96 Ch.) Ch.) (E2) (120 Ch.)

Third level

44.736 Mbit/s (DS3) (672 Ch.) (T3)

32.064 Mbit/s (480 Ch.)

Fourth level

274.176 Mbit/s (DS4) (4032 Ch.)

97.728 Mbit/s (1440 139.264 Mbit/s Ch.) (2048 Ch.) (E4)

Fifth level

400.352 Mbit/s (DS5) (5760 Ch.)

565.148 Mbit/s (8192 565.148 Mbit/s Ch.) (8192 Ch.) (E5)

-

-

34.368 Mbit/s (512 Ch.) (E3)

Note 1: The DS designations are used in connection with the North American hierarchy only. Technically a DS1 is the data carried on a T1 circuit, and likewise for a DS3 and a T3, but the terms are almost always used interchangeably.

Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system that operates at 90 Mbit/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development. Higher data rates are now often achieved by using Synchronous optical networking, SONET or Synchronous digital hierarchy, SDH.

E1 An E1 link operates over two separate sets of wires, usually coaxial cable. A nominal 2.4 volt 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 time slots, each being allocated 8 bits in turn. Thus each time slot 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. A T1/E1 Multiplexer is a T/E carrier network element that uses Time Division Multiplexing to combine digital input signals into one high-speed output signal. The T1 multiplexer, based on the US T-carrier spec, has a 1.544 Mbps DS-1 output signal, while the E1 multiplexer, based on the European E-carrier spec, has a 2.048 Mbps E1 output signal. T1/E1 muxes are typically used to aggregate 64-Kbps digital voice channels, 24 for T1 and 30 for E1, and in this configuration are called Channel Banks. Therefore, T1/E1 Multiplexers played a key role in the early stages of the digitalization of the public telephone networks worldwide. T1/E1 Multiplexers can also accommodate other types of digital traffic such as data and video with the installation of line cards. Because of this feature, T1 multiplexers also gained popularity in its use by enterprise customers to aggregate enterprise network traffic, interconnect LANs, and gain high-speed access to the Internet.