E1 and T1 are digital telecommunication carrier standards, initially developed in different continents to carry voice conversations simultaneously using time division multiplexing. Both the standards use transmit and receive paths separately to achieve full duplex communication. E1 is the European hierarchy, which was called as CEPT30+2 (European Conference of Postal and Telecommunications Administrations) prior to 1988, while T1 is used as North American standard. Structure and the frames of the E1 and T1 carriers have significant differences.
What is E1?
E1 consists of 32 channels, which can be used to carry simultaneous voice calls, and each channel is called as a Time Slot (TS). As per the ITU-T recommendations, 2 time slots are reserved for signalling and synchronization. Therefore, E1 can carry 30 voice calls or data communications simultaneously. Each Time Slot of E1 has a bandwidth of 64 Kbps, which leads to 2048 Kbps total speed for an E1 carrier. Time division multiplexing is used to separate the channels from each other. In general E1 time slots are designed to send Pulse Code Modulated (PCM) voice signals, which have a sampling frequency of 8000 samples per second. Due to this reason, each E1 frame is designed to send 1 sample from each channel and the size of E1 frame is limited to 125 µs (1s/8000). So, within this 125µs frame interval, 32 samples should be sent, which has 8 bits in each sample. Therefore, the total number of bits that should be transferred in a single frame is 256 bits. Two types of physical delivery methods are available as per the E1 standard, which is called as balanced physical delivery and unbalanced physical delivery. Balanced physical delivery is the most popular method, which uses 4 copper wires grouped as two pairs for transmit and receive paths.
What is T1?
T1 is the North American digital communication carrier standard that consists of 24 channels, which has 64Kbps bandwidth each. Initially each 64Kbps channel is designed to transfer pulse code modulated voice signals. As per the North American standard PCM with µ-Law is used with T1 carrier. Duration of the T1 time frame is decided based on the sampling frequency of PCM, because within one second each channel of T1 frame should transfer 8000 samples. In other words, 1 sample within 125µS (1s/8000 samples). As per the ANSI specification, each T1 consists of 24 channels, which are multiplexed into 125µS time frame. Other than these channels, T1 frame consists of a Framing bit, which denotes the end of the frame, is also used for signalling. Altogether, T1 frame consists of 193 bits (24 samples x 8 bits per sample + 1 frame bit) that need to be transferred within 125µS. Therefore, data rate of T1 carrier is 1.544 Mbps (193 bits/125µS). Physical transmission of T1 channels are done using 4 copper wires grouped into two pairs.
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What is the difference between E1 and T1? E1 and T1 are digital telecommunication carrier standards; in other words, multi-channel telecommunication systems, which are time multiplexed into a single carrier to transmit and receive. Both standards use two pairs of wires for transmit and receive paths to achieve full duplex communication. Initially, both methods are developed in order to send voice channels over copper wires simultaneously, which leads to less transmission cost. – Data rate of E1 is 2048kbps as per the recommendations of ITU-T, while the data rate of T1 is 1.544Mbps as per the ANSI recommendations. – E1 comprises of 32 simultaneous channels, while T1 consists of 24 simultaneous channels, which has 64kbps data rate in each channel. – Since both systems initially designed to transmit PCM voice, frame rate of both carriers are designed as 8000 frames per second to support 8kHz sampling rate of PCM. – Even though both E1 and T1 have same 125µS frame interval, E1 transmit 256 bits, while T1 transmit 193 bits within the same period. – In general E1 uses European standard of PCM called A-law while T1 uses North American standard of PCM known as µ-Law as voice channel modulation method. – Both E1 and T1 carrier methods are initially developed to transmit and receive pulse code modulated voice signals over time multiplexed copper wires. – Key difference of E1 and T1 is the number of channels, which can be transmitted simultaneously over the given physical medium.
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