E1 and T1 at the primary level
At the primary PDH level, the European E1 and North American T1 differ in frame structure, capacity, signaling, physical line coding, and G.711 speech companding. These differences complicated interconnection in legacy international networks and contributed to the adoption of SDH/SONET transport.
E1 and T1: parameter comparison
PCM-30/32 (E1) and PCM-24 (T1) use different frame allocations and physical interfaces. A DS1 frame repeats 8000 times per second, so 193 bits per frame produce the 1.544 Mbit/s line rate. Speech companding and physical line coding are separate operations.
| Parameter | PCM-24 (T1, North America) | PCM-30/32 (E1, Europe) |
|---|---|---|
| Total Bit Rate | 1.544 Mbit/s | 2.048 Mbit/s |
| Frame Length | 125 µs (193 bits) | 125 µs (256 bits) |
| User Channels per Frame | 24 | 30 |
| Frame Organization | 24 DS0 octets plus one framing F bit | 32 timeslots, with TS0 framing and TS16 signaling or payload |
| Signaling Method | CAS with robbed-bit signaling in traditional voice DS1, or CCS | CAS in TS16, or CCS / 64 kbit/s payload |
| Line Code | AMI or B8ZS | HDB3 |
| G.711 Companding | µ-law, traditionally | A-law, traditionally |
Differences in frame, signaling, and coding
- Frame structure and bit rate. E1 carries 256 bits every 125 µs, including 30 8-bit channel slots and framing overhead. T1 carries 193 bits every 125 µs, including 24 8-bit channel slots and one framing bit. Traditional DS1 framing uses a 12-frame superframe or a 24-frame extended superframe. The resulting line rates are 2.048 Mbit/s and 1.544 Mbit/s.
- Signaling method. The two systems allocate signaling information differently.
- E1 uses Channel Associated Signaling (CAS) in PCM-30/32, dedicating TS16 to signaling. This leaves the voice-channel octets in their own timeslots. G.704 also permits common-channel signaling in TS16 or use of TS16 as a 64 kbit/s payload channel.
- T1 uses . The system periodically uses the least significant bit of a voice channel for signaling. This preserves 24 channel positions and avoids a dedicated signaling timeslot, while slightly affecting the available voice payload bits.
- G.704 also defines T1 common-channel signaling with a dedicated 64 kbit/s channel. In the 24-frame CAS arrangement, A, B, C, and D states occur in selected frames, typically every sixth frame, giving four 333 bit/s signaling channels. A 12-frame arrangement carries A and B at 667 bit/s each.
- Line coding. G.703 specifies AMI or B8ZS for the 1.544 Mbit/s interface and HDB3 for the 2.048 Mbit/s interface. AMI alternates the polarity of marks. B8ZS replaces eight consecutive zeros with a bipolar-violation pattern, while HDB3 applies a corresponding substitution after four zeros. These are physical-layer codes, separate from speech samples.
- Speech companding. Both systems traditionally carry G.711 PCM voice sampled at 8 kHz with 8 bits per sample, giving 64 kbit/s per channel.
- North America and Japan traditionally use µ-law.
- Europe and many other regions use A-law.