Monday, October 20, 2008

E1 Link for ATM Physical Interface

E1 ATM Physical Interface


For information on testing E1 links

The E1 interface operates at 2 Mbps over coax cables, compliant with ATM Forum UNI specifications. It supports both PLCP and direct cell mapping and complies with the following standards: G.704, G.706, G.732. The interface has BNC connectors.

The E1 transmission link consists of 32 transmission channels (0-31), each of which is 64 Kbits/sec. The overall transmission rate is 2.048 Mbits/sec. Channels 0 and 16 are reserved for transmission management, while all other channels are used for payload. The payload bandwidth is thus 1.920 Mbits/sec. Since ATM uses 48 out of the possible 53 bytes for payload transmission, the net transmission rate becomes 1.738 Mbits/sec.

Channel 0 carries F3-OAM information, signals loss of frame or synchronization, and is responsible for transferring FERF and LOC messages. Channel 16 is reserved for signalling.

Direct Mapping

The direct mapping of ATM cells onto E1 transmission frames is specified in CCITT recommendation G.804. This specifies that ATM cells are to be carried in bits 9-28 and 137-256 (corresponding to channels 1-15 and 17-31).

The following is an illustration of the E1 frame format when direct mapping of ATM cells is used. The 53 byte ATM cell begins with a header and wraps around consecutive E1 frames.


PLCP Cell Mapping

The PLCP format for E1 is described in ETSI document ETS 300 213, where an E1 PLCP frame is specified as consisting of ten rows of 57 bytes each. Four bytes are added to the cell length of 53 bytes to provide the various overhead functions.

The E1 frame structure with PLCP cell mapping is illustrated in the following diagram:



A-bits
Separator bytes.

P-bits
Path overhead identifier.

C1
Pad bit counter.

M-bits
SIP layer 1 management information.

G1
PLCP path status.

B1
Bit-interleaved parity 8 (BIP-8).

F1
PLCP path user channel.

Z-bits
For future use.

Thirty of the available 32 E1 channels are used for transporting the PLCP frame. The remaining two channels are reserved for E1 framing and signalling functions. The PLCP frame is octet aligned to the channel boundaries in the E1 frame; thus the A1 octet of the first row of the PLCP frame is inserted into time slot 1 of the E1 frame.

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