Zhone EtherXtend 3300 Series, Руководство пользователя

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Features and Capabilities
26
EtherXtend User’s Guide
2BASE-TL is based on the SHDSL standard for symmetric DSL services. The
data rates over a single copper pair range between 1 Mbps and 5.7 Mbps.
2BASE-TL can serve distances up to 2,700 meters. Theoretically, up to 32
copper pairs can be bonded together to increase data rates and resiliency. In
practice, no more than 8 pairs are bounded.
The standard closely resembles the SHDSL specification for the PMA and
PMD sublayers. The PMA sublayer consists of a framer/deframer and a
scrambler/descrambler. The PMA sublayer then sends a framed and
scrambled bit stream to the PMD layer for modulation over the single copper
pair.
The PMD sublayer first passes the bit stream through a convolutional
encoder/decoder and then through the PCM modulator/demodulator. The
PMD sublayer then passes electrical signals across a single pair of voice grade
copper lines.
Zhone EFM features
Zhone’s EtherXtend equipment adds enhancements to the EFM standard.
Some of these enhancements are:
•
PME aggregation on page 26
•
Loop bonding on page 27
PME aggregation
Physical Medium Entities (PME) aggregation uses frame fragmentation to
divide, load balance, and transmit MAC frames across up to 32 parallel links.
Frame fragmentation is used to so that MAC frames are broken up into
smaller pieces and sent in parallel across multiple links. This increases
effective throughput while minimizing latency and jitter across the bonded
links.
Fragment sizes must be between 64 and 512 bytes and must be multiples of 64
bytes. All fragments must be at least 64 bytes long. The algorithm for
fragmenting MAC frames is left up to the implementer so there can be
differences in performance between different vendors in transmitting data
across bonded links.
Table 3: 2BASE-TL protocol
Protocol Point Method Standard Media Mode Data Rate Distance Bonding
2BASE-TL Point-to-Point SHDSL Copper Symmetric 1 - 5.7
Mbps 2,700
meters 32 pairs; 8
pairs
(actual)