ABOUT THE TECHNOLOGY A

This appendix provides more information about the bridging and ADSL transmission features of the Megabit Modem 310F and 320F.

MAC LAYER BRIDGING

A bridge moves information across an internetwork from a source to a destination at the link layer (of an OSI reference model). The information is forwarded or filtered based on its Media Access Control (MAC) address.

The Megabit Modem 310F and 320F provide transparent Ethernet MAC level bridging which includes learning, forwarding, filtering, and hashing/buffer management.

Forwarding performance is at full DMT rate and filtering performance is at full Ethernet rate of 14 kpps for 64-byte frames (minimum size).

Neither the Megabit Modem 310F nor 320F support the Spanning Tree protocol. Therefore, the modems cannot be used to provide link redundancy to a LAN segment.

DATA ENCAPSULATION

Data transmitted over the ADSL link comprises HDLC encapsulated Ethernet MAC frames as shown in the figure below.

 

1

1

6

6

2

46-1500

2

 

Flag

Address

Control

Destination Address

Source Address

Length/Type

Data

FCS

Flag

0x7E

0xFF

0x03

 

 

 

 

 

 

One or more flag patterns (0x7E) occur between frames to indicate the frame boundaries. Zero bit insertion is used for data transparency. This means that a zero is inserted after every occurrence of five consecutive ones (1s) in the data field. This prevents a flag pattern from occurring in the data.

Megabit Modem 310F and 320F User Manual

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