ZyXEL Communications P-861H-G1, Руководство пользователя, Страница: 267 / 341

Prestige 861H Series User’s Guide
Chapter 32 VDSL Commands 266
nrt-VBR (non-real-time Variable Bit Rate) is commonly used for “bursty” traffic typical on
LANs. PCR and MBS define the burst levels, SCR defines the minimum level. An example
application is multimedia e-mail.
Unspecified Bit Rate (UBR) does not guarantee bandwidth or throughput. Cells are dropped if
there is not enough bandwidth. Only the PCR is set. An example application is background file
transfer.
Table 83 Service Characteristic
CHARACTERISTIC CBR RT-VBR NRT-VBR UBR
Guaranteed Bandwidth Yes Yes Yes No
Good for Real-Time Traffic Yes Yes No No
Suitable for Bursty Traffic No No Yes Yes
32.4 Interleave Delay
Interleave delay is the wait (in milliseconds) that determines the size of a single block of data
to be interleaved (assembled) and then transmitted. Interleave delay is used when transmission
error correction (Reed- Solomon) is necessary due to a less than ideal telephone line. The
bigger the delay, the bigger the data block size, allowing better error correction to be
performed. This may sometimes be referred to as a “slow channel”.
An interleave delay of “0” means no interleaving takes place and transmission is faster (a “fast
channel”). This would be suitable if you have a good line where little error correction is
necessary.
Reed-Solomon codes are block-based error correcting codes with a wide range of applications.
The Reed-Solomon encoder takes a block of digital data and adds extra "redundant" bits. The
Reed-Solomon decoder processes each block and attempts to correct errors and recover the
original data.
32.5 SNR (Signal-to-Noise-Ratio)
The Prestige uses SNR(Signal-to-Noise-Ratio) to determine line quality. SNR is the ratio of
the amplitude of the actual signal to the amplitude of noise signals at a given point in time. A
slow SNR indicates poor line quality. When SNR (upstream or downstream) falls below a pre-
determined threshold, the Prestige then uses rate adaptation.