LightSand S-2500B, Installation And User Manual

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1-2 S-2500B Gateway
SONET
SONET (Synchronous Optical NETwork) is the system of fiber optic cables, interface and
transport mechanisms that carries data and voice throughout the United States. SONET is the
recognized Telcordia GR-253 standard for the high speed optical transmission of digital
information. SONET is universally distributed, and SONET-based signals are readily passed
from carrier to carrier throughout the country.
A SONET network is typically deployed in a double ring configuration, with one ring serving
as a backup that assumes control within 50 milliseconds if the primary ring fails. Linking to the
network is accomplished by means of add/drop multiplexers (“network elements”) connected
simultaneously to both rings.
SONET defines the network speed of its optical carrier (OC) signals in terms of integer
multiples of a basic rate, 51.84 Mb/s. For example, OC-3 indicates a line rate of 3 times the
basic rate, or 155.52 Mb/s. OC-3 is typically the lowest network speed, and higher rates are
formed by bundling OC-3 channels together in successive groups of 4 to form OC-12, OC-48,
etc. When more bandwidth is required for one single channel, SONET has the ability to
combine several channels into a single rate, which is designated with a “c” for “concatenated.”
Signal processing and switching in SONET are handled electronically (not optically), so data
is converted into light signals for transport after it is first grouped into electronic frames. The
rates for electronic data processing and switching match the optical rates, and are similarly
defined in terms of integer multiples of the basic 51.84 Mb/s rate. Electronic data rates are
designated as the “synchronous transport signal” rate, or STS-n.
SONET is based on synchronous frame communication principals. Data is packaged into
separate frames with a fixed number of bytes at the start of each frame used to synchronize and
control transmission. Data paths are provisioned and remain static. Once data is multiplexed
into the synchronous envelope, it can be transported and switched without having to be
examined or demultiplexed at intermediate points. Each dedicated channel is given a
guaranteed fixed bandwidth for its data.
Fiber Channel Over SONET
The choice of SONET for SAN extension enables the benefits of FC flow control to be
extended between distant sites. The purpose of flow control is to regulate traffic sent to a
congested node. Credit buffering is a vital aspect of traffic control in a FC-based SAN. As
distance (one component of latency) is added to the system, credit buffering becomes an even
more crucial aspect of system design to guarantee that data is never dropped due to congestion.