Information Technology Reference
In-Depth Information
SONET and SDH
SONET is an ANSI standard that defines physical interface rates, which allows data streams at
different rates to be multiplexed. SONET defines Optical Carrier (OC) levels. North America
uses SONET. The rates are as follows:
•
OC-1 = 51.85 Mbps
•
OC-3 = 155.52 Mbps
•
OC-12 = 622.08 Mbps
•
OC-24 = 1.244 Gbps
•
OC-48 = 2.488 Gbps
•
OC-192 = 9.952 Gbps
SONET uses the Synchronous Transport Signal (STS) as its frame format and STS level 1
(STS-1) is the basic signal rate, at 51.84 Mbps. Each SONET frame is constructed of 9 rows by
90 columns of octets for a total of 810 octets (9 rows
×
90 columns = 810 octets). These 810
octets are transmitted in 125
µ
secs, or 8000 frames/second (8000
×
125
µ
sec = 1 second). The
basic rate for the STS-1 channel is 51.84 Mbps (810 bytes/frame
×
8 bits/byte
×
8000 frames/
second = 51.84 Mbps).
SDH is the international standard defined by the International Telecommunication Union (ITU)
for transmission over fiber optics. It defines the hierarchy of rates starting at 155.52 Mbps. The
rates are as follows:
•
STM-1 = 155.52 Mbps
•
STM-4 = 622.08 Mbps
•
STM-16 = 2.488 Gbps
One way to transmit network layer packets over a SONET/SDH network is to use ATM to
establish connections and provide traffic management over the SONET network.
An alternative is to use Packet over SONET (PoS), which typically adds less overhead from
frame headers than ATM does with cells. PoS maps IP directly onto SONET/SDH. PoS has
three main components: a link-layer protocol, octet framing to map onto the SONET payload,
and data scrambling for data security and reliability. For the link layer, PPP over SONET/SDH
is defined in RFC 2615. SONET/SDH links are provisioned as point-to-point circuits, making
PPP a suitable choice for the link-layer protocol. As RFC 2615 describes, PPP treats SONET/
SDH as octet-oriented synchronous links. Octet-oriented framing (PPP with HDLC-like framing)
is defined in RFC 1662. Data scrambling is defined in RFC 2615 and prevents packets with bit
patterns that might cause synchronization problems, emulates the SDH set-reset scrambler
pattern, and replicates the STS-N frame alignment word.
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