Introduction
- SONET, the Synchronous Optical Network.
- Initial standards developed by Bellcore in 1985.
- The CCITT (now ITU) joined with the SDH effort in 1987.
- SONET characteristics.
- SONET-SDH is a global telecommunications standard.
- It's a voice-oriented protocol.
- It's a OSI layer 2-3 protocol set.
- It defines point-to-point connections.
- Motivations:
- Too much multiplexing overhead (faster networks lead to more
multiplexing leads to more overhead).
- No global standard (expensive interconnection equipment).
- The reliance on bit-stuffing causes problems excessive demultiplexing.
- Insufficient management tools.
- No framing standards leads to interoperability problems.
- Objectives:
- Improve on current technology, particularly mltiplexing.
- Improve vender interoperability.
- Prepare for high-speed, Gbits/sec networks and associated protocls
(ATM).
- Develop a survivable-ring standard.
- Provide better network operations and administration management.
- SONET data rates.
- SONET specifies two data rates: the opical rates (optical carrier or
OC) and the electrical rates (synchronous transport signal or STS).
- The OC and STS rates are identical.
- The basic rate is OC-1 (STS-1) at 51.84 Mbits/sec.
- The OC-n (STS-n) rate is a multiple of the OC-1 rate:
n*51.84 Mbit/sec.
- For example, the OC-24 rate is 24*51.84 = 1244.16 Mbits/sec.
- Sonet networks.
- Sonet hardware comprises switches, (de-)mulplexers, and repeaters.
- A section is a point-to-point connection between devices.
- A line is a path between multiplexers.
- A path is an end-to-end connection.
- The multiplexers are known as add-drop multiplexers.
- They can add and remove data on the fly, without holding and
regenerating.
- Sonet frames.
- The SONET frame is two-dimentional: 9 rows x 90 bytes per row.
- (51.84 Mbits/sec)/(9*90 bytes/frame)*(8 bits/byte) = 8000 frames/sec.
- (1 sec)/(8000 frames) = 0.000125 sec/frame = 125 msec/frame).
- This corresponds to 28 DS-1 lines or one DS-3 line.
- Transmission is from left to right and top to bottom.
- The first three frame columns contain system management information.
- The first three rows are section information; the remainder are
line information.
- The remaining 87 columns, known as the synchronous payload envelope
(SPE), contain user data.
- Inter-frame multiplexing.
- Higher data rates are formed by multiplexing (interleaving) lower rate
frame streams.
- OC-n contains n interleaved OC-1 streams.
- The c data rates (e.g. OC-12c) are formed by straight frame catenation.
- Lower-bit rate streams, called virtual tributaries, can also be
multiplexed within a frame.
- Synchrony.
- The "synchronous" means Sonet components share a common clock.
- This is expensive and difficult, but useful.
- But even so, frames drift, particularly in virtual tributaries.
- This is called plestochrony.
- Let components float in the payload portion of the frame.
- The overhead portion of the frame contains start pointers into the
payload.
- Sonet network topologies.
- Sonet networks for a ring, called the bidirectional line-switched ring
(BLSR).
- Bidirectional rings protect against ring breaks.
- A BLSR/n network uses n fibers.
- And has a maximum of 16 nodes.
- Propagation delay is capped at 6 msec (1200 km, 745 miles).
- Synchronous Digital Hierarchy.
- Developed by the CCITT (now ITU), published in 1989.
- It's a global standard; SONET (a North American standard) is a subset.
- The T vs E rates.
- DS-1 = 24*DS-0 = 24*(64 kbits/sec) = 1.45 Mbits/sec.
- E-1 = 32*E-0 = 32*(64 kbits/sec) = 2.048 Mbits/sec.
- The SDH base rate, called synchronous transport module 1 or STM-1, is
155 Mbit/sec.
- And lo, OC-3 = STM-1, almost.
This page last modified on 14 November 2004.