Transcript SONET - Oswego

SONET
Synchronous Optical Network
Overview
ECSA – Exchange Carrier Standards Association
ANSI – American National Standards Institute
SONET was formulated by ECSA for ANSI.
SONET was formulated to set industry standards in the US for
telecommunications and are expected to provide transport
infrastructure for world wide telecomm for at least 3-4 decades
Standard for connecting fiber systems together
Clocking
Synchronous: Transitions in the signals occur at the exact same time
Plesiochronous: Transitions occur at almost the same time
Asynchronous: Transitions of signals do not necessarily occur at the
same time
• In digital transmission clocking is one of the most important considerations.
• Internal clock of a SONET terminal serves as a master for other SONET
nodes for timing. (BITS)
• For SONET, the average frequency of all clocks in the system will be
synchronous or plesiochronous
Framing
•STS: basic transmission rate
•STS-1 = 51.84
• Higher level signals are multiples
• STS-3 = 3 x 51.84 = 155.52
DS-X
Date
Rate
DS0
Multiple
TCarrier
DS0
64
Kbps
1
DS1
1.544M
bps
24
T-1
DS2
6.321
Mbps
96
T-2
DS3
44.736
Mbps
672
T-3
Digital Transmission Rates based on DS0 (normally used for telephone voice
channel)
NOTE: 3 STS-1 frames fit in 1 STS-3 frame. Hmm. Allows for low speed links to be
multiplexed into a high speed link.
Frame Structure
• 9 rows of 90bytes
• First 3 bytes of each row are overhead
• First 2 bytes help received determine start of frame (special pattern)
• Rest is data transferred by link
No bit stuffing
More Framing
If no bit stuffing, can’t this pattern randomly happen?
Yes. However the receiver looks for pattern consistently hoping to find
it every (9 x 90 = ) 810Bytes. When this happens, the receiver assumes
it is in synch.
Overhead
Overhead = Complicated!
SONET runs over optical networks not just a single link.
• SONET encoded NRZ (1-high, 0-low)
• To ensure the receiver has enough time to recover (enough
transitions) payload bytes are scrambled.
• XOR data with well known (127 bit) pattern.
Enough information is contained in the overhead to allow the network to
operate and allow OAM&P communications between an intelligent network
controller and the individual nodes.
Interleaving
Bytes from first are transmitted, then the second, and so on.
Allows for smooth 51Mbps frames to the receiver
STS-3c – viewed as one pipe (instead of 3 STS-1)
Payload Shift: Simplifies the task of synchronizing clocks throughout
carriers (big worry for carriers)
SONET Alarm Structure
SONET frame structure has been designed to contain a large amount of
overhead information that provides a variety of management and other
functions.
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error performance monitoring
pointer adjustment information
path status
path trace
section trace
remote defect, error, and failure indications
signal labels
new data flag indications
data communications channels (DCC)
automatic protection switching (APS) control
synchronization status message
SONET Alarm Structure
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anomaly—This is the smallest discrepancy that can be
observed between the actual and desired characteristics
of an item. The occurrence of a single anomaly does not
constitute an interruption in the ability to perform a
required function.
defect—The density of anomalies has reached a level
where the ability to perform a required function has
been interrupted. Defects are used as input for
performance monitoring, the control of consequent
actions, and the determination of fault cause.
failure—This is the inability of a function to perform a
required action persisted beyond the maximum time
allocated.
Digital Loop Carriers
• DLC’s are concentrators for low-speed services before they reach the CO.
If this were not done, the CO would serve a lot less people
• DLC’s are systems of multiplexers and switches which forward signals to
the dial office then to the CO.
• SONET multiplexers are deployed at customers prem where DLC is
intended for service at a CO.
• Integrated Digital Loop Carrier (IDLC) which consists of a remote digital
terminal (RDT) connected with a digital line to an integrated digital
terminal (IDT)
• IDLC’s are designed for a more efficient integration of DLC systems with
existing digital switches.
Network Configurations
Point-to-Point
Path Terminating Equipment: (PTE) Terminating systems that access, generate,
and process path overhead.
Regenerator: device that restores a degraded digital signal for continued
transmission; also called a repeater
• SONET multiplexer acts as an forwarder of DS-Xs.
• Here, (simplest configuration) two terminal multiplexers are linked by
fiber with a regenerator.
Network Configurations
(cont)
Point-to-Multipoint
Point-to-multipoint expands on point-to-point by adding dropping circuits along
the way.
• Add/Drop Multiplexer: (ADM) Specifically designed for facilitating add/drop
channels at intermediate points along the network.
Network Configurations
(cont)
Hub Network
Digital Cross-Connect System: (DCS) Has access to lower-rate channels in
higher-rate multiplexed signals and can electronically rearrange those channels
Allow for simpler unexpected growth. A hub concentrates traffic at a central
site and allows easy reprovisioning of the circuits.
Network Configurations
(cont)
Ring Architecture
• Multiple ADMs are put in a ring with bidirectional or unidirectional traffic.
Survivability: if a fiber cable is cut, the multiplexers are smart enough to sent the
service effected via alternative route without interruption.
• The demand for survivable services, diverse routing of fiber facilities, flexibility to
rearrange services to alternate serving nodes, as well as automatic restoration within
seconds, have made rings a popular SONET topology.
Network Configs
using two or more ADMs, and a wideband
cross-connect switch, which allows crossconnecting the tributary services at the
tributary level
 using a broadband digital cross-connect
switch, which allows cross-connecting at
both the SONET level and the tributary
level

advantages