SONET Synchronous Optical Network

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 Overview 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 = Higher level signals are multiples STS-3 = 3 x = DS-X Date Rate DS0 Multiple T- Carrier DS0 64 Kbps 1 DS M bps 24T-1 DS Mbps 96T-2 DS Mbps 672T-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 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 frame structure has been designed to contain a large amount of overhead information that provides a variety of management and other functions.

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. 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. 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. failure—This is the inability of a function to perform a required action persisted beyond the maximum time allocated. SONET Alarm Structure

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.

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 Network Configurations (cont) Allow for simpler unexpected growth. A hub concentrates traffic at a central site and allows easy reprovisioning of the circuits. Digital Cross-Connect System: (DCS) Has access to lower-rate channels in higher-rate multiplexed signals and can electronically rearrange those channels

Ring Architecture Network Configurations (cont) 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 cross- connecting the tributary services at the tributary level using two or more ADMs, and a wideband cross-connect switch, which allows cross- connecting 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 using a broadband digital cross-connect switch, which allows cross-connecting at both the SONET level and the tributary level

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