Networks and internet

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Chapter 1: Introduction

        In today's fast driven business world, each industry is trying to find different means to deliver Products, Services and information in more timely manner in the most cost effective way. SONET (Synchronous Optical Network) provides one of the most effective solutions to the high demands of the data communication requirements of these fast phased business organization as well as general data communication needs.

        SONET is a transport technology, designed to provide enterprise and government users as well as service providers a network infrastructure with survivability characteristics, which helps business operations continue uninterrupted. The method was primarily developed for replacing the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic from different sources over the same fiber wire without synchronization problems. SONET allows multiple technologies and vendor products to interoperate by defining standard physical network interfaces. It provides the unification of voice, data and video over the same transport service. It is a powerful, highly scalable technology. At the current time SONET/SDH are being implemented for long-haul traffic but there is no difficulty in implementing it for the short distance traffic as well. Fiber is the physical medium of choice in SONET. The advantage of optical fibers is that they are not susceptible to interference and they has a very high transmission speed.


        The foundation of SONET was laid by Telecordia (previously known as Bellcore) in 1980s. They saw the need of replacing the Plesiochronus Digital Hierarchy (PDH) system that was using during that period for telecommunication, with a synchronous network that allows the transportation of circuit mode communications (such as T1 and T3) from a variety of different sources. Until that time PHD had evolved in a ad hock fashion and Telecordia took the step to change this by Planning on SONET and implementing a standard that allowed higher data transmission and properly planned network management facilities. This move was also backed by Regional Bell Operating Companies and inter-exchange carriers (IXCs) in the U.S., Canada, Korea, Taiwan, and Hong Kong as they found that the proprietary systems that were in use during that period was not flexible for interconnection between them. Furthermore they planned the SONET to be able to time lock the digital signal being carried out in the network so that individual lower rate channels could be accessed directly without the need to breakdown the PDH signal by hierarchy level. SONET provided the solution for the synchronization problems that were facing when needed to transmit traffic from different sources in a single medium. Because the circuits if the different transmission sources were operating at a slightly different rate with different phase. SONET architecture allowed the simultaneous transport of traffic from different circuit sources with different origin in a single framing protocol. In this manner SONET it more of a Transport protocol than a communication protocol.

        As mentioned in the introduction chapter, SONET has two variations; SONET itself and SDH, which is based on SONET. American National Standards Institute (ANSI) in association with Exchange Carriers Standards Association (ECSA) and Committee T1 which is sponsored by the Alliance for Telecommunications Industry Solutions (ATIS) made some amendments to initial SONET architecture documentation and standardized it in America as SONET. The European and international version, which is SDH was published and standardized by International Telecommunications Union (ITU).

Chapter 2: SOnet characteristics


        SONET is a transmission standard that uses synchronous signaling. In synchronous signal transmission networks, the digital transitions in the signals occur at exactly the same rate. There may be a possibility of phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may occur due to propagation time delays or jitter introduced into the transmission network. In a synchronous network like SONET, all the clocks are traceable to one Stratum 1 Primary Reference Clock (PRC). The accuracy of the PRC is better than ±1 in 1011 and is derived from a cesium atomic standard.

        SONET facilitates transmission using lasers or light-emitting diodes (LEDs) for carrying many signals (mostly asynchronous) of different capacities through a synchronous, flexible, optical hierarchy which is accomplished through a byte-interleaved multiplexing scheme. Byte-interleaving simplifies multiplexing, and offers end-to-end network management. Following figure shows the fundamental model of SONET architecture.

As shown in the figure multiple user devices (usually many different carriers such as T1 and T1) are connected to SONET multiplexer. Multiplexer converts the incoming signals to frames (STS-1, STS-3 etc)that are placed in the fiber ring.

        The self healing fiber optic characteristic of SONET facilitates automatic network recovery from the failures that may cause by Fiber optic cable cut, Loss or degraded signal (due to aging fiber) or failures in the nodes or system.

        The first phase of SONET multiplexing process involves the generation of the lowest level or base signal. This signal is referred to as Synchronous Transport Signal level-1 or more commonly STS-1. Signals generated at STS-1 operates at 51.84 Mb/s. Higher SONET signal levels creates lights streams with higher data rates. These higher level signals are named using integer multiples of STS-1. Different STS signals are composed of different number of byte interleaved STS-1 Signals.

        The "line rate" is the maximum bit rate carried over the optical fiber. A portion of the bits transferred over the line is overhead data. The overhead carries information that provides OAM&P (Operations, Administration, Maintenance, and Provisioning) services such as status, trace, framing, multiplexing, and performance monitoring. Payload is the bandwidth available for transferring user data such as packets or ATM cells. It is the difference between Line rate and payload rate.

        The signals transmitted in DS carriers are asynchronous in nature. In an asynchronous transmission the transitions of the signals do not necessarily occur at the same nominal rate. What SONET does is convert these asynchronous signals in to synchronous signals and place them in the fiber ring for transmission. SONET architecture was designed to be highly fault tolerant. For this purpose it uses considerable amount of bandwidth for transmitting control and error correction information along with actual payload. The amount of overhead data amount at each STS signal level can be understood.


        SONET frame contains three basic parts. The line overhead, the section overhead, and the synchronous payload. The path overhead is carried within the payload. Each SONET STS-1 (Synchronous Transport Signal) frame contains nine rows with total of 90 bytes. Within 126 microseconds, SONET frame is transmitted row by row. SONET STS-1 level transmission has a throughput of 51.84 Mbits/sec. That is achieved as follows.

  • 9 rows x 90Bytes = 810 bytes or 6480 bits. Each frame is sampled once every 125 microseconds or 8000 frames/sec.
  • 8000 frames/sec x 6480 bits/frame = 51.84 Mbits/sec (Speed of STS-1 SONET payload)

SONETSTS-1 frames contains Synchronous Payload Envelope (SPE) with data overhead which account for 87 bytes. An actual packet or cell within an SPE can span multiple SONET Frames. The first 3 columns In SONET Frames, make up the SONET Transport Overhead (Section and Line overhead), which accounts for 27 bytes (3 bytes X 9 rows). This 27 bytes is divided between 18 bytes for Line overhead and 9 bytes for Section overhead.

  • The Section Overhead contains data for framing, performance monitoring and a voice channel for maintenance personnel and a channel for OAMP. This information is used for section to section communication, such as repeater to repeater communication.
  • The line overhead information is destined for line termination equipments. These include equipment such as Add/Drop terminals. Payload pointer; which indicated the position of the payload inside the frame, also contain in Line overhead section. Furthermore it contains additional OAMP data, line performance monitoring and another voice channel for maintenance personnel.
  • The Payload contains the actual data being transmitted. Section of it contains Path overhead. The reason for carrying path overhead within payload is, the only time it is created and looked at is when payload enters or exits the SONET network. Path overhead's duty is, end-to-end monitoring of the payload and it's performance during the journey across the network and also it make sure that the connection was made correctly, identifies the payload type and provides a user channel for the service providers information.

Each path within any single SONET frame carries data of the same format. This called as mapping for SPE. ATM is one such available SPE mapping. Each path within the SONET stream is treated as a separate logical entity. Thus, each single path can be mapped differently from the other paths carried in that SONET stream.

Each path in a SONET data stream is capable of carrying a number of embedded streams which are of lower transport rates. One STS-1 SONET stream (like the Path 1 shown in the OC-3 frame of Figure 2.3) has the ability to carry twenty-eight 1.728 Mbit/s channels. When the SPE mapping is ATM, the separate paths are collapsed and formed in to a single concatenated path; for an instance, the SONET OC3c type carries data rate of 155.52 Mbit/s . In OC3c, both the line rate and the path rate are 155.52 Mbit/s as all paths are collapsed and concatenated in to one path. Following figure shows the collapsed single path frame format of OC3c.


        As mentioned earlier, SONET was developed having the idea to provide a high speed pathways to multiple data streams through a multiplexing mechanism. The primary task of SONET was to multiplex different PDH channels and transmit their data. PDH data streams are not synchronous in nature. SONET poses the capability to handle these tributary signals such as PDH and multiplex them and place in to higher order synchronous data streams to transport through SONET networks.

SONET transmits 8000 frames per second at 1 frame taking 125 microseconds to transmit. The width of the frame is the difference between various STS level SONET frames. All these types of frames still contain 9 rows, but width of the rows differs according to the specifications for each STS type. The bytes are transmitted from left to right starting from the top most row and one at a time.

Payload pointer plays a critical role in SONET frame. It is the function that make whole concept of SONET a reality. The incoming tributaries in to a SONET multiplexer are mostly non synchronous thus they are not aligned with each other or with the clock in the multiplexer . SONET is a synchronous network, so in order to properly transmit the multiplexed data, SONET multiplexer finds the beginning of a frame for each tributary. It then calculates a pointer designating where in the STS-1 frame it has placed the beginning of the tributary frame. This way, it is not necessary for the multiplexer to either get the signals in sync or stuff the frame with bits. If a tributary signal's clock slips over time with respect to the multiplexer's clock, the SONET multiplexer handle this by recalculating the pointer.

        STM-3 or faster frame creation generally involves multiplexing slower frame types (usually STM-1) into the larger frame. This process is different than creating STS-1 frames. Each byte from the incoming tributaries is interleaved byte-by-byte into the larger frame. Each byte of the tributary signal, and thus the tributary signal itself is visible in the frame. This makes it relatively easy to pull a tributary signal out of the main signal. As an instance, an STS-12 signal is formed by byte-interleaving 12 STS-1 frames. Each byte is separately visible. The various STS-1's could be carrying different types of traffic (voice, data...) and can be heading to different destinations.


SONET uses following functions for Multiplexing.

  • Mapping - Mapping is used when tributaries are adapted into Virtual Tributaries (VTs) by adding justification bits and Path Overhead (POH) information.
  • Aligning - SONET places pointers in the STS Path or VT Path Overhead, to indicate the location of the first byte of the Virtual Tributary. This process of placing pointers is referred to as aligning.
  • Multiplexing - when the higher order path signals are adapted into the Line Overhead or multiple lower-order path-layer signals are adapted into a higher-order path signal Multiplexing is used.
  • Stuffing - SONET has the capability to handle various asynchronous tributary input signal which are of various rates. SONET being a synchronous technology, As the tributary signals are multiplexed and aligned, some spare space has been included into the SONET frame to provide enough space for all these various tributary rates so all the input signals can be transformed in to uniform frames. At certain points in the multiplexing hierarchy this void space capacity is filled with "fixed stuffing" bits that carry no information, but are required to fill up in order to form the complete frame. This process is known as Stuffing.

        SONET is designed to carry large payloads at one time. However, one of the main goals of designing SONET is to facilitate the transmission of existing digital hierarchy signals. As digital hierarchy signals are normally smaller than the smallest SONET transport format, which is STS-1 that operates at 51.840 Mb/s, STS Synchronous Payload Envelope is sub-divided into smaller components or structures, known as Virtual Tributaries (VTs) for the purpose of transporting and switching payloads smaller than the STS-1 rate such as digital hierarchy signals. All services below DS3 rate are transported in the VT structure.

        A service adapter maps the signal into the payload envelope of the STS-1 or virtual tributary (VT). Service adapters has the capability to accept any type of services such as voice, high-speed data and video. New services and signals can be transported by adding new service adapters at the edge of the SONET network.

All Input signals are converted to a base format of a synchronous STS-1 signal (51.84 Mb/s or higher) Except for concatenated signals. All lower speed inputs such as DS1s are first bit- or byte-multiplexed into virtual tributaries. Several synchronous STS-1s are then multiplexed together in either a single- or two-stage process to form an electrical STS-N signal (N = 1 or more).

Byte Interleave Synchronous Multiplexer performs the STS multiplexing. Basically, the bytes are interleaved together in a format such that the low-speed signals are visible. This characteristic makes it easier if needed to extract the lower speed signal from the SONET frame. Once this process is done before placing the data in the fiber medium, the only function takes place is direct conversion of input electrical signal to optical to form an OC-N signal.

Chapter 3: SOnet Topologies

SONET technology can be deployed using 4 different types of topologies. These topologies can be deployed depending on the survivability and bandwidth efficiencies needed as well as cost. The SONES topologies include,

  • Point-to-point configuration
  • Hubbed configuration
  • Linear Add/Drop configuration
  • Self healing Ring configuration - Self healing Ring configurations has two variants; Bi-directional Line Switched Self-Healing Ring (BLSR) and Unidirectional Path-Switched Ring (UPSR)
  • Dual Ring Interconnection (DRI)
  • Folded Rings.


Following figure shows the basic multiplexing structure of SONET.

Point-to-point configurations are typically deployed in transport applications, which require a single SONET multiplexer in a single route. Point-to-point SONET network is a simple topology that terminates a SONET payload at each point of a fiber optic cable span. The survivability of Point-to-point SOENT networks can be enhanced by deploying a second fiber span over a different path between two or more SONET multiplexers.

Following figure shows the basic multiplexing structure of SONET.

Device configuration in a Point to Point configuration is shown in the above diagram. In this type of a configuration, SONET path terminating terminal multiplexer (PTE) acts as a concentrator of DS1s as well as other tributaries. The simplest form of this configuration deployment involves two terminal multiplexers linked by fiber with or without a regenerator (REG) in the link. In a point to point configuration, In a point to point configuration, and the Service path (DS1 or DS3 links end-to-end) are identical and this synchronous island can exist within an asynchronous network world. In future networks, Point to point service path connections will span across the whole network and will always originate and terminate in a multiplexer.


Following figure shows the basic multiplexing structure of SONET.

In a hubbed configuration SONET network, traffic from multiple sites are consolidated on to a single optical channel which is then forwarded to another site. This topology is often used in situations where traffic from multiple satellite sites needs to consolidated to a single site (such as corporate headquarters) before extending it to somewhere else like a central office. Using this topology, a multisite topology can be created using lesser amount of equipments thus reducing number of hops. This network architecture accommodates unexpected growth and changes more easily than simple point-to-point networks.

In a hub configuration SONET architecture, (as shown in above figure) A hub concentrates traffic at a central site and allows easy reprovisioning of the circuits.

This type of network can be implemented in the following two ways.

  • 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.


        SONET linear Add/Drop configuration eliminates the need to process (multiplex/demultiplex) the entire optical signal for pass-through traffic. This is done by enabling direct access to VTS/STS channels at each intermediate site along a fiber optic path.

        In this kind of a configuration, circuits are added and dropped c along the way. The SONET ADM (add/drop multiplexer) is a unique network element specifically designed for this task. It avoids the current cumbersome network architecture of demultiplexing, cross connecting, adding and dropping channels, and than re-multiplexing. The adding and dropping of tributary channels at intermediate points in the network is facilitated by placing ADM along a SONET link.


        A mechanism called Automatic Protection Switching is employed in Self-Healing Ring configuration networks. There are two variants of protection ring topologies. They are UPSR (unidirectional Path Switched Ring) and the other is BLSR (Bi-directional Line Switched Ring). Each of these ring topologies is discussed later. Ring configuration poses high survivability, which is a main advantage. If a fiber cable is cut, the multiplexers have the intelligence to send the services affected via an alternate path through the ring without interruption. The switching to alternate path or the secondary ring happens with seconds that the downtime is negligible, this feature as well as routing of fiber facilities, flexibility to rearrange services to alternate serving nodes have made Ring architecture a Popular SONET topology.

        In this configuration, a backup fiber span (protection ring) is enabled when and if there is a failure within the fiber span currently carrying traffic on a SONET network. Both fiber spans are always active during normal operating conditions and SONET multiplexer selects which fiber span to receive traffic, based on an internal algorithm. The SONET standard specifies, In the event of a failure on the current fiber ring that carries data, the protection ring should automatically become the fiber span (ring) the SONET multiplexer receives traffic from within 60 milliseconds which is barely noticeable to the user.

In the following type of operational problems that can occur, the protection ring will take over and become the fiber span (ring):

  • A break in the fiber cable
  • Signal failure (e.g. laser problem)
  • Signal degrade (which can happen when an old laser fail due to age)
  • Node failure


        In this type of SONET configuration exists, duplicate, geographically diverse paths for each SONET service which uses a closed-loop transport design.

        The closed-loop design implements bi-directional fiber spans, where each dual fiber span handles one direction of SONET traffic. Each direction of SONET traffic carries a duplicate copy of the traffic, one on the working ring and the other on the protection ring. Along the path, each multiplexer compares the integrity the SONET signal. BSLSR configuration automatically switches traffic to the surviving fiber span, When cable cuts, signal failures or signal degrades are detected. Typical application area of BSLR is interoffice applications where distributed meshed or linear node-to-node configurations are implemented.


        A UPSR configuration is made up of a single fiber optic pair that interconnects neighboring SONET multiplexers. Some SONET multiplexers (e.g. DDM-2000) support the Unidirectional Path Switched Ring (UPSR) configuration to support critical ring applications. In UPSR networks, survivability is ensured using a closed-loop transport that automatically protects against cable cuts, signal failure, or signal degrades by implementing duplicate, geographically diverse paths for each SONET service.

        In a UPSR rings configuration as shown in the figure, Traffic in the working path flows in one direction while traffic on the protection path flows in the opposite direction. A SONET multiplexer continuously monitor the Traffic on each path. Traffic is automatically diverted to the backup fiber protection path when there is a fiber failure or signal degradation is detected on the working fiber path. UPSR configurations are usually implemented in SONET access networks where traffic is terminated at a central office.


        DRI configuration is typically deployed in central office topologies. This type of configuration allows multiple rings sharing traffic to be resilient from a SONET multiplexer node failure. It is designed to route the traffic through the operational SONET multiplexer If a catastrophe takes out a SONET multiplexer system. Thus it provides the network continuity and business operation even in a situation where a failure has occurred.


        In folded ring configuration equipment costs is minimized and provides a simple configuration. , a single path ring provides simplicity and minimizes equipment costs. A two-fiber span path switched ring can be configured in SONET folded ring configuration when route diversity is not available to setup the network.


        Due to its scalability, high performance, and inherent reliability and survivability characteristics, Many utility, civil government and large enterprise customers have selected to deploy SONET on their enterprise backbones. These government and enterprise customers either bypass their local carriers and self-manage the entire SONET infrastructure, or they tie into a Central Office (CO) to allow their local service provider to offer WAN connectivity or network management services using high speed SONET networks. Major Service Providers such as SPRINT-SONET Sphere, U.S. West Network 21, and MCI are deploying significant number of SONET equipment in to the field.

        The primary application areas of SONET is transportation of DS-1, DS-3, ATM, FDDI, Frame Relay and IP traffic. In North America SONET equipment is being highly deployed for transportation of these types of traffic. SONET is used in many long-haul networks built by IXCs or LECs entering the inter-LATA market. A long-haul SONET network deploys an OC-n (usually OC-48, or the even higher OC-192 on occasion) high bandwidth transport network over an existing point-to-point fiber network (usually a 565 Mbps or other speed FOTS network with multiple DS-3s on a nonstandard fiber system).

        Another area of deploying SONET is Satellite and Cable TV distribution. In this method, a higher capacity (perhaps an OC-48) SONET link can be deployed to connect to another SONET switching node which could have access to an existing microwave or cellular network, and existing FOTS facilities. This node could function as the entry point for a cable TV head end; thus, SONET would provide the distribution network for cable TV video signals.

        In private organizations, SONET fiber rings are used as backbones. One such situation is the use of SONET fiber ring as the backbone for network architectures such as Frame relay switches.

        SONET technology is also highly used in transferring ATM traffic for metro and long-haul applications.

Another application area of SONET is the interconnection of LANs. Today the LANs are no longer restricted to older 4,10 and 16 Mbps Ethernet and Token rings. Fast Ethernet (100 Mbps) and Gigabit Ethernet (1000 Mbps) is already deployed in may LANs in use today. Interconnecting these type of high-speed LANs require a high-speed backbone. SONET has the potential to provide that.

Chapter 4: SONET equipments and functionality

        Synchronous networks like SONET should poses the ability to transmit plesiochronous signals and at the same time be capable of handling services such as ATM. These functions require the use of various network equipments. This section describes the equipments used in SONET networks and their functionality.


        SONET consists of 3 different equipment Layers which facilitate the end-to-end connection. This includes Path Terminating Equipment (PTE), Line Terminating Equipment (LTE), and Section Terminating Equipment (STE).


        SONET line terminating equipment provides the functionality of originating and terminates line signals. SONET line terminating equipment has the ability to originate, access, modify, or terminate line overhead in any combination.


        Multiplexing and demultiplexing functions are provided by STS (Synchronous Transport Signal) path terminating equipments. Path terminating equipment poses the ability to originate, access, modify, or terminate path overhead in any combination.


        In a SONET configuration, any two neighboring SONET network elements is referred to as a "section". A SONET section terminating equipment can be a network element or a SONET regenerator. These section terminating equipment have the ability to originate, access, modify, or terminate section overhead in any combination.


        Terminal Multiplexer is a path terminating element (PTE). The role of path terminating terminal multiplexer is to acts as a concentrator of DS1s as well as other tributary signals. Its simplest deployment would involve two terminal multiplexers linked by fiber with or without a regenerator in the link. This implementation represents the simplest SONET link (a Section, Line, and Path all in one link).


        As the distance between two multiplexers grows, the signal level of the fiber becomes low. The regenerator clocks itself off of the received signal and replaces the Section Overhead bytes and re-transmitting the signal. The Line Overhead, payload, and Path Overhead are not altered. Its function is similar to that of a Repeater in a LAN.


        Through the use of ADMs, Plesiochronous and lower bit rate synchronous signals can be dropped from or inserted into SONET bit streams without affecting the other traffic. This feature makes it possible to set up ring structures, which have the advantage that automatic back-up path switching is possible using protection bandwidth in the ring in the event of a fault.


        A SONET cross-connect accepts various optical carrier rates, accesses the STS-1 signals, and switches at this level. The most ideal place to use cross-connects is at a SONET hub. A cross-connect may be used to interconnect a much larger number of STS-1s. This is the major difference between a cross-connect and an add-drop multiplexer. Consolidating or segregating of STS-1s or broadband traffic management can be done by using Cross connects. For example, it may be used to separate high-bandwidth from low-bandwidth traffic and send them separately to the high-bandwidth (e.g., video) switch and a low-bandwidth (voice) switch. Wideband Digital Cross Connects is the synchronous equivalent of a DS3 digital cross-connect and supports hubbed network architectures. It is similar to a DS3/1 cross-connect as it accepts DS1s, DS3s, and the built-in optical interfaces allows it to accept optical carrier signals.

A W-DCS accepts OC-N signals as well as STS-1, DS-1 and DS-3 signals. Switching is at DS-1 and VT1.5. A cross connect has the ability to drop containers from any OC-N signal and also The received signals can be connected from any input port to any output port at the different levels even with asynchronous signals.

At W-DCS only the required tributaries are accessed and switched because of this less demultiplexing and multiplexing is required. This is one major advantage of wideband digital cross-connects.


        The Broadband Digital Cross-Connect interfaces various SONET signals and DS3s. At STS-1 level, it access signals and switches. except the fact that broadband digital cross-connect accepts optical signals and allows overhead to be maintained for integrated OAM&P (asynchronous systems prevent overhead from being passed from optical signal to signal), Broadband digital cross connects is the synchronous equivalent of the DS3 digital cross-connect.

At the DS3, STS-1, and STS-Nc levels, The Broadband Digital Cross-Connect can make two-way cross-connections. B-DCS is best used as a SONET hub where it can be deployed for grooming STS-1s, Broadband restoration or for routing traffic.


        The Digital Loop Carrier (DLC) can be thought of as a concentrator of low speed services before they are brought into the local central office for distribution. The purpose of this concentration is to eliminate the limitation of service capability of the SONET service provide company. If this concentration were not performed, the number of subscribers (or lines) that a central office could serve would be capped to the number of lines served by the Company. The DLC itself is a system of multiplexers and switches that perform concentration from the remote terminals to the community dial office and, from there, to the central office. A DLC is usually deployed for service in the central office or a controlled environment vault (CEV) that belongs to the SONET carrier company.

        Generic DLC is consist of intelligent Remote Digital Terminals (RDTs) and digital switch elements called Integrated Digital Terminals (IDTs), which are connected by a digital line (OC-1, OC-3 etc).


        An Add/Drop Multiplexer (ADM) is a form of digital cross-connect system which multiplexes STS (Synchronous Transport Signals) input signals (e.g. DS0, DS1 signals) into Optical Carrier channels (signals). A single-stage multiplexer/demultiplexer perform the multiplexing of various inputs into an OC-N signal. At an add/drop site, only the signals that need to be accessed are dropped or inserted. The adding and dropping of payloads onto one of two channels are referred to as Add and Drop. The input signals (traffic) not dropped onto the channel at a given site are passed directly through an Add/Drop Multiplexer without being processed.

An ADM has the ability to consolidate or segregate traffic from multiple sites. The above figure shows, an Add/Drop Multiplexer (ADM) system where certain DS1 & DS3 signals are added and dropped to one site, while other traffic is sent on through the SONET network.

SONET supports drop and repeat (also known as drop and continue): a key capability in both telephony and cable TV applications. Using drop and repeat, a signal terminates at one node, is duplicated (repeated), and is then sent to the next and subsequent nodes. In ring-survivability applications, drop and repeat provides alternate routing for traffic passing through interconnecting rings in a "matched-nodes" configuration. If the connection cannot be made through one of the nodes, the signal is repeated and passed along an alternate route to the destination node.

Chapter 5: Advantages and COST Comparison

        The increased configuration flexibility and availability of high bandwidth of SONET provides significant advantages. These include;

  • Increased network reliability while reducing the equipment requirements
  • The overhead bytes permit centralized fault sectionalization through the management of the payload bytes on an individual basis.
  • Simplification of interface to digital switches, digital cross-connect switches and add-drop multiplexers through definition of synchronous multiplexing format for carrying lower level digital signals (such as DS1, DS3) and a synchronous structure.
  • Facilitates set of generic standards which enable products from different vendors to be connected. This solves the interconnection problems of proprietary standards.
  • Facilitates a flexible architecture capable of accommodating future applications, with a variety of transmission rates
  • SONET is based on the principal of direct synchronous multiplexing. In this method separate, slower signals can be multiplexed directly onto higher speed SONET signals without intermediate stages of multiplexing.
  • Both SONET and SDH have the ability to transport signals for all the networks in existence today and it has the flexibility to facilitate service for any networks defined in future. This is a huge advantage of SONET.
  • SONET can be used in the three traditional telecommunications areas: long-haul networks, local loop carrier networks. It can also facilitate the carriage of CATV video traffic.
  • Currently, SONET is the ideal platform for services ranging from POTS, ISDN and mobile radio through to data communications (LAN,WAN, etc.), and it is able to handle new, upcoming services such as video on demand and digital video broadcasting via ATM.
  • With SONET, network providers can react quickly and easily to the requirements of their customers. For example, leased lines can be switched in a matter of minutes. The network provider can use standardized network elements that can be controlled and monitored these features result in high availability and capacity matching.

        The speed and cost of SONET make the technology competitive with alternatives like ATM. This is doubled by the unmatched survivability posses in SONET networks.

        However the Emerging technologies like Gigabit Ethernet and 10 Gbit Ethernet is posing threats to SONET in terms of Cost and throughput. SONET compared to Ethernet is highly expensive. But the benefits offered by SONET is unmatched by Ethernet. SONET is generally considered superior to Ethernet in both general robustness and in technological areas related to timing and synchronization, which given the essentially time-sensitive data payloads often associated with the MAN market is definitely a selling point for SONET.

        The scalability of SONET is far from simple when compared to Ethernet; if needs to double or triple the performance of a SONET ring, the challenge involved will be much more complex than for an Ethernet network of comparable initial performance. And SONET is expensive - at least compared to Ethernet. Initial costs and operating expenses necessarily vary by vendors and circumstances, but many have found Ethernet to be roughly one-fourth the cost of SONET for a given set of services.

Chapter 6: Summery and Conclution

        SONET is an international standard that is being widely adopted. It will continue to penetrate to new application areas as the demand for high bandwidth critical applications continues to emerge every day. The capability of SONET standard shows form the fact that it has the ability to transport all signals currently defined in the world today.

        The emerging Synchronous Optical Network standard (SONET) specifies common rules to ensure efficient, high volume transport of digitized voice, image or data communications on fiber-optic networks. This allows communication between different vendors' equipment in a standard form without conflicts.

        The explosive growth of the Internet and World Wide Web, have created an unprecedented demand for higher bandwidth among ISPs, telephone companies, and even Fortune 500 companies. Furthermore, Applications emerging from the convergence of telephony and data communications, such as video-on-demand, HDTV, video teleconferencing, Voice over Internet (VOIP) and other applications with high bandwidth requirements have made the demand for high speed, high bandwidth data transport technologies such as SONET is a must rather than an option. Large ISPs are continuously in the battle of providing best possible bandwidth for multimedia rich web applications that are emerging. SONET has the potential to accommodate all that high demand and provide high speed data transfer between networks across countries and across the continents.

        The ability of SONET to provide high bandwidth of up to 10Gbps for voice and data transmission has made SONET the standard for encapsulating and transporting data for telecommunications carrier products.

        Predictions indicate that the global market for SONET will grow at 17.4% compounded rate annually over the next five years. Despite the higher cost of setting up SONET architectures, it has been adopted by many long haul telecommunication carriers as well as large organizations due to it's superior stability and survivability characteristics. As the demand for internet applications such as on demand video and Hi-definition and 3d video capabilities continue to be build in to web applications, the demand for SONET networks will go further higher. Furthermore the next big thing in the internet, which is the emergence of Cloud computing will certainly boost the demand for high speed networking technologies. This will result in more and more SONET and other high speed networks being created.



  • SONET - Walter J. Goralski, McGraw-Hill Osborne, 2000, p 489


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