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Cisco's ISR 3303
Cisco's new ISR 3303 is a platform targeted at service providers with fiber-based infrastructures in metro areas and enables delivery of voice services and provisioned Internet protocol services. Cisco hopes to attract competitive local exchange carriers, regional Bell Operating companies and cable companies because of the lower cost of ownership. (See Organizations and Industries for more information.)
Packet over SONET
Many start-up companies who are beginning to develop access products at 155 Mbps are utilizing the strengths of Packet over SONET (PoS). They are trying to provide higher-speed access ramps, for example, for enterprises who use Frame Relay and are looking for a next-step migration solution from their current T1 or sub-T1 bandwidth crunches. It would be logical for such companies to keep in step with a perfect partner, namely, by accessing an uplink of 155 Mbps to connect to ISPs who are already running PoS networks.
The transport network using SONET provides much more powerful networking capabilities than existing asynchronous systems. The key benefits provided by SONET are:
As a result of SONET transmission, the network's clocks are referenced to a highly stable reference point. Therefore, the need to align the data streams or synchronize clocks is unnecessary. Therefore, a lower rate signal such as DS1 is accessible, and demultiplexing is not needed to access the bitstreams. Also, the signals can be stacked together without bit stuffing.
For those situations in which reference frequencies may vary, SONET uses pointers to allow the streams to "float" within the payload envelope. Synchronous clocking is the key to pointers. It allows a very flexible allocation and alignment of the payload within the transmission envelope.
Separate M13 multiplexers (DS1 to DS3) and fiber optic transmission system terminals are used to multiplex a DS1 signal to a DS2, DS2 to DS3, and then DS3 to an optical line rate. The next stage is a mechanically integrated fiber/multiplex terminal.
In the existing asynchronous format, care must be taken when routing circuits in order to avoid multiplexing and demultiplexing too many times since electronics (and their associated capital cost) are required every time a DS1 signal is processed. With SONET, DS1s can be multiplexed directly to the OC-N rate. Because of synchronization, an entire optical signal doesn't have to be demultiplexed, only the VT or STS signals that need to be accessed.
Because of different optical formats among vendors' asynchronous products, it's not possible to optically connect one vendor's fiber terminal to another. For example, one manufacturer may use 417 Mb/s line rate, another 565 Mb/s.
A major SONET value is that it allows mid-span meet with multi-vendor compatibility. Today's SONET standards contain definitions for fiber-to-fiber interfaces at the physical level. They determine the optical line rate, wavelength, power levels, pulse shapes, and coding. Current standards also fully define the frame structure, overhead, and payload mappings. Enhancements are being developed to define the messages in the overhead channels to provide increased OAM&P functionality.
SONET allows optical interconnection between network providers regardless of who makes the equipment. The network provider can purchase one vendor's equipment and conveniently interface with other vendors' SONET equipment at either the different carrier locations or customer premises sites. Users may now obtain the OC-N equipment of their choice and meet with their network provider of choice at that OC-N level.
The difference between point-to-point and multipoint systems was shown previously in Figures 26 and 27. Most existing asynchronous systems are only suitable for point-to-point, whereas SONET supports a multipoint or hub configuration.
A hub is an intermediate site from which traffic is distributed to three or more spurs. The hub allows the four nodes or sites to communicate as a single network instead of three separate systems. Hubbing reduces requirements for back-to-back multiplexing and demultiplexing, and helps realize the benefits of traffic grooming.
Network providers no longer need to own and maintain customer-located equipment. A multi-point implementation permits OC-N interconnects or mid-span meet, allowing network providers and their customers to optimize their shared use of the SONET infrastructure.
Convergence is the trend toward delivery of audio, data, images, and video through diverse transmission and switching systems that supply high-speed transportation over any medium to any location. Tektronix is pursuing every opportunity to lead the market providing test and measurement equipment to markets that process or transmit audio, data, image, and video signals over high-speed networks.
With its modular, service-independent architecture, SONET provides vast capabilities in terms of service flexibility. Many of the new broadband services may use Asynchronous Transfer Mode (ATM) -- a fast packet-switching technique using short, fixed-length packets called cells. Asynchronous Transfer Mode multiplexes the payload into cells that may be generated and routed as necessary. Because of the bandwidth capacity it offers, SONET is a logical carrier for ATM.
Figure 30. ATM cell consists of two parts: a five-byte header and a 48-byte information field.
An ATM-based network is bandwidth-transparent, which allows handling of a dynamically variable mixture of services at different bandwidths. ATM also easily accommodates traffic of variable speeds. An example of an application that requires the benefits of variable-rate traffic is that of a video CODEC. The video signals can be packed within ATM cells for transport.
Grooming refers to either consolidating or segregating traffic to make more efficient use of the facilities. Consolidation means combining traffic from different locations onto one facility.
Segregation is the separation of traffic. With existing systems, the cumbersome technique of back-hauling might be used to reduce the expense of repeated multiplexing and demultiplexing.
Grooming eliminates inefficient techniques like back-hauling. It's possible to groom traffic on asynchronous systems, however to do so requires expensive back-to-back configurations and manual DSX panels or electronic cross-connects. By contrast, a SONET system can segregate traffic at either an STS-1 or VT level to send it to the appropriate nodes.
Grooming can also provide segregation of services. For example, at an interconnect point, an incoming SONET line may contain different types of traffic, such as switched voice, data, or video. A SONET network can conveniently segregate the switched and non-switched traffic.
Asynchronous systems are dominated by back-to-back terminals because the asynchronous fiber-optic transmission system architecture is inefficient for other than point-to-point networks. Excessive multiplexing and demultiplexing are used to transport a signal from one end to another, and many bays of DSX-1 cross-connect and DSX-3 panels are required to interconnect the systems. Associated expenses are the panel, bays, cabling, the labor installation, and the inconveniences of increased floor space and congested cable racks.
The corresponding SONET system allows a hub configuration, reducing the need for back-to-back terminals. Grooming is performed electronically so DSX panels are not used except when required to interface with existing asynchronous equipment.
SONET allows integrated network OAM&P (also known as OA&M), in accordance with the philosophy of single-ended maintenance. In other words, one connection can reach all network elements (within a given architecture); separate links are not required for each network element. Remote provisioning provides centralized maintenance and reduced travel for maintenance personnel which translates to expense savings.
Substantial overhead information is provided in SONET to allow quicker troubleshooting and detection of failures before they degrade to serious levels.
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