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Synchronous Networks

One of the most attractive aspects of SONET deals with how network components send and receive traffic to and from each other. The original, first-generation digital transport networks were designed to work as asynchronous (or more accurately, nearly synchronous) systems. With this approach, each device in the network runs with its own clock, or devices may be clocked from more than one source. That is, the clocks are not synchronized from a central source.6

The purpose of the terminal clock is to locate precisely the digital 1s and 0s in the incoming data stream on the link attached to the terminal—a very important operation in a digital network. Obviously, if bits are lost in certain user traffic (user traffic is called payload in this book) then the traffic may be unintelligible to the receiver. Equally important, the loss of bits or the inability to locate them accurately can cause further synchronization problems. When this situation occurs, the receiver usually does not deliver the traffic to the end user because it is simpler to discard the traffic than to initiate retransmission efforts.

To give the reader an idea of how precise the timing must be, consider a T1 system that operates at a modest 1.544 Mbit/s. Obviously, a receiver must be able to detect each bit as it "shows itself" at the link interface at the receiving machine. Each bit is only 648 ns in duration (1 sec/1544000 = .000000648). This means that the receiver's clock must be aligned accurately with the transmitter's clock.

Because a sender's clock may run independently of the receiver's clock in an asynchronous network, large variations can occur between the sender's clock (machine 1) and the rate at which the bits are received and then transmitted by the receiver's clock (machine 2). The problem is not at the receiving link at machine 2, since machine 2 can "lock" onto machine 1's incoming signal and accept the traffic. In this regard, machine 2 extracts the clock from machine 1's signal.

The problem occurs when machine 2 then prepares that traffic for transmission onto the next outgoing link. If it is using its own clock, it usually varies from the rate that was received from machine 1. These different timing operations can create a big headache for the network administrator. For example, experience has demonstrated that a T3 signal may experience a variation of up to 1789 bit/s for a 44.736 Mbit/s signal in a network that does not have precise and accurate timing.

The Perils of Bit Stuffing

Moreover, T1 signals such as DS1s are multiplexed in stages up to DS2, DS3, etc., and extra bits are added to the stream of data to account for timing variations in each stream. The process is called bit stuffing. The lower level signals, such as DS1, are not accessible nor visible at the higher rates. Consequently, the original stream of traffic must be demultiplexed if these signals are to be accessed. The demultiplexing process is very expensive and adds delay and overhead to the network.

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