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In a syschronus communication between two DTEs, the frames sent are composed of an initial burst of bits (0s and 1s) to force the reciever DTE to synchronize its clock. Is it possible that the network interface controller of one of the DTEs has a (tangible) higher clock speed than the other? If so, How this synchronus communication is achived? |
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First, remember that you actually have some sort of DCE device between the two DTE's that are trying to communicate. So, a generic example: Router --> CSU/DSU --> Network Cloud --> Remote CSU/DSU --> Remote Router In the "real world", you would never just place two random DTE devices in synchronous communication with each other without configuring an agreed clock source prior to activation. In other words, there must be a source driving a clock. Both DCE devices (assuming a point-to-point connection) must agree on what will be driving the clock. There are three generally accepted methods of clocking: internal, external (also called network or master/slave), and derived (also called recovered). By far, the prevalent source of clocking is external clocking. This permits your ISP/carrier/telco to set the clock rate and your device follows. Thus, with all respect, your question is slightly misworded/misguided. When a new line is brought up, if (and only if) external (or master/slave) clocking is configured, the slave clock will be "forced" to a certain clock rate by the master clock. It's possible they two clocks had different speeds originally, but they will be "forced" into alignment by the nature of the master/slave relationship. Now, let's take your question and assume an internal clocking configuration. Since each CSU (or DCE) runs their own clock, they will never synchronize. Remember this is at layer 1: there is no inherent "negotiation" ability. Recovery and fallback requires on the transmitting and receiving hardware. FWIW: Troubleshooting clocking issues takes up a large percentage of my WAN troubleshooting. Add scenarios when jitter (short-term) and wander (long-term) deviations occur, and clock issues can be very difficult to troubleshoot. |
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Sometimes the data Terminating Equipment, or the Data Set (modem, switch, hub) can operate at different frequencies (600,1200,2400 … 96000 … 10M, 100M, 1G etc) so there needs to be some way to know. As well as frequency synchronisation, there is phase synchronisation: align to start of packet, byte, bit. There are a number of ways to do this, the sending special bit sequences at the start is one way. May be migrate to http://electronics.stackexchange.com/ as this is now the edge of my knowledge, and I think you may find more knowledgeable people on an electronics site. |
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