QUESTION:
What is the difference between an E1 and 2048 kHz signal?
ANSWER:
The primary differences between an E1 (output) signal and a 2048 kHz (output) signal are the format and electrical characteristics.
E1 is a bit stream (1s and 0s) that has a framing pattern associated with it so that the bit-stream can be broken down into frames and octets within the frames correspond to channels, and so forth. Electrically, as it appears on the cable pair, an E1 signal comprises bipolar (i.e. +ve and -ve voltage polarity) pulses that are "return-to-zero". Generally speaking the presence of a pulse, of either polarity is considered a binary "1" and absence of a pluse interpreted as a binary "0". Also, the polarity of pulses used for sending a binary "1"s alternate as in Alternate Mark Inversion (AMI). However, if there is a (long) string of binary "0"s, there will be "silence" (lack of pulses) which in turn will affect clock recovery. To avoid this situation E1 uses a scheme called HDB3 in order to suppress the occurence of more than 2 absent pulses (which could occur if there were 3 three or more "0"s in the binary information bit-stream) whereby pulses are inserted for binary "0" data and a bipolar-rule-violation method used to identify where this substitution has taken place. Since it can carry actual information, an E1 stream can, if necessary, carry sync status messages (SSM). In an E1 input module special circuitry is required to extract the underlying clock signal from this bipolar voltage signal.
A 2048 kHz output signal is just that. It comprises a square wave of nominally 50% duty cycle and in most implementations a zero dc bias. There is no binary (data) information transferred, just the clock waveform. The notion of clock extraction does not arise since the waveform is the clock signal itself.
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