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Saturday, February 10, 2007

ECE124 Data Communications - Research Work No. 3 - Synchronization

ECE 124

Research Work No. 3

Synchronization

Synchronization is a problem in timekeeping which requires the coordination of events to operate a system in unison. Systems operating with all their parts in synchrony are said to be synchronous or in sync. Some systems may be only approximately synchronized, or plesiochronous. For some applications relative offsets between events need to be determined, for others only the order of the event is important. Today, synchronization can occur on a global basis due to GPS-enabled timekeeping systems. For digital logic and data transfer, a synchronous object requires a clock signal. Timekeeping technologies such as the GPS satellites and Network time protocol (NTP) provide real-time access to a close approximation to the UTC timescale, and are used for many terrestrial synchronization applications.

Serialized data is not generally sent at a uniform rate through a channel. Instead, there is usually a burst of regularly spaced binary data bits followed by a pause, after which the data flow resumes. Packets of binary data are sent in this manner, possibly with variable-length pauses between packets, until the message has been fully transmitted. In order for the receiving end to know the proper moment to read individual binary bits from the channel, it must know exactly when a packet begins and how much time elapses between bits. When this timing information is known, the receiver is said to be synchronized with the transmitter, and accurate data transfer becomes possible. Failure to remain synchronized throughout a transmission will cause data to be corrupted or lost.

Two basic techniques are employed to ensure correct synchronization. In synchronous systems, separate channels are used to transmit data and timing information. The timing channel transmits clock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the data channel and latches the bit value found on the channel at that moment. The data channel is not read again until the next clock pulse arrives. Because the transmitter originates both the data and the timing pulses, the receiver will read the data channel only when told to do so by the transmitter (via the clock pulse), and synchronization is guaranteed.

Techniques exist to merge the timing signal with the data so that only a single channel is required. This is especially useful when synchronous transmissions are to be sent through a modem. Two methods in which a data signal is self-timed are nonreturn-to-zero and biphase Manchester coding. These both refer to methods for encoding a data stream into an electrical waveform for transmission.

Assuming that we are sending a collection of bits across the medium in bit-serial mode, regardless of the underlying transmission characteristics, the receiver will receive a bit pattern representing the message. Therefore, the receiver needs to determine:

» the start of each bit cell - bit or clock synchronization

» the start and end of each element (character, byte or octet) - character or byte synchronization

» the start and end of each complete message block (frame) - block or frame synchronization

The complete frame (block) of characters is transmitted as a contiguous stream of bits and the receiver endeavors to keep in synchronization with the incoming bit stream for the duration of the complete block (no start stop codes). There is a need to synchronize at three levels:

» transmitted bit stream, encoded so receiver can be kept in synchronization (digital encoding we discussed last time)

» frames are preceded by one or more reserved bytes to ensure receiver reliably interprets received bit stream on correct byte boundary

» contents of each frame encapsulated between pair of reserved characters for frame synchronization

Between frames either (1) continuous idle bytes or (2) special synch characters are sent to maintain synchronization.

Error Control

– Need mechanism to detect when a bit is in error (parity bits, block checksums, etc)

– Need protocol to request retransmitted copy

Flow Control

– The receiver may not be able to handle all the information sent due to limited processing power, buffer space, etc. A mechanism is needed to negotiate the flow of information.

Data Link Protocols

– We need to agree on format of data being exchanged (bits per byte and encoding) and type and order of messages to achieve reliability.

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