Preamble (communication)
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In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identify the start of a frame, and is also called ''reference signal'' or ''midamble'' in wireless communications. Prefix codes allow unambiguous identification of synchronization sequences and may serve as self-synchronizing code.


Examples

In an audio receiver receiving a bit stream of data, an example of a syncword is 0x0B77 for an AC-3 encoded stream. An
Ethernet packet In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a network packet, data unit on an Ethernet link transports an Ethernet frame ...
with the Ethernet preamble, 56 bits of alternating 1 and 0 bits, allowing the receiver to synchronize its clock to the transmitter, followed by a one-octet start frame delimiter byte and then the header. All USB packets begin with a sync field (8 bits long at low speed, 32 bits long at high speed) used to synchronize the receiver's clock to the transmitter's clock. A receiver uses a physical layer preamble, also called a physical layer training sequence, to synchronize on the signal by estimating frequency and clock offsets. Some documentation uses "preamble" to refer to a signal used to announce a transmission, to wake-up receivers in a low-power mode. While some systems use exactly the same signal for both physical-layer training and wake-up functions, others use 2 different signals at 2 different times for these 2 functions, or have only one or the other of these signals. The Bisync protocol of the 1960s used a minimum of two ASCII " SYN" characters (0x16…0x16) to achieve character synchronization in an undifferentiated bit stream, then other special characters to synchronize to the beginning of a frame of characters. The syncwords can be seen as a kind of
delimiter A delimiter is a sequence of one or more characters for specifying the boundary between separate, independent regions in plain text, mathematical expressions or other data streams. An example of a delimiter is the comma character, which acts a ...
. Various techniques are used to avoid delimiter collision, orin other wordsto "disguise" bytes of data at the data link layer that might otherwise be incorrectly recognized as the syncword. For example,
HDLC High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both c ...
uses bit stuffing or "octet stuffing", while other systems use ASCII armor or
Consistent Overhead Byte Stuffing Consistent Overhead Byte Stuffing (COBS) is an algorithm for encoding data bytes that results in efficient, reliable, unambiguous packet framing regardless of packet content, thus making it easy for receiving applications to recover from malformed ...
(COBS).


Alternatives

In some communication systems, a receiver can achieve character synchronization from an undifferentiated bit stream, or start-of-header synchronization from a byte stream, without the overhead of an explicit syncword. For example, the FSK441 protocol achieves character synchronization by synchronizing on any " space" characters in the messagein effect, every "space" character in the message does double duty as a syncword. For example, CRC-based framing achieves character and start-of-header synchronization. In a self-synchronizing code, every character is, in effect, a syncword, and can be used to achieve character synchronization in an undifferentiated bit stream.


Preamble

In digital communication, preamble is a sequence of known bits sent in each frame. It is used for frame synchronization such as in Ethernet frames, as well as for channel estimation. In Ethernet and other protocols, the preamble can also be used for automatic baud rate detection.


See also

* * Magic number (programming) *
Out-of-band data In computer networking, out-of-band data is the data transferred through a stream that is independent from the main ''in-band'' data stream. An out-of-band data mechanism provides a conceptually independent channel, which allows any data sent via th ...
*
Start Frame Delimiter In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload. ...
* Synchronous idle * Pilot signal


References

{{reflist Synchronization Data transmission Packets (information technology)