computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

s, a syncword, sync character, sync sequence or preamble is used to synchronize a

data transmission Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point or ...

by indicating the end of header information and the start of data. The syncword is a known

sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...

of data used to identify the start of a frame, and is also called ''reference signal'' or ''midamble'' in

wireless communications Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...

. Prefix codes allow unambiguous identification of synchronization sequences and may serve as

self-synchronizing code In coding theory, especially in telecommunications, a self-synchronizing code is a uniquely decodable code in which the symbol stream formed by a portion of one code word, or by the overlapped portion of any two adjacent code words, is not a val ...

Examples

In an

audio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The ...

receiving a

bit stream A bitstream (or bit stream), also known as binary sequence, is a sequence of bits. A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet ma ...

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 data unit on an Ethernet link transports an Ethernet frame as its payload. ...

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 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 as ...

, orin other wordsto "disguise" bytes of data at the

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...

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 bot ...

uses

bit stuffing In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits. Bit stuffing is used for ...

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 Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...

" 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

, every character is, in effect, a syncword, and can be used to achieve character synchronization in an undifferentiated bit stream.

Preamble

digital communication Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point or ...

, preamble is a sequence of known bits sent in each frame. It is used for

frame synchronization In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals (i.e., a distinctive bit sequences or syncwords) are identified (that is, distinguished fro ...

such as in

Ethernet frame 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 ...

s, as well as for

channel estimation In wireless communications, channel state information (CSI) is the known channel properties of a communication link. This information describes how a signal propagates from the transmitter to the receiver and represents the combined effect of, for ...

. In Ethernet and other protocols, the preamble can also be used for automatic baud rate detection.

References

{{reflist Synchronization Data transmission Packets (information technology)

Examples

Alternatives

Preamble

See also

References