computer network A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...

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

data transmission Data communication, including data transmission and data reception, is the transfer of data, signal transmission, transmitted and received over a Point-to-point (telecommunications), point-to-point or point-to-multipoint communication chann ...

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 cal ...

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

Prefix code A prefix code is a type of code system distinguished by its possession of the prefix property, which requires that there is no whole Code word (communication), code word in the system that is a prefix (computer science), prefix (initial segment) of ...

s 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 ...

Examples

In an audio receiver 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 may ...

of data, an example of a syncword is 0x0B77 for an AC-3 encoded stream. An Ethernet packet 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 Universal Serial Bus (USB) is an industry standard, developed by USB Implementers Forum (USB-IF), for digital data transmission and power delivery between many types of electronics. It specifies the architecture, in particular the physical ...

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 Character (computing), characters for specifying the boundary between separate, independent regions in plain text, Expression (mathematics), mathematical expressions or other Data stream, data streams. An ...

. 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 ...

, 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 communication protocol used for transmitting data between devices in Telecommunications, telecommunication and Computer network, networking. Developed by the International Organization for Standardization ...

uses

bit stuffing In data transmission and telecommunications, 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 f ...

or "octet stuffing", while other systems use

ASCII armor A binary-to-text encoding is encoding of data in plain text. More precisely, it is an encoding of binary data in a sequence of printable characters. These encodings are necessary for transmission of data when the communication channel does not a ...

or Consistent Overhead Byte Stuffing (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 a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...

" 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 communication, including data transmission and data reception, is the transfer of data, transmitted and received over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical ...

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

frame synchronization In telecommunications, frame synchronization or framing is the process by which, while receiving a stream of fixed-length frames, the receiver identifies the frame boundaries, permitting the data bits within the frame to be extracted for decodin ...

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 paylo ...

s, as well as for channel estimation. 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