Frame (networking)

A frame is a digital data transmission unit in computer networking and telecommunications. In packet switched systems, a frame is a simple container for a single network packet. In other telecommunications systems, a frame is a repeating structure supporting time-division multiplexing.

A frame typically includes frame synchronization features consisting of a sequence of bits or symbols that indicate to the receiver the beginning and end of the payload data within the stream of symbols or bits it receives. If a receiver is connected to the system during frame transmission, it ignores the data until it detects a new frame synchronization sequence.

Packet switching

In the OSI model of computer networking, a frame is the protocol data unit at the data link layer. Frames are the result of the final layer of encapsulation before the data is transmitted over the physical layer.^[1] A frame is "the unit of transmission in a link layer protocol, and consists of a link layer header followed by a packet."^[2] Each frame is separated from the next by an interframe gap. A frame is a series of bits generally composed of frame synchronization bits, the packet payload, and a frame check sequence. Examples are Ethernet frames, Point-to-Point Protocol (PPP) frames, Fibre Channel frames, and V.42 modem frames.

Often, frames of several different sizes are nested inside each other. For example, when using Point-to-Point Protocol (PPP) over asynchronous serial communication, the eight bits of each individual byte are framed by start and stop bits,^[3][4] the payload data bytes in a network packet are framed by the header and footer, and several packets can be framed with frame boundary octets.^[5]

Time-division multiplex

In telecommunications, specifically in time-division multiplex (TDM) and time-division multiple access (TDMA) variants, a frame is a cyclically repeated data block that consists of a fixed number of time slots, one for each logical TDM channel or TDMA transmitter. In this context, a frame is typically an entity at the physical layer. TDM application examples are SONET/SDH and the ISDN circuit-switched B-channel, while TDMA examples are Circuit Switched Data used in early cellular voice services. The frame is also an entity for time-division duplex, where the mobile terminal may transmit during some time slots and receive during others.

See also

• Application-layer framing
• Datagram
• Jumbo frame
• Multiplex techniques
• Overhead bit

References

1. "Data Link Layer (Layer 2)". The TCP/IP Guide. 2005-09-20. Retrieved 2010-01-31.
2. R. Braden, ed. (October 1989). Requirements for Internet Hosts -- Communication Layers. Network Working Group. doi:10.17487/RFC1122. STD 3. RFC 1122. Internet Standard 3. Updated by RFC 1349, 4379, 5884, 6093, 6298, 6633, 6864, 8029 and 9293.
3. David S.Lawyer and Greg Hankins. "Serial HOWTO". Section "20.4 Forming a Byte (Framing)". 2011. quote: "... a start bit and a stop bit to mark the beginning and end of a byte. This is called framing ... Don't confuse this type of framing with the framing used for a packet of bytes on a network."
4. MATLAB External Interfaces. Section "Serial Data Format". quote: "... one start bit... parity bit ... stop bit[s] ... called framing bits because they frame the data bits."
5. W. Simpson, ed. (July 1994). The Point-to-Point Protocol (PPP). Network Working Group. doi:10.17487/RFC1661. STD 51. RFC 1661. Internet Standard 51. Obsoletes RFC 1548. Updated by RFC 2153. A packet is usually mapped to a frame; the exceptions are when data link layer fragmentation is being performed, or when multiple packets are incorporated into a single frame.