Frame (networking)

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A frame is a digital data transmission unit in computer networking and telecommunication. 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[edit]

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[edit]

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[edit]


  1. ^ "Data Link Layer (Layer 2)". The TCP/IP Guide. 2005-09-20. Retrieved 2010-01-31.
  2. ^ "RFC 1122: Requirements for Internet Hosts — Communication Layers". IETF. October 1989. p. 18. doi:10.17487/RFC1122. RFC 1122. Retrieved 2010-06-07.
  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. ^ RFC 1661 "The Point-to-Point Protocol (PPP)" quote: "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."