Frame (networking)

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A frame is a digital data transmission unit in computer networking and telecommunication. 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 in the middle of a frame transmission, it ignores the data until it detects a new frame synchronization sequence.

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 framing 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.

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 the 2G and 3G circuit-switched cellular voice services. The frame is also an entity for time-division duplex, where the mobile terminal may transmit during some timeslots and receive during others.

Often frames of several different sizes are nested inside each other. For example, when people use Point-to-Point Protocol over asynchronous serial communication, the 8 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]

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References[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. RFC 1122. https://tools.ietf.org/html/rfc1122#page-18. 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."