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Intelecommunications andcomputer networking, anetwork packet is a formatted unit ofdata carried by apacket-switched network. A packet consists of control information and user data;[1] the latter is also known as thepayload. Control information provides data for delivering the payload (e.g., source and destinationnetwork addresses,error detection codes, or sequencing information). Typically, control information is found in packetheaders andtrailers.
Inpacket switching, thebandwidth of thetransmission medium is shared between multiple communication sessions, in contrast tocircuit switching, in which circuits are preallocated for the duration of one session and data is typically transmitted as a continuousbit stream.
In the seven-layerOSI model ofcomputer networking,packet strictly refers to aprotocol data unit at layer 3, thenetwork layer.[2] A data unit at layer 2, thedata link layer, is aframe. In layer 4, thetransport layer, the data units aresegments anddatagrams. Thus, in the example of TCP/IP communication overEthernet, aTCP segment is carried in one or moreIP packets, which are each carried in one or moreEthernet frames.
The basis of the packet concept is the postal letter: the header is like the envelope, the payload is the entire content inside the envelope, and the footer would be your signature at the bottom.[3]
Network design can achieve two major results by using packets:error detection andmultiple host addressing.[4]
Communications protocols use various conventions for distinguishing the elements of a packet and for formatting the user data. For example, inPoint-to-Point Protocol, the packet is formatted in 8-bit bytes, and special characters are used to delimit elements. Other protocols, like Ethernet, establish the start of the header and data elements by their location relative to the start of the packet. Some protocols format the information at abit level instead of abyte level.[5]
A packet may contain any of the following components:
IP packets are composed of a header and payload. The header consists of fixed and optional fields. The payload appears immediately after the header. An IP packet has no trailer. However, an IP packet is often carried as the payload inside an Ethernet frame, which has its own header and trailer.
Per theend-to-end principle, IP networks do not provide guarantees of delivery, non-duplication, or in-order delivery of packets. However, it is common practice to layer areliabletransport protocol such asTransmission Control Protocol on top of the packet service to provide such protection.
The Consultative Committee for Space Data Systems (CCSDS) packet telemetry standard defines the protocol used for the transmission of spacecraft instrument data over the deep-space channel. Under this standard, an image or other data sent from a spacecraft instrument is transmitted using one or more packets.
Packetized elementary stream (PES) is a specification associated with theMPEG-2 standard that allows anelementary stream to be divided into packets. The elementary stream is packetized by encapsulating sequential data bytes from the elementary stream between PES packet headers.
A typical method of transmitting elementary stream data from a video or audio encoder is to first create PES packets from the elementary stream data and then to encapsulate these PES packets inside anMPEG transport stream (TS) packets or anMPEG program stream (PS). The TS packets can then be transmitted using broadcasting techniques, such as those used in anATSC andDVB.
In order to providemonocompatibility, theNICAM signal is transmitted on asubcarrier alongside the sound carrier. This means that theFM orAM regular mono sound carrier is left alone for reception by monaural receivers. The NICAM packet (except for the header) is scrambled with a nine-bit pseudo-random bit-generator before transmission. Making the NICAM bitstream look more likewhite noise is important because this reduces signal patterning on adjacent TV channels.
Packet: A group of bits that includes data plus control information. Generally refers to a network layer (OSI layer 3) protocol data unit.
