Packet (information technology)

In information technology, a packet is a formatted unit of data carried by a packet mode computer network. Computer communications links that do not support packets, such as traditional point-to-point telecommunications links, simply transmit data as a series of bytes, characters, or bits alone. When data is formatted into packets, the bitrate of the communication medium can be better shared among users than if the network were circuit switched. By using packet switched networking it is also harder to guarantee a lowest possible bitrate.

Packet framing

A packet consists of two kinds of data: control information and user data (also known as payload). The control information provides data the network needs to deliver the user data, for example: source and destination addresses, error detection codes like checksums, and sequencing information. Typically, control information is found in packet headers and trailers, with user data in between.

Different communications protocols use different conventions for distinguishing between the elements and for formatting the data. In Binary Synchronous Transmission, the packet is formatted in 8-bit bytes, and special characters are used to delimit the different 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 a bit level instead of a byte level.

A good analogy is to consider a packet to be like a letter: the header is like the envelope, and the data area is whatever the person puts inside the envelope. A difference, however, is that some networks can break a larger packet into smaller packets when necessary (note that these smaller data elements are still formatted as packets).

A network design can achieve two major results by using packets: error detection and multiple host addressing.

Error detection

It is more efficient and reliable to calculate a checksum or cyclic redundancy check over the contents of a packet than to check errors using character-by-character parity bit checking.

The packet trailer often contains error checking data to detect errors that occur during transmission.

Host addressing

Modern networks usually connect three or more host computers together; in such cases the packet header generally contains addressing information so that the packet is received by the correct host computer. In complex networks constructed of multiple routing and switching nodes, like the ARPANET and the modern Internet, a series of packets sent from one host computer to another may follow different routes to reach the same destination. This technology is called packet switching.

Packets vs. datagrams

In general, the term packet applies to any message formatted as a packet, while the term datagram is generally reserved for packets of an "unreliable" service. A "reliable" service is one that notifies the user if delivery fails, while an "unreliable" one does not notify the user if delivery fails. For example, IP provides an unreliable service. Together, TCP and IP provide a reliable service, whereas UDP and IP provide an unreliable one. All these protocols use packets, but UDP packets are generally called datagrams. |- | || ||Note: The above 4 bytes is called the 32 bit start code. |- |PES Packet length||2 bytes||Can be zero as in not specified for video streams in MPEG transport streams |- |Optional PES header||variable length|| |- |Stuffing bytes||variable length|| |- |Data|| ||See elementary stream. In the case of private streams the first byte of the payload is the sub-stream number. |}

Optional PES header

{|class="wikitable" !Name!!Number of Bits!!Description |- |Marker bits||2||10 binary or 0x2 hex |- |Scrambling control||2||00 implies not scrambled |- |Priority||1|| |- |Data alignment indicator||1||1 indicates that the PES packet header is immediately followed by the video start code or audio syncword |- |Copyright||1||1 implies copyrighted |- |Original or Copy||1||1 implies original |- |PTS DTS indicator||2||11 = both present, 10 = only PTS |- |ESCR flag||1|| |- |ES rate flag||1|| |- |DSM trick mode flag||1|| |- |Additional copy info flag||1|| |- |CRC flag||1|| |- |extension flag||1|| |- |PES header length||8||gives the length of the remainder of the PES header |- |Optional fields||variable length||presence is determined by flag bits above |- |Stuffing Bytes||variable length||0xff |}

NICAM

In order to provide mono "compatibility", the NICAM signal is transmitted on a subcarrier alongside the sound carrier. This means that the FM or AM regular mono sound carrier is left alone for reception by monaural receivers.

A NICAM-based stereo-TV infrastructure can transmit a stereo TV programme as well as the mono "compatibility" sound at the same time, or can transmit two or three entirely different sound streams. This latter mode could be used to transmit audio in different languages, in a similar manner to that used for in-flight movies on international flights. In this mode, the user can select which soundtrack to listen to when watching the content by operating a "sound-select" control on the receiver.

NICAM offers the following possibilities. The mode is auto-selected by the inclusion of a 3-bit type field in the data-stream

One digital stereo sound channel.

Two completely different digital mono sound channels.

One digital mono sound channel and a 352 kbit/s data channel.

One 704 kbit/s data channel.

The four other options could be implemented at a later date. Only the first two of the ones listed are known to be in general use however.

NICAM packet transmission

The NICAM packet (except for the header) is scrambled with a nine-bit pseudo-random bit-generator before transmission.

The topology of this pseudo-random generator yields a bitstream with a repetition period of 511 bits.

The pseudo-random generator's polynomial is: x^9 + x^4 + 1.

The pseudo-random generator is initialized with: 111111111.

Making the NICAM bitstream look more like white noise is important because this reduces signal patterning on adjacent TV channels.

The NICAM header is not subject to scrambling. This is necessary so as to aid in locking on to the NICAM data stream and resynchronisation of the data stream at the receiver.

At the start of each NICAM packet the pseudo-random bit generator's shift-register is reset to all-ones.

See also

Statistical multiplexing

Anti-replay

Packet sniffer

Gigapackets

Mangled packet

TCP segment

TCP/IP

DHCP server

Packet generation model

Louis Pouzin

Tail drop

References

Dean, Tamara (2006). Network+ Guide to Networks. Boston, Massachusetts: Thomson Course Technology.

Category:Units of information