Network Notes

Quadrature Amplitude Modulation - (QAM) A method for encoding digital data in an analog signal in which each combination of phase and amplitude represents one of sixteen four bit patterns. This is required for fax transmission at 9600 bits per second. Like all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two waves, 90 degrees out-of-phase with each other (in quadrature) are changed (modulated or keyed) to represent the data signal. Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the magnitude of the modulating signal is a constant, with only the phase varying. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded a special case of phase modulation.

Frequency-division multiplexing (FDM) is a form of signal multiplexing(simultaneous transmission of many messages on a single channel) where multiple baseband signals are modulated(change) on different frequency carrier waves and added together to create a composite signal.

[edit] Non telephone FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal. A television channel is divided into subcarrier frequencies for video, color, and audio. DSL uses different frequencies for voice and for upstream and downstream data transmission on the same conductors, which is also an example of frequency duplex. Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA). FDMA is the traditional way of separating radio signals from different transmitters. In the 1860 and 70s, several inventors attempted FDM under the names of Acoustic telegraphy and Harmonic telegraphy. Practical FDM was only achieved in the electronic age. Meanwhile their efforts led to an elementary understanding of electroacoustic technology, resulting in the invention of the telephone.

[edit] Telephone For long distance telephone connections, 20th century telephone companies used Lcarrier and similar co-axial cable systems carrying thousands of voice circuits multiplexed in multiple stages. For short distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier. Those cables didn't allow such large bandwidths, so only 12 voice channels (Double Sideband) and later 24 (Single Sideband) were multiplexed into four wires, one pair for each direction with repeaters every several miles, approximately 10 km. By the end of the 20th Century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM. Since the late twentieth century Digital Subscriber Lines have used a Discrete multitone (DMT) system to divide their spectrum into frequency channels. The concept corresponding to frequency division multiplexing in the optical domain is known as wavelength division multiplexing.

Cyclic redundancy check From Wikipedia, the free encyclopedia (Redirected from Cyclical redundancy checking) Jump to: navigation, search A cyclic redundancy check (CRC) is a type of function that takes as input a data stream of any length, and produces as output a value of a certain space, commonly a 32-bit integer. The term CRC is often used to denote either the function or the function's output. A CRC can be used as a checksum to detect alteration of data during transmission or storage. CRCs are popular because they are simple to implement in binary hardware, are easy to analyze mathematically, and are particularly good at detecting common errors caused by noise in transmission channels. The CRC was invented by W. Wesley Peterson, and published in his 1961 paper[1]. The first appearance of the CRC-32, now employed in Ethernet, other applications, and IEEE recommended, in the literature was in the paper, Evaluation of Error Detection Polynomial Performance on the AUTOVON Channel, K. Brayer, J.L. Hammond, IEEE National Telecommunications Conference, New Orleans, LA, December 1975. It is the generator polynomial of a Hamming code and was selected for its error detection performance.

Computer network From Wikipedia, the free encyclopedia Jump to: navigation, search A computer network is a group of interconnected computers. Networks may be classified according to a wide variety of characteristics. This article provides a general overview of some types and categories and presents the basic components of a network.

Types of networks Below is a list of the most common types of computer networks in order of scale.

[edit] Personal Area Network (PAN) Main article: Personal area network A personal area network (PAN) is a computer network used for communication among computer devices close to one person. Some examples of devices that are used in a PAN are printers, fax machines, telephones, PDAs or scanners. The reach of a PAN is typically within about 20-30 feet (approximately 6-9 metres). Personal area networks may be wired with computer buses such as USB[1] and FireWire. A wireless personal area network (WPAN) can also be made possible with network technologies such as IrDA and Bluetooth..

[edit] Local Area Network (LAN) Main article: Local Area Network A network covering a small geographic area, like a home, office, or building. Current LANs are most likely to be based on Ethernet technology. For example, a library may have a wired or wireless LAN for users to interconnect local devices (e.g., printers and servers) and to connect to the internet. On a wired LAN, PCs in the library are typically connected by category 5 (Cat5) cable, running the IEEE 802.3 protocol through a system of interconnection devices and eventually connect to the internet. The cables to the servers are typically on Cat 5e enhanced cable, which will support IEEE 802.3 at 1 Gbit/s. A wireless LAN may exist using a different IEEE protocol, 802.11b or 802.11g. The staff computers (bright green in the figure) can get to the color printer, checkout records, and the academic network and the Internet. All user computers can get to the Internet and the card catalog. Each workgroup can get to its local printer. Note that the printers are not accessible from outside their workgroup.

Typical library network, in a branching tree topology and controlled access to resources All interconnected devices must understand the network layer (layer 3), because they are handling multiple subnets (the different colors). Those inside the library, which have only 10/100 Mbit/s Ethernet connections to the user device and a Gigabit Ethernet connection to the central router, could be called "layer 3 switches" because they only have Ethernet interfaces and must understand IP. It would be more correct to call them access routers, where the router at the top is a distribution router that connects to the Internet and academic networks' customer access routers. The defining characteristics of LANs, in contrast to WANs (wide area networks), include their higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects investigating the standardization of 100 Gbit/s, and possibly 40 Gbit/s.

[edit] Campus Area Network (CAN) Main article: Campus Area Network A network that connects two or more LANs but that is limited to a specific and contiguous geographical area such as a college campus, industrial complex, or a military base. A CAN may be considered a type of MAN (metropolitan area network), but is generally limited to an area that is smaller than a typical MAN. This term is most often used to discuss the implementation of networks for a contiguous area. This should not be confused with a Controller Area Network. A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet.

[edit] Metropolitan Area Network (MAN) Main article: Metropolitan Area Network A Metropolitan Area Network is a network that connects two or more Local Area Networks or Campus Area Networks together but does not extend beyond the boundaries of the immediate town/city. Routers, switches and hubs are connected to create a Metropolitan Area Network.

[edit] Wide Area Network (WAN) Main article: Wide Area Network

A WAN is a data communications network that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.

[edit] Global Area Network (GAN) Main article: Global Area Network Global area networks (GAN) specifications are in development by several groups, and there is no common definition. In general, however, a GAN is a model for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is "handing off" the user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial Wireless local area networks (WLAN).[2]

[edit] Internetwork Main article: Internetwork Two or more networks or network segments connected using devices that operate at layer 3 (the 'network' layer) of the OSI Basic Reference Model, such as a router. Any interconnection among or between public, private, commercial, industrial, or governmental networks may also be defined as an internetwork. In modern practice, the interconnected networks use the Internet Protocol. There are at least three variants of internetwork, depending on who administers and who participates in them: • • •

Intranet Extranet Internet

Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet is not considered to be a part of the intranet or extranet, although it may serve as a portal for access to portions of an extranet.

[edit] Intranet Main article: Intranet An intranet is a set of interconnected networks, using the Internet Protocol and uses IPbased tools such as web browsers and ftp tools, that is under the control of a single administrative entity. That administrative entity closes the intranet to the rest of the world, and allows only specific users. Most commonly, an intranet is the internal network

of a company or other enterprise. A large intranet will typically have its own web server to provide users with browseable information.

[edit] Extranet Main article: Extranet An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g. a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.

[edit] Internet Main article: Internet A specific internetwork, consisting of a worldwide interconnection of governmental, academic, public, and private networks based upon the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the U.S. Department of Defense – also home to the World Wide Web (WWW) and referred to as the 'Internet' with a capital 'I' to distinguish it from other generic internetworks. Participants in the Internet use the Internet Protocol Suite and IP Addresses allocated by address registries. Service providers and large enterprises exchange information about the reachability of their address ranges through the Border Gateway Protocol (BGP).

TCP/IP Architecture Model: 4-Layers vs. OSI 7 Layers TCP/IP architecture does not exactly follow the OSI model. Unfortunately, there is no universal agreement regarding how to describe TCP/IP with a layered model. It is generally agreed that TCP/IP has fewer levels (from three to five layers) than the seven layers of the OSI model. We adopt a four layers model for the TCP/IP architecture. TCP/IP architecture omits some features found under the OSI model, combines the features of some adjacent OSI layers and splits other layers apart. The 4-layer structure of TCP/IP is built as information is passed down from applications to the physical network layer. When data is sent, each layer treats all of the information it receives from the upper layer as data, adds control information (header) to the front of that data and then pass it to the lower layer. When data is received, the opposite procedure takes place as each layer processes and removes its header before passing the data to the upper layer. The TCP/IP 4-layer model and the key functions of each layer is described below: Application Layer The Application Layer in TCP/IP groups the functions of OSI Application, Presentation Layer and Session Layer. Therefore any process above the transport layer is called an Application in the TCP/IP architecture. In TCP/IP socket and port are used to describe the path over which applications communicate. Most application level protocols are associated with one or more port number. Transport Layer In TCP/IP architecture, there are two Transport Layer protocols. The Transmission Control Protocol (TCP) guarantees information transmission. The User Datagram Protocol (UDP) transports datagram switch out end-to-end reliability checking. Both protocols are useful for different applications. Network Layer The Internet Protocol (IP) is the primary protocol in the TCP/IP Network Layer. All upper and lower layer communications must travel through IP as they are passed through the TCP/IP protocol stack. In addition, there are many supporting protocols in the Network Layer, such as ICMP, to facilitate and manage the routing process. Network Access Layer In the TCP/IP architecture, the Data Link Layer and Physical Layer are normally grouped together to become the Network Access layer. TCP/IP makes use of existing Data Link and Physical Layer standards rather than defining its own. Many RFCs describe how IP

utilizes and interfaces with the existing data link protocols such as Ethernet, Token Ring, FDDI, HSSI, and ATM. The physical layer, which defines the hardware communication properties, is not often directly interfaced with the TCP/IP protocols in the network layer and above.

TCP/IP Architecture Model: 4-Layers vs. OSI 7 Layers

High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The original ISO standards for HDLC were: • • • •

ISO 3309 — Frame Structure ISO 4335 — Elements of Procedure ISO 6159 — Unbalanced Classes of Procedure ISO 6256 — Balanced Classes of Procedure

The current standard for HDLC is ISO 13239, which replaces all of those standards. HDLC provides both connection-oriented and connectionless service. HDLC can be used for point to multipoint connections, but is now used almost exclusively to connect one device to another, using what is known as Asynchronous Balanced Mode (ABM). The original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM) are rarely used.

HDLC Protocol Description HDLC [High-level Data Link Control] is a group of protocols for transmitting [synchronous] data [Packets] between [Point-to-Point] nodes. In HDLC, data is organized into a frame. HDLC protocol resides with Layer 2 of the OSI model, the data link layer. HDLC uses zero insertion/deletion process [bit stuffing] to ensure that the bit pattern of the delimiter flag does not occur in the fields between flags. The HDLC frame is synchronous and therefore relies on the physical layer to provide method of clocking and synchronizing the transmission and reception of frames.

HDLC Frame Structure HDLC uses the term "frame" to indicate an entity of data(or a protocol data unit) transmitted from one station to another. Figure 3 below is a graphical representation of a HDLC frame with an information field. Figure 3 An HDLC frame with an information field.

Field Name Flag Field( F ) Address Field( A ) Control Field( C ) Information Field( I ) Frame Check Sequence( FCS ) Closing Flag Field( F )

Size(in bits) 8 bits 8 bits 8 or 16 bits Variable; Not used in some frames 16 or 32 bits 8 bits

HDLC Framing Frame Synchronisation Once an HDLC link has been started, bits are transmitted continuously, even when the link is idle, in which case the flag sequence - 01111110 (or 0x7E) is transmitted. Frames are transmitted within the gaps between flags, so the receiver can determine when a frame starts and stops by synchronising with the flags. To ensure transparency, a 0 bit is inserted by the transmitter after 5 continuous 1s within the frame contents, and removed by the receiver whenever it detects 5 continuous 1s followed by a 0. This prevents data within the frame being confused with the flags, and is known as "bit-stuffing". Normal frames are terminated with a flag - any frame which terminates with 7 1s or more is assumed to be aborted, and discarded. The transmitter can deliberately abort a frame if it chooses - this can sometimes happen if frames need to be retransmitted, and the transmitter knows that the frame currently being sent will be discarded by the receiver. Only a single flag is necessary between frames - the ending flag of one frame can be the starting flag of the next. The flag is therefore not actually part of the HDLC frame itself.

HDLC Frame Structure The HDLC Frame (before transmission - i.e. before bit-stuffing and wrapping with flags) consists of the following fields: Address field Control field Information field

Frame Check Sequence The frame is transmitted with the lowest order bit of each byte first - and indeed the frame structure within the X.25 specification shows the field encodings such that the lowest order bit is on the left, which can be rather confusing to those used to normal computing notation. Since all the fields are in reality byte-aligned, this guide uses standard ordering of each byte - for example, the LAPB Address fields are described as 0x03 and 0x01.

Address Field The Address field in theory can be number of bytes; if the byte value is even, that means that another address byte follows. The Address field for LAPB is always a single byte, and takes the value 0x01 or 0x03, as follows: 0x03 0x01

Command frames DCE to DTE Response frames DTE to DCE Command frames DTE to DCE Response Frames DCE to DTE

Note that the different address field values mean that a LAPB link is not symmetrical one end must be a DTE and one a DCE, and thus the link will not work if misconfigured. To get round this, the FarSync X.25 software employs rather clever algorithm which causes one end of the link to reconfigure itself if it encounters the situation in which both ends of the link are configured in the same way, but this only happens if Auto Link Role is enabled.

Control Field The Control Field is usually 1 byte, but can be 2 bytes long for LAPB. This depends on the frame type, and whether normal (modulo-8) or extended (modulo-128) sequence numbers are used. Modulo-8 sequence numbers are used most of the time, in which case the Control Field is always 1 byte.

Information Field The Information Field contains the higher layer data being carried by the Link Layer - for X.25, this is typically the X.25 Packet.

Frame Check Sequence (CRC) The Frame Check Sequence is 16-bits long for LAPB (a 32-bit FCS is possible for other types of HDLC). It contains a Cyclic Redundancy Check sequence, and thus is often referred to as the CRC instead of the FCS. The FCS is generated when a frame is transmitted (before any bit-stuffing) and is checked when a frame is received. If one or more bits have been modified during transmission by a line error, then the CRC check should fail, in which case the entire frame is discarded.