♥OSI Basic Reference Model♥

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative[1]. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.
A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.
Even though it has been largely superseded by newer IETF, IEEE, and indeed OSI protocol developments (subsequent to the publication of the original architectural standards), the basic OSI model is considered an excellent place to begin the study of network architecture. Not understanding that the pure seven-layer model is more historic than current, many beginners make the mistake of trying to fit every protocol under study into one of the seven basic layers. Especially the attempts of cross-layer optimization break the boundaries of the original layer scheme. Describing the actual layer concept with implemented systems is not always easy to do as most of the protocols in use on the Internet were designed as part of the TCP/IP model, and may not fit cleanly into the OSI Model.
Contents [hide]1 History 2 Description of OSI layers 2.1 Layer 7: Application Layer 2.2 Layer 6: Presentation Layer 2.3 Layer 5: Session Layer 2.4 Layer 4: Transport Layer 2.5 Layer 3: Network Layer 2.6 Layer 2: Data Link Layer 2.6.1 WAN Protocol architecture 2.6.2 IEEE 802 LAN architecture 2.7 Layer 1: Physical Layer 3 Interfaces 4 Examples 5 Comparison with TCP/IP 6 Remembering The OSI Layers 7 References 8 External links
HistoryIn 1977, work on a layered model of network architecture was started, and the International Organization for Standardization (ISO) began to develop its OSI framework architecture. The ISO is a worldwide federation of national standards bodies from some 130 countries, one from each country.[citation needed] OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model) and a set of specific protocols. The standard documents that describe the OSI model can be freely downloaded from the ITU-T as the X.200-series of recommendations [2]. A number of the protocol specifications are also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from the ISO, but only some of the ISO/IEC standards are available as cost-free downloads.[3]
Some aspects of OSI design evolved from experiences with the CYCLADES network, which also influenced Internet design. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.
Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host.
Description of OSI layersOSI Model Data unit Layer Function Hostlayers Data 7. Application Network process to application 6. Presentation Data representation and encryption 5. Session Interhost communication Segment/Datagram 4. Transport End-to-end connections and reliability Medialayers Packet 3. Network Path determination and logical addressing Frame 2. Data Link Physical addressing (MAC & LLC) Bit 1. Physical Media, signal and binary transmission
Layer 7: Application LayerMain article: Application LayerThe application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
Layer 6: Presentation LayerMain article: Presentation LayerThe Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack.
The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML. ASN.1 has a set of cryptographic encoding rules that allows end-to-end encryption between application entities.
Layer 5: Session LayerMain article: Session LayerThe Session Layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls (RPCs).
Layer 4: Transport LayerMain article: Transport LayerThe Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail.
Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, the best known examples of a Layer 4 protocol are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Of the actual OSI protocols, there are five classes of transport protocols ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (which is also known as TP4 and is designed for less reliable networks, similar to the Internet). Class 0 is closest to UDP. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Detailed characteristics of TP0-4 classes are shown in the following table:[4]
Feature Name TP0 TP1 TP2 TP3 TP4 Connection oriented - - - Yes Yes Connectionless - - - No Yes Segmentation/Fragmentation Yes Yes Yes Yes Yes Reassembly Yes Yes Yes Yes Yes Error Recovery No Yes No No Yes Reinitiate connection (if an excessive number of PDUs are unacknowledged) No Yes No Yes Yes multiplexing and demultiplexing over a single virtual circuit No No Yes Yes Yes Reliable Transport Service No Yes Yes Yes Yes
Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the Transport Layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a Network Layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet.
Layer 3: Network LayerMain article: Network LayerThe Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.
The best-known example of a Layer 3 protocol is the Internet Protocol (IP). It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for fragmenting into sufficiently small packets that the medium can accept it.
A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the Network Layer. These include routing protocols, multicast group management, Network Layer information and error, and Network Layer address assignment. It is the function of the payload that makes these belong to the Network Layer, not the protocol that carries them.
Layer 2: Data Link LayerMain article: Data Link LayerThe Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work, in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages.
Both WAN and LAN services arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the FDDI bit stream is not used by the Data Link Layer.
WAN Protocol architectureConnection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They also are capable of controlling the rate of transmission. A WAN Data Link Layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD.
IEEE 802 LAN architecturePractical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a Media Access Control sublayer. Above this MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer, which deals with addressing and multiplexing on multiaccess media.
While IEEE 802.3 is the dominant wired LAN protocol and IEEE 802.11 the wireless LAN protocol, obsolescent MAC layers include Token Ring and FDDI. The MAC sublayer detects but does not correct errors.
Layer 1: Physical LayerMain article: Physical LayerThe Physical Layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.
To understand the function of the Physical Layer in contrast to the functions of the Data Link Layer, think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, where the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Obsolescent Physical Layer standards such as RS-232 do use physical wires to control access to the medium.
The major functions and services performed by the Physical Layer are:
Establishment and termination of a connection to a communications medium. Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link. Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as Token ring, FDDI, and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.
InterfacesNeither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers are implementation-specific.
For example, Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfaces between the media (Layer 2) and the network protocol (Layer 3).
Interface standards, except for the Physical Layer to media, are approximate implementations of OSI Service Specifications.
ExamplesLayer Misc. examples TCP/IP suite SS7[5] AppleTalk suite OSI suite IPX suite SNA UMTS # Name 7 Application HL7, Modbus NNTP, SIP, SSI, DNS, FTP, Gopher, HTTP, NFS, NTP, DHCP, SMPP, SMTP, SNMP, Telnet INAP, MAP, TCAP, ISUP, TUP AFP, ZIP, RTMP, NBP FTAM, X.400, X.500, DAP, ROSE, RTSE, ACSE RIP, SAP APPC 6 Presentation TDI, ASCII, EBCDIC, MIDI, MPEG MIME, XDR, SSL, TLS (Not a separate layer) AFP ISO/IEC 8823, X.226, ISO/IEC 9576-1, X.236 5 Session Named Pipes, NetBIOS, SAP, Half Duplex,Full Duplex,Simplex,SDP Sockets. Session establishment in TCP. SIP. (Not a separate layer with standardized API.) ASP, ADSP, PAP ISO/IEC 8327, X.225, ISO/IEC 9548-1, X.235 NWLink DLC? 4 Transport NBF, nanoTCP, nanoUDP TCP, UDP,PPTP, L2TP, SCTP, RTP DDP ISO/IEC 8073, TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC 8602, X.234 SPX 3 Network NBF, Q.931 IP, IPsec, ARP, ICMP, RIP, OSPF, BGP, IGMP, IS-IS SCCP, MTP ATP (TokenTalk or EtherTalk) ISO/IEC 8208, X.25 (PLP), ISO/IEC 8878, X.223, ISO/IEC 8473-1, CLNP X.233. IPX RRC (Radio Resource Control) Packet Data Convergence Protocol (PDCP) and BMC (Broadcast/Multicast Control) 2 Data Link 802.3 (Ethernet), 802.11a/b/g/n MAC/LLC, 802.1Q (VLAN), ATM, HDP, FDDI, Fibre Channel, Frame Relay, HDLC, ISL, PPP, Q.921, Token Ring, CDP PPP, SLIP MTP, Q.710 LocalTalk,AppleTalk Remote Access, PPP ISO/IEC 7666, X.25 (LAPB), Token Bus, X.222, ISO/IEC 8802-2 LLC Type 1 and 2 IEEE 802.3 framing, Ethernet II framing SDLC LLC (Logical Link Control), MAC (Media Access Control) 1 Physical RS-232, V.35, V.34, I.430, I.431, T1, E1, 10BASE-T, 100BASE-TX, POTS, SONET, SDH,DSL, 802.11a/b/g/n PHY MTP, Q.710 RS-232, RS-422, STP, PhoneNet X.25 (X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703) Twinax UMTS L1 (UMTS Physical Layer)
Comparison with TCP/IPIn the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.[6] RFC 3439 contains a section entitled "Layering considered harmful." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network.
Even though the concept is different than in OSI, these layers are nevertheless often compared with the OSI layering scheme in the following way: The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its end-to-end Transport Layer includes the graceful close function of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer (Internet Layer) is a subset of the OSI Network Layer, while the Link Layer includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document.
The presumably strict consumer/producer layering of OSI as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right.
The TCP/IP design generally favors decisions based on simplicity, efficiency and ease of implementation.
Remembering The OSI LayersVarious mnemonics have been created over the years to help remember the order of the OSI layers. Often cited are the following:
Please Do Not Throw Sausage Pizza Away All People Seem To Need Data Processing

♥Local Area Network♥

Latest LAN



LAN" redirects here. For other uses, see LAN (disambiguation).
A local-area network is a computer network covering a small geographic area, like a home, office, or group of buildings e.g. a school. The defining characteristics of LANs, in contrast to wide-area networks (WANs), include their much higher data-transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines.

Ethernet over unshielded twisted pair cabling, and Wi-Fi are the two most common technologies currently, but ARCNET, Token Ring and many others have been used in the past.


History
As larger universities and research labs obtained more computers during the late 1960s, there was increasing pressure to provide high-speed interconnections. A report in 1970 from the Lawrence Radiation Laboratory detailing the growth of their "Octopus" network, gives a good indication of the situation.


[edit] Early systems
Cambridge University's Cambridge Ring was started in 1974 but was never developed into a successful commercial product.

Ethernet was developed at Xerox PARC in 1973–1975,[4] and filed as U.S. Patent 4,063,220 . In 1976, after the system was deployed at PARC, Metcalfe and Boggs published their seminal paper - "Ethernet: Distributed Packet-Switching For Local Computer Networks"

ARCNET was developed by Datapoint Corporation in 1976 and announced in 1977 - and had the first commercial installation in December 1977 at Chase Manhattan Bank in New York


[edit] The Personal Computer
The development and proliferation of CP/M-based personal computers from the late 1970s and then DOS-based personal computers a from 1981 meant that a single site began to have dozens or even hundreds of computers. The initial attraction of networking these was generally to share disk space and laser printers, which were both very expensive at the time. There was much enthusiasm for the concept and for several years, from about 1983 onward, computer industry pundits would regularly declare the coming year to be “the year of the LAN”.

In reality, the concept was marred by proliferation of incompatible physical layer and network protocol implementations, and confusion over how best to share resources. Typically, each vendor would have its own type of network card, cabling, protocol, and network operating system. A solution appeared with the advent of Novell NetWare which provided even-handed support for the 40 or so competing card/cable types, and a much more sophisticated operating system than most of its competitors. Netware dominated[8] the personal computer LAN business from early after its introduction in 1983 until the mid 1990s when Microsoft introduced Windows NT Advanced Server and Windows for Workgroups.

Of the competitors to NetWare, only Banyan Vines had comparable technical strengths, but Banyan never gained a secure base. Microsoft and 3Com worked together to create a simple network operating system which formed the base of 3Com's 3+Share, Microsoft's LAN Manager and IBM's LAN Server. None of these were particularly successful.

In this same timeframe, Unix computer workstations from vendors such as Sun Microsystems, Hewlett-Packard, Silicon Graphics, Intergraph, NeXT and Apollo were using TCP/IP based networking. Although this market segment is now much reduced, the technologies developed in this area continue to be influential on the Internet and in both Linux and Apple Mac OS X networking—and the TCP/IP protocol has now almost completely replaced IPX, AppleTalk, NBF and other protocols used by the early PC LANs.


Cabling
Early LAN cabling had always been based on various grades of co-axial cable, but IBM's Token Ring used shielded twisted pair cabling of their own design, and in 1984 StarLAN showed the potential of simple Cat3 unshielded twisted pair—the same simple cable used for telephone systems. This led to the development of 10Base-T (and it's successors) and structured cabling which is still the basis of most LANs today.


Technical aspects
Although switched Ethernet is now the most common data link layer protocol and IP as a network layer protocol, many different options have been used, and some continue to be popular in niche areas. Smaller LANs generally consist of a one or more switches linked to each other - often with one connected to a router, cable modem, or DSL modem for Internet access.

Larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loops, their ability to manage differing traffic types via quality of service (QoS), and to segregate traffic via VLANs. Larger LANS also contain a wide variety of network devices such as switches, firewalls, routers, load balancers, sensors and so on.[9]

LANs may have connections with other LANs via leased lines, leased services, or by 'tunneling' across the Internet using VPN technologies. Depending on how the connections are made, secured, and the distance involved they become a Metropolitan Area Network (MAN), a Wide Area Network (WAN), or a part of the internet.

♥Networking Topology♥

Network topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network, especially the physical (real) and logical (virtual) interconnections between nodes.




A local area network (LAN) is one example of a network that exhibits both a physical topology and a logical topology. Any given node in the LAN will have one or more links to one or more other nodes in the network and the mapping of these links and nodes onto a graph results in a geometrical shape that determines the physical topology of the network. Likewise, the mapping of the flow of data between the nodes in the network determines the logical topology of the network. It is important to note that the physical and logical topologies might be identical in any particular network but they also may be different.

Any particular network topology is determined only by the graphical mapping of the configuration of physical and/or logical connections between nodes. LAN Network Topology is, therefore, technically a part of graph theory. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ in two networks and yet their topologies may be identical

Basic types of topologies
The arrangement or mapping of the elements of a network gives rise to certain basic topologies which may then be combined to form more complex topologies (hybrid topologies). The most common of these basic types of topologies are:

Bus (Linear, Linear Bus)
Star
Ring
Mesh
partially connected mesh (or simply 'mesh')
fully connected mesh
Tree
Hybrid
Point to Point


Classification of network topologies
There are also three basic categories of network topologies:

physical topologies
signal topologies
logical topologies
The terms signal topology and logical topology are often used interchangeably even though there is a subtle difference between the two and the distinction is not often made between the two.


Physical topologies
The mapping of the nodes of a network and the physical connections between them – i.e., the layout of wiring, cables, the locations of nodes, and the interconnections between the nodes and the cabling or wiring system.

Classification of physical topologies

Point-to-point
The simplest topology is a permanent link between two endpoints. Switched point-to-point topologies are the basic model of conventional telephony. The value of a permanent point-to-point network is the value of guaranteed, or nearly so, communications between the two endpoints. The value of an on-demand point-to-point connection is proportional to the number of potential pairs of subscribers, and has been expressed as Metcalfe's Law.

Permanent (dedicated)
Easiest to understand, of the variations of point-to-point topology, is a point-to-point communications channel that appears, to the user, to be permanently associated with the two endpoints. Children's "tin-can telephone" is one example, with a microphone to a single public address speaker is another. These are examples of physical dedicated channels.
Within many switched telecommunications systems, it is possible to establish a permanent circuit. One example might be a telephone in the lobby of a public building, which is programmed to ring only the number of a telephone dispatcher. "Nailing down" a switched connection saves the cost of running a physical circuit between the two points. The resources in such a connection can be released when no longer needed, as, for example, a television circuit from a parade route back to the studio.
Switched:
Using circuit-switching or packet-switching technologies, a point-to-point circuit can be set up dynamically, and dropped when no longer needed. This is the basic mode of conventional telephony.

Bus
Linear bus
The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the 'bus', which is also commonly referred to as the backbone, or trunk) – all data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network virtually simultaneously (disregarding propagation delays).
Note: The two endpoints of the common transmission medium are normally terminated with a device called a terminator that exhibits the characteristic impedance of the transmission medium and which dissipates or absorbs the energy that remains in the signal to prevent the signal from being reflected or propagated back onto the transmission medium in the opposite direction, which would cause interference with and degradation of the signals on the transmission medium (See Electrical termination).
Distributed bus
The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology (i.e., all nodes share a common transmission medium).
Notes:
1.) All of the endpoints of the common transmission medium are normally terminated with a device called a 'terminator' (see the note under linear bus).
2.) The physical linear bus topology is sometimes considered to be a special case of the physical distributed bus topology – i.e., a distributed bus with no branching segments.
3.) The physical distributed bus topology is sometimes incorrectly referred to as a physical tree topology – however, although the physical distributed bus topology resembles the physical tree topology, it differs from the physical tree topology in that there is no central node to which any other nodes are connected, since this hierarchical functionality is replaced by the common bus.

Star
The type of network topology in which each of the nodes of the network is connected to a central node with a point-to-point link in a 'hub' and 'spoke' fashion, the central node being the 'hub' and the nodes that are attached to the central node being the 'spokes' (e.g., a collection of point-to-point links from the peripheral nodes that converge at a central node) – all data that is transmitted between nodes in the network is transmitted to this central node, which is usually some type of device that then retransmits the data to some or all of the other nodes in the network, although the central node may also be a simple common connection point (such as a 'punch-down' block) without any active device to repeat the signals.
Notes:
1.) A point-to-point link (described above) is sometimes categorized as a special instance of the physical star topology – therefore, the simplest type of network that is based upon the physical star topology would consist of one node with a single point-to-point link to a second node, the choice of which node is the 'hub' and which node is the 'spoke' being arbitrary[1].
2.) After the special case of the point-to-point link, as in note 1.) above, the next simplest type of network that is based upon the physical star topology would consist of one central node – the 'hub' – with two separate point-to-point links to two peripheral nodes – the 'spokes'.
3.) Although most networks that are based upon the physical star topology are commonly implemented using a special device such as a hub or switch as the central node (i.e., the 'hub' of the star), it is also possible to implement a network that is based upon the physical star topology using a computer or even a simple common connection point as the 'hub' or central node – however, since many illustrations of the physical star network topology depict the central node as one of these special devices, some confusion is possible, since this practice may lead to the misconception that a physical star network requires the central node to be one of these special devices, which is not true because a simple network consisting of three computers connected as in note 2.) above also has the topology of the physical star.
4.) Star networks may also be described as either broadcast multi-access or nonbroadcast multi-access (NBMA), depending on whether the technology of the network either automatically propagates a signal at the hub to all spokes, or only addresses individual spokes with each communication.
Extended star
A type of network topology in which a network that is based upon the physical star topology has one or more repeaters between the central node (the 'hub' of the star) and the peripheral or 'spoke' nodes, the repeaters being used to extend the maximum transmission distance of the point-to-point links between the central node and the peripheral nodes beyond that which is supported by the transmitter power of the central node or beyond that which is supported by the standard upon which the physical layer of the physical star network is based.
Note: If the repeaters in a network that is based upon the physical extended star topology are replaced with hubs or switches, then a hybrid network topology is created that is referred to as a physical hierarchical star topology, although some texts make no distinction between the two topologies.
Distributed Star
A type of network topology that is composed of individual networks that are based upon the physical star topology connected together in a linear fashion – i.e., 'daisy-chained' – with no central or top level connection point (e.g., two or more 'stacked' hubs, along with their associated star connected nodes or 'spokes').

Ring
The type of network topology in which each of the nodes of the network is connected to two other nodes in the network and with the first and last nodes being connected to each other, forming a ring – all data that is transmitted between nodes in the network travels from one node to the next node in a circular manner and the data generally flows in a single direction only.
Dual-ring
The type of network topology in which each of the nodes of the network is connected to two other nodes in the network, with two connections to each of these nodes, and with the first and last nodes being connected to each other with two connections, forming a double ring – the data flows in opposite directions around the two rings, although, generally, only one of the rings carries data during normal operation, and the two rings are independent unless there is a failure or break in one of the rings, at which time the two rings are joined (by the stations on either side of the fault) to enable the flow of data to continue using a segment of the second ring to bypass the fault in the primary ring.

Mesh
The value of fully meshed networks is proportional to the exponent of the number of subscribers, assuming that communicating groups of any two endpoints, up to and including all the endpoints, is approximated by Reed's Law.

Full
Fully connected
The type of network topology in which each of the nodes of the network is connected to each of the other nodes in the network with a point-to-point link – this makes it possible for data to be simultaneously transmitted from any single node to all of the other nodes.
Note: The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected[3].
Partial
Partially connected
The type of network topology in which some of the nodes of the network are connected to more than one other node in the network with a point-to-point link – this makes it possible to take advantage of some of the redundancy that is provided by a physical fully connected mesh topology without the expense and complexity required for a connection between every node in the network.
Note: In most practical networks that are based upon the physical partially connected mesh topology, all of the data that is transmitted between nodes in the network takes the shortest path (or an approximation of the shortest path) between nodes, except in the case of a failure or break in one of the links, in which case the data takes an alternate path to the destination. This requires that the nodes of the network possess some type of logical 'routing' algorithm to determine the correct path to use at any particular time.

Tree (also known as hierarchical):
The type of network topology in which a central 'root' node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy – the hierarchy of the tree is symmetrical, each node in the network having a specific fixed number, f, of nodes connected to it at the next lower level in the hierarchy, the number, f, being referred to as the 'branching factor' of the hierarchical tree.
Notes:
1.) A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree, since a network with a central 'root' node and only one hierarchical level below it would exhibit the physical topology of a star.
2.) A network that is based upon the physical hierarchical topology and with a branching factor of 1 would be classified as a physical linear topology.
3.) The branching factor, f, is independent of the total number of nodes in the network and, therefore, if the nodes in the network require ports for connection to other nodes the total number of ports per node may be kept low even though the total number of nodes is large – this makes the effect of the cost of adding ports to each node totally dependent upon the branching factor and may therefore be kept as low as required without any effect upon the total number of nodes that are possible.
4.) The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less than the total number of nodes in the network.
5.) If the nodes in a network that is based upon the physical hierarchical topology are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy.

Hybrid network topologies
The hybrid topology is a type of network topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies or a type of network topology that is composed of one or more interconnections of two or more networks that are based upon the same physical topology, but where the physical topology of the network resulting from such an interconnection does not meet the definition of the original physical topology of the interconnected networks (e.g., the physical topology of a network that would result from an interconnection of two or more networks that are based upon the physical star topology might create a hybrid topology which resembles a mixture of the physical star and physical bus topologies or a mixture of the physical star and the physical tree topologies, depending upon how the individual networks are interconnected, while the physical topology of a network that would result from an interconnection of two or more networks that are based upon the physical distributed bus network retains the topology of a physical distributed bus network).

Star-bus
A type of network topology in which the central nodes of one or more individual networks that are based upon the physical star topology are connected together using a common 'bus' network whose physical topology is based upon the physical linear bus topology, the endpoints of the common 'bus' being terminated with the characteristic impedance of the transmission medium where required – e.g., two or more hubs connected to a common backbone with drop cables through the port on the hub that is provided for that purpose (e.g., a properly configured 'uplink' port) would comprise the physical bus portion of the physical star-bus topology, while each of the individual hubs, combined with the individual nodes which are connected to them, would comprise the physical star portion of the physical star-bus topology.
Star-of-stars
Hierarchical star
A type of network topology that is composed of an interconnection of individual networks that are based upon the physical star topology connected together in a hierarchical fashion to form a more complex network – e.g., a top level central node which is the 'hub' of the top level physical star topology and to which other second level central nodes are attached as the 'spoke' nodes, each of which, in turn, may also become the central nodes of a third level physical star topology.
Notes:
1.) The physical hierarchical star topology is not a combination of the physical linear bus and the physical star topologies, as cited in some texts, as there is no common linear bus within the topology, although the top level 'hub' which is the beginning of the physical hierarchical star topology may be connected to the backbone of another network, such as a common carrier, which is, topologically, not considered to be a part of the local network – if the top level central node is connected to a backbone that is considered to be a part of the local network, then the resulting network topology would be considered to be a hybrid topology that is a mixture of the topology of the backbone network and the physical hierarchical star topology.
2.) The physical hierarchical star topology is also sometimes incorrectly referred to as a physical tree topology, since its physical topology is hierarchical, however, the physical hierarchical star topology does not have a structure that is determined by a branching factor, as is the case with the physical tree topology and, therefore, nodes may be added to, or removed from, any node that is the 'hub' of one of the individual physical star topology networks within a network that is based upon the physical hierarchical star topology.
3.) The physical hierarchical star topology is commonly used in 'outside plant' (OSP) cabling to connect various buildings to a central connection facility, which may also house the 'demarcation point' for the connection to the data transmission facilities of a common carrier, and in 'inside plant' (ISP) cabling to connect multiple wiring closets within a building to a common wiring closet within the same building, which is also generally where the main backbone or trunk that connects to a larger network, if any, enters the building.
Star-wired ring
A type of hybrid physical network topology that is a combination of the physical star topology and the physical ring topology, the physical star portion of the topology consisting of a network in which each of the nodes of which the network is composed are connected to a central node with a point-to-point link in a 'hub' and 'spoke' fashion, the central node being the 'hub' and the nodes that are attached to the central node being the 'spokes' (e.g., a collection of point-to-point links from the peripheral nodes that converge at a central node) in a fashion that is identical to the physical star topology, while the physical ring portion of the topology consists of circuitry within the central node which routes the signals on the network to each of the connected nodes sequentially, in a circular fashion.
Note: In an 802.5 Token Ring network the central node is called a Multistation Access Unit (MAU).
Hybrid mesh
A type of hybrid physical network topology that is a combination of the physical partially connected topology and one or more other physical topologies the mesh portion of the topology consisting of redundant or alternate connections between some of the nodes in the network – the physical hybrid mesh topology is commonly used in networks which require a high degree of availability..

Signal topology
The mapping of the actual connections between the nodes of a network, as evidenced by the path that the signals take when propagating between the nodes.

Note: The term 'signal topology' is often used synonymously with the term 'logical topology', however, some confusion may result from this practice in certain situations since, by definition, the term 'logical topology' refers to the apparent path that the data takes between nodes in a network while the term 'signal topology' generally refers to the actual path that the signals (e.g., optical, electrical, electromagnetic, etc.) take when propagating between nodes.
Example
In an 802.4 Token Bus network, the physical topology may be a physical bus, a physical star, or a hybrid physical topology, while the signal topology is a bus (i.e., the electrical signal propagates to all nodes simultaneously [ignoring propagation delays and network latency] ), and the logical topology is a ring (i.e., the data flows from one node to the next in a circular manner according to the protocol).

Logical topology
The mapping of the apparent connections between the nodes of a network, as evidenced by the path that data appears to take when traveling between the nodes.


Classification of logical topologies
The logical classification of network topologies generally follows the same classifications as those in the physical classifications of network topologies, the path that the data takes between nodes being used to determine the topology as opposed to the actual physical connections being used to determine the topology.

Notes:
1.) Logical topologies are often closely associated with media access control (MAC) methods and protocols.
2.) The logical topologies are generally determined by network protocols as opposed to being determined by the physical layout of cables, wires, and network devices or by the flow of the electrical signals, although in many cases the paths that the electrical signals take between nodes may closely match the logical flow of data, hence the convention of using the terms 'logical topology' and 'signal topology' interchangeably.
3.) Logical topologies are able to be dynamically reconfigured by special types of equipment such as routers and switches.

Daisy chains
Except for star-based networks, the easiest way to add more computers into a network is by daisy-chaining, or connecting each computer in series to the next. If a message is intended for a computer partway down the line, each system bounces it along in sequence until it reaches the destination. A daisy-chained network can take two basic forms: linear and ring.

A linear topology puts a two-way link between one computer and the next. However, this was expensive in the early days of computing, since each computer (except for the ones at each end) required two receivers and two transmitters.
By connecting the computers at each end, a ring topology can be formed. An advantage of the ring is that the number of transmitters and receivers can be cut in half, since a message will eventually loop all of the way around. When a node sends a message, the message is processed by each computer in the ring. If a computer is not the destination node, it will pass the message to the next node, until the message arrives at its destination. If the message is not accepted by any node on the network, it will travel around the entire ring and return to the sender. This potentially results in a doubling of travel time for data, but since it is traveling at a fairly insignificant multiple of the speed of light, the loss is usually negligible.

Centralization
The star topology reduces the probability of a network failure by connecting all of the peripheral nodes (computers, etc.) to a central node. When the physical star topology is applied to a logical bus network such as Ethernet, this central node (traditionally a hub) rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the remaining peripheral nodes will be unaffected. However, the disadvantage is that the failure of the central node will cause the failure of all of the peripheral nodes also.

If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way round trip transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

A tree topology (a.k.a. hierarchical topology) can be viewed as a collection of star networks arranged in a hierarchy. This tree has individual peripheral nodes (e.g. leaves) which are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the functionality of the central node may be distributed.

As in the conventional star network, individual nodes may thus still be isolated from the network by a single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.

In order to alleviate the amount of network traffic that comes from broadcasting all signals to all nodes, more advanced central nodes were developed that are able to keep track of the identities of the nodes that are connected to the network. These network switches will "learn" the layout of the network by "listening" on each port during normal data transmission, examining the data packets and recording the address/identifier of each connected node and which port it's connected to in a lookup table held in memory. This lookup table then allows future transmissions to be forwarded to the intended destination only.


Decentralization
In a mesh topology (i.e., a partially connected mesh topology), there are at least two nodes with two or more paths between them to provide redundant paths to be used in case the link providing one of the paths fails. This decentralization is often used to advantage to compensate for the single-point-failure disadvantage that is present when using a single device as a central node (e.g., in star and tree networks). A special kind of mesh, limiting the number of hops between two nodes, is a hypercube. The number of arbitrary forks in mesh networks makes them more difficult to design and implement, but their decentralized nature makes them very useful. This is similar in some ways to a grid network, where a linear or ring topology is used to connect systems in multiple directions. A multi-dimensional ring has a toroidal topology, for instance.

A fully connected network, complete topology or full mesh topology is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are n(n-1)/2 direct links. Networks designed with this topology are usually very expensive to set up, but provide a high degree of reliability due to the multiple paths for data that are provided by the large number of redundant links between nodes. This topology is mostly seen in military applications. However, it can also be seen in the file sharing protocol BitTorrent in which users connect to other users in the "swarm" by allowing each user sharing the file to connect to other users also involved. Often in actual usage of BitTorrent any given individual node is rarely connected to every single other node as in a true fully connected network but the protocol does allow for the possibility for any one node to connect to any other node when sharing files.


Hybrids
Hybrid networks use a combination of any two or more topologies in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For example, a tree network connected to a tree network is still a tree network, but two star networks connected together exhibit a hybrid network topology. A hybrid topology is always produced when two different basic network topologies are connected. Two common examples for Hybrid network are: star ring network and star bus network

A Star ring network consists of two or more star topologies connected using a multistation access unit (MAU) as a centralized hub.
A Star Bus network consists of two or more star topologies connected using a bus trunk (the bus trunk serves as the network's backbone).
While grid networks have found popularity in high-performance computing applications, some systems have used genetic algorithms to design custom networks that have the fewest possible hops in between different nodes. Some of the resulting layouts are nearly incomprehensible, although they function quite well.

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