Mesh Topoloy

Mesh Topoloy

In this topology, every node has a dedicated point-to-point connection to every other node on the network. A fully connected mesh network has n(n-1)/2 channels to link 'n' devices. Therefore, every device on the network must have 'n-1' input/output (I/O) ports.

In mesh network, each node is directly connected to all nodes on the network. This type of network involves the concept of routes. In this type of network, each node may send message to destination through multiple paths. It means that each node of mesh network has several possible paths to send (or to receive) message, but in Bus, Star, Ring and Tree topologies each node has only one path.

Advantages

Mesh topology has the following advantages:

• It has multiple links, so if one route is blocked then other routes can be used for data communication.
• Each connection can have its own data load, so the traffic problem is eliminated.
• It ensures the data privacy or security, because every message travels along a dedicated link.
• Troubleshooting of this topology is easy as compared to other networks.
• Its performance is not affected with heavy load of data transmission.

Disadvantages

Mesh topology has the following disadvantages:

• It becomes very expensive because a large number of cabling and 110 ports are required.
• It is difficult to install.

Tree Topology

Tree Topology

In tree network, the nodes are connected to each other in such a way that forms a tree like structure. Typically to form a tree network, multiple star topologies are combined together. This type of network has combined features of bus and star topology.

On tree topology the hubs of each star topology are connected to the central hub that controls the entire network. However, some nodes can be directly connected to the central hub. The tree topology configuration is shown in figure below.

The central Hub in the tree network is an active hub. It contains a repeater (a hardware device), which re-generates the received bit patterns. The secondary hubs usually are passive hubs. The passive hub controls the nodes directly connected to it and exchange data to other devices connected to the other secondary hubs (or same hub) through the central hub. The secondary hub may also be active hub if another secondary hub is directly connected to it. The cable TV network is an example of tree topology, where main cable is divided into, branches and each branch is further divided into smaller branches and so on. The hub is used when a branch is created.

Advantages

The tree topology has the same advantages as star topology but it has some additional advantages. These are.

• It allows more devices to be connected to the central Hub.

Disadvantages

The tree topology also has the same disadvantages as star topology built has some additional disadvantages such as:

• It because more expansive because more hubs are required to install the network.

Hybrid Topology

A ring topology is a network topology or circuit arrangement in which each network device is attached along the same signal path to two other devices, forming a path in the shape of a ring. Each device in the network that is also referred to as node handles every message that flows through the ring. Each node in the ring has a unique address. Since in a ring topology there is only one pathway between any two nodes, ring networks are generally disrupted by the failure of a single link.

The redundant topologies are used to eliminate network downtime caused by a single point of failure. All networks need redundancy for enhanced reliability. Network reliability is achieved through reliable equipment and network designs that are tolerant to failures and faults. The FDDI networks overcome the disruption in the network by sending data on a clockwise and a counterclockwise ring. In case there is a break in data flow,the data is wrapped back onto the complementary ring before it reaches the end of the cable thereby maintaining a path to every node within the complementary ring.

The most well known example of a ring topology is Token Ring.

[edit]

Advantages

• An orderly network where every device has access to the token and the opportunity to transmit
• Under heavy network load performs better than a start topology.
• To manage the connectivity between the computers it doesnt need network server.

[edit]

Disadvantages

• One malfunctioning workstation can throw away the entire network.
• Moves, adds and changes of devices can affect the entire network .
• It is slower than an Ethernet network.

• Unlike Ethernet, Token Ring uses a ring topology whereby the data is sent from one machine to the next and so on around the ring until it ends up back where it started. It also uses a token passing protocol which means that a machine can only use the network when it has control of the Token, this ensures that there are no collisions because only one machine can use the network at any given time. The Basics Here is an animated GIF that shows the basic operation of a Token Ring, and below is an explanation of what is going on.

  Although 16Mbps is the standard ring speed these days (and Fast Token Ring is being developed) we will consider a 4Mbps Token Ring in this tutorial to explain the basic concepts.
  
  

  Hit 'Refresh' on your browser to start the animation from the beginning

  At the start, a free Token is circulating on the ring, this is a data frame which to all intents and purposes is an empty vessel for transporting data. To use the network, a machine first has to capture the free Token and replace the data with its own message.

  In the example above, machine 1 wants to send some data to machine 4, so it first has to capture the free Token. It then writes its data and the recipient's address onto the Token (represented by the yellow flashing screen).

  The packet of data is then sent to machine 2 who reads the address, realizes it is not its own, so passes it on to machine 3. Machine 3 does the same and passes the Token on to machine 4.

  This time it is the correct address and so number 4 reads the message (represented by the yellow flashing screen). It cannot, however, release a free Token on to the ring, it must first send the message back to number 1 with an acknowledgement to say that it has received the data (represented by the purple flashing screen).

  The receipt is then sent to machine 5 who checks the address, realizes that it is not its own and so forwards it on to the next machine in the ring, number 6.

  Machine 6 does the same and forwards the data to number 1, who sent the original message.

  Machine 1 recognizes the address, reads the acknowledgement from number 4 (represented by the purple flashing screen) and then releases the free Token back on to the ring ready for the next machine to use.

  That's the basics of Token Ring and it shows how data is sent, received and acknowledged, but Token Ring also has a built in management and recovery system which makes it very fault tolerant. Below is a brief outline of Token Ring's self maintenance system.
  

  Token Ring Self Maintenance
  When a Token Ring network starts up, the machines all take part in a negotiation to decide who will control the ring, or become the 'Active Monitor' to give it its proper title. This is won by the machine with the highest MAC address who is participating in the contention procedure, and all other machines become 'Standby Monitors'.

  The job of the Active Monitor is to make sure that none of the machines are causing problems on the network, and to re-establish the ring after a break or an error has occurred. The Active Monitor performs Ring Polling every seven seconds and ring purges when there appears to be a problem. The ring polling allows all machines on the network to find out who is participating in the ring and to learn the address of their Nearest Active Upstream Neighbour (NAUN). Ring purges reset the ring after an interruption or loss of data is reported.

  Each machine knows the address of its Nearest Active Upstream Neighbour. This is an important function in a Token Ring as it updates the information required to re-establish itself when machines enter or leave the ring.

  When a machine enters the ring it performs a lobe test to verify that its own connection is working properly, if it passes, it sends a voltage to the hub which operates a relay to insert it into the ring.

  If a problem occurs anywhere on the ring, the machine that is immediately after the fault will cease to receive signals. If this situation continues for a short period of time it initiates a recovery procedure which assumes that its NAUN is at fault, the outcome of this procedure either removes its neighbour from the ring or it removes itself.
  

  Token Ring Operation using a Hub

  
  

  A Token Ring hub simply changes the topology from a physical ring to a star wired ring. The Token still circulates around the network and is still controlled in the same manner, however, using a hub or a switch greatly improves reliability because the hub can automatically bypass any ports that are disconnected or have a cabling fault. Further advancements have been made in recent years with regard to Token Ring technology, such as early Token release and Token Ring switching but as this site is primarily concerned with cabling issues we will not go into any more detail here.

BUS topology

http://ucan.us/doyetech/images/bustop.jpg

BUS TOPOLOGY

A bus network uses a multi-drop transmission medium, all node on the network share a common bus and thus share communication. This allows only one device to transmit at a time. A distributed access protocol determines which station is to transmit. Data frames contain source and destination addresses, where each station monitors the bus and copies frames addressed to itself.

( a typical bus topology)

A bus topology connects each computer (nodes) to a single segment trunk (a communication line, typically coax cable, that is referred to as the 'bus'. The signal travels from one end of the bus to the other. A terminator is required at each to absorb the signal so as it does not reflect back across the bus. A media access method called CSMA/MA is used to handle the collision that occur when two signals placed on the wire at the same time. The bus topology is passive. In other words, the computers on the bus simply 'listen' for a signal; they are not responsible for moving the signal along.

Advantages: Failure of one of the station does not affect others.

Good compromise over the other two topologies as it allows relatively high rate of data tansmittion.

Well suited for temporary networks that must be set up in a hurry.

Easy to implement and extend.

Disadvantage: Require a network to detect when two nodes are transmitting at the same time.

Does not cope well with heavy traffic rates

Difficult to administer/troubleshoot.

Limited cable length and number of stations.

A cable brake can disable the entire network; no redundancy.

Maintenance cost may be higher in the long run.

Performance degrade as additional computers are added.

stra topologi

Star network

From Wikipedia, the free encyclopedia

Star network layout

Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages.^[1]Thus, the hub and leaf nodes, and the transmission lines between them, form a graph with the topology of astar. 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 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.

The star topology reduces the chance of network failure by connecting all of the systems to a central node. When applied to a bus-based network, this central 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 rest of the systems will be unaffected. ^[2]

Contents [hide] 1 Advantages 2 Disadvantages 3 References 4 See also

[edit]Advantages

• Better performance: The star topology prevents the passing of data packets through an excessive number of nodes. At most, 3 devices and 2 links are involved in any communication between any two devices. Although this topology places a huge overhead on the central hub, with adequate capacity, the hub can handle very high utilization by one device without affecting others.
• Isolation of devices: Each device is inherently isolated by the link that connects it to the hub. This makes the isolation of individual devices straightforward and amounts to disconnecting each device from the others. This isolation also prevents any non-centralized failure from affecting the network.
• Benefits from centralization: As the central hub is the bottleneck, increasing its capacity, or connecting additional devices to it, increases the size of the network very easily. Centralization also allows the inspection of traffic through the network. This facilitates analysis of the traffic and detection of suspicious behavior.
• Simplicity: This topology is easy to understand, establish, and navigate. Its simplicity obviates the need for complex routing or message passing protocols. Also, as noted earlier, the isolation and centralization it allows simplify fault detection, as each link or device can be probed individually.

[edit]Disadvantages

The primary disadvantage of a star topology is the high dependence of the system on the functioning of the central hub. While the failure of an individual link only results in the isolation of a single node, the failure of the central hub renders the network inoperable, immediately isolating all nodes. The performance and scalability of the network also depend on the capabilities of the hub. Network size is limited by the number of connections that can be made to the hub, and performance for the entire network is capped by its throughput. While in theory traffic between the hub and a node is isolated from other nodes on the network, other nodes may see a performance drop if traffic to another node occupies a significant portion of the central node's processing capability or throughput. Furthermore, wiring up of the system can be very complex.

toplogies n thier types

What is Topology?

A topology is configuration of communication networksand is of two types, Physical and Logical. Physicaltopology refers to configuration of computers, cables, devices and mostly depends on various factors. A logical topology is a method of transmitting or passing data between workstations.

Types of Physical Topologies

1. Bus Network (also known as Liner Bus)
2. Star Topology (Centralization)
3. Ring Topology (also known as Star-Wired or Token Ring Network)
4. Tree
5. Mesh Topology

Bus Network

A bus network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet networks.

Star Topology

In a Star Topology each computer is directly connected to the centralized Hub or a Switch. In this way, when computer A sends a data packet for computer B, the data flows through the Hub or Switch to which both computer A and B are connected. Different types of cables can be used in this scenario like coaxial cable, fibre optic cable and twisted pair cable.

Token Ring / Star-Wired

A token ring topology is architecturally similar to star topology. The only difference here is that it is created of wiring that would allow transfer of data from one computer to another in a ring (or circle). A token ring network will pass information based on token system.

Tree

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network with ease.

Mesh Topology

A fully connected or complete 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)/2direct links. Synonym fully connected mesh network.

In a mesh topology, there are at least two nodes with two or more paths between them. 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 (torus) topology, for instance.

Considerations

Consider the following when choosing a topology:-

• Future Growth: Is the network for temporary use or will undergo lot of growth in the future. Plan it accordingly.
• Money: What is your budget? What will be the purpose of this network?
• Cable Media: Type of cable that should be used as per the standards.
• Length of the cable: How far are your systems placed in the network? Is it the same building that your systems will be placed in or if your office is in two floors which should be connected?

Summary

See the below table for a quick understanding and comparison of topologies, cable media and protocols used.

Topologies Comparison Table

Topology	Cable Media	Protocols Used
Linear Bus	Twisted Pair Coaxial Fiber	Ethernet LocalTalk
Star	Twisted Pair Fiber	Ethernet LocalTalk
Token / Star-Wired Ring	Twisted Pair	Token Ring
Tree	Twisted Pair Coaxial Fiber	Ethernet

links of osi model

Here are some links with explanation on what each layer does:

• http://www.dummies.com/how-to/content/getting-to-know-the-osi-model-for-the-ccna-exam.html
• http://www.webopedia.com/quick_ref/OSI_Layers.asp

OSI model

The OSI Model
7. Application Layer
NNTP · SIP · SSI · DNS · FTP · Gopher · HTTP · NFS · NTP · SMPP · SMTP ·SNMP · Telnet (more)
6. Presentation Layer
MIME · XDR ·
5. Session Layer
Named Pipes · NetBIOS · SAP
4. Transport Layer
TCP · UDP · PPTP · SCTP · SSL · TLS
3. Network Layer
IP · ICMP · IPsec · IGMP
2. Data Link Layer
ARP · CSLIP · SLIP · Ethernet · Frame relay · ITU-T G.hn DLL · L2TP · PPP
1. Physical Layer
RS-232 · V.35 · V.34 · I.430 · I.431 · T1 · E1 · POTS · SONET/SDH · OTN ·DSL · 802.11a/b/g/n PHY · ITU-T G.hn PHY
This box: view • talk • edit

The Open System Interconnection 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. On each layer an instance provides services to the instances at the layer above and requests service from the layer below. 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. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.

Communication in the OSI-Model (Example with layers 3 to 5)

Contents [hide] 1 History 2 Description of OSI layers 2.1 Layer 1: Physical Layer 2.2 Layer 2: Data Link Layer 2.2.1 WAN Protocol architecture 2.2.2 IEEE 802 LAN architecture 2.3 Layer 3: Network Layer 2.4 Layer 4: Transport Layer 2.5 Layer 5: Session Layer 2.6 Layer 6: Presentation Layer 2.7 Layer 7: Application Layer 3 Interfaces 4 Examples 5 Comparison with TCP/IP 6 See also 7 References 8 External links

[edit]History

In 1970, work on a layered model of network architecture was started and the International Organization for Standardization (ISO) began to develop its OSI framework architecture. OSI has two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols.

Note: 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 ISO, but only some of them at no charge.^[3]

All 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.

[edit]Description of OSI layers

OSI Model
	Data unit	Layer	Function
Host layers	Data	7. Application	Network process to application
		6. Presentation	Data representation and encryption
		5. Session	Interhost communication
	Segment	4. Transport	End-to-end connections and reliability
Media layers	Packet	3. Network	Path determination andlogical addressing
	Frame	2. Data Link	Physical addressing
	Bit	1. Physical	Media, signal and binary transmission

[edit]Layer 1: Physical Layer

Main article: Physical Layer

The Physical Layer defines 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, cablespecifications, 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). 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, contentionresolution 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, ITU-T G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

[edit]Layer 2: Data Link Layer

Main article: Data Link Layer

The 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 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 the 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.

The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete Data Link Layer which provides both error correction and flow control by means of a selective repeat Sliding Window Protocol.

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 Layer.

[edit]WAN Protocol architecture

Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They are also 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.

[edit]IEEE 802 LAN architecture

Practical, 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 Ringand FDDI. The MAC sublayer detects but does not correct errors.

[edit]Layer 3: Network Layer

Main article: Network Layer

The 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 networkrouting 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 the packet into sufficiently small packets that the medium can accept.

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.

[edit]Layer 4: Transport Layer

Main article: Transport Layer

The 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, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries, both of which TCP is incapable. Detailed characteristics of TP0-4 classes are shown in the following table:^[4]

Feature Name	TP0	TP1	TP2	TP3	TP4
Connection oriented network	Yes	Yes	Yes	Yes	Yes
Connectionless network	No	No	No	No	Yes
Concatenation and separation	No	Yes	Yes	Yes	Yes
Segmentation and reassembly	Yes	Yes	Yes	Yes	Yes
Error Recovery	No	Yes	No	Yes	Yes
Reinitiate connection (if an excessive number of PDUs are unacknowledged)	No	Yes	No	Yes	No
multiplexing and demultiplexing over a single virtual circuit	No	No	Yes	Yes	Yes
Explicit flow control	No	No	Yes	Yes	Yes
Retransmission on timeout	No	No	No	No	Yes
Reliable Transport Service	No	Yes	No	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 protocolsoperate 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.

[edit]Layer 5: Session Layer

Main article: Session Layer

The Session Layer controls the dialogues (connections) 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 theTransmission Control Protocol, 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.

[edit]Layer 6: Presentation Layer

Main article: Presentation Layer

The 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.

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

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 serialization of objects and other data structures from and to XML.

[edit]Layer 7: Application Layer

Main article: Application Layer

The 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 or 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 Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and X.400 Mail...

[edit]Interfaces

Neither 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.

[edit]Examples

Layer		OSI protocols	TCP/IP protocols	Signaling System 7[5]	AppleTalk	IPX	SNA	UMTS	Misc. examples
#	Name
7	Application	FTAM, X.400,X.500, DAP,ROSE, RTSE,ACSE	NNTP, SIP, SSI,DNS, FTP, Gopher,HTTP, NFS, NTP,DHCP, SMPP,SMTP, SNMP,Telnet, RIP, BGP	INAP,MAP,TCAP,ISUP,TUP	AFP, ZIP,RTMP,NBP	RIP,SAP	APPC		HL7, Modbus
6	Presentation	ISO/IEC 8823, X.226, ISO/IEC 9576-1, X.236	MIME, SSL, TLS,XDR	AFP				TDI, ASCII, EBCDIC,MIDI, MPEG
5	Session	ISO/IEC 8327, X.225, ISO/IEC 9548-1, X.235	Sockets. Session establishment inTCP, SIP, RTP		ASP,ADSP,PAP	NWLink	DLC?		Named pipes, NetBIOS,SAP, half duplex, full duplex, simplex, SDP,RPC
4	Transport	ISO/IEC 8073, TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC 8602, X.234	TCP, UDP, SCTP			DDP,SPX			NBF
3	Network	ISO/IEC 8208,X.25 (PLP), ISO/IEC 8878,X.223, ISO/IEC 8473-1, CLNP X.233.	IP, IPsec, ICMP,IGMP, OSPF	SCCP,MTP	ATP(TokenTalkorEtherTalk)	IPX		RRC (Radio Resource Control)Packet Data Convergence Protocol (PDCP) and BMC(Broadcast/Multicast Control)	NBF, Q.931, IS-IS
2	Data Link	ISO/IEC 7666,X.25 (LAPB),Token Bus, X.222, ISO/IEC 8802-2 LLC Type 1 and 2	PPP, SLIP, PPTP,L2TP	MTP,Q.710	LocalTalk,AppleTalk Remote Access,PPP	IEEE 802.3framing,Ethernet II framing	SDLC	LLC (Logical Link Control), MAC(Media Access Control)	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, ARP(maps layer 3 to layer 2 address), ITU-T G.hn DLL
1	Physical	X.25 (X.21bis,EIA/TIA-232,EIA/TIA-449,EIA-530,G.703)		MTP,Q.710	RS-232,RS-422,STP,PhoneNet		Twinax	UMTS L1 (UMTS Physical Layer)	RS-232, Full duplex,RJ45, V.35, V.34, I.430,I.431, T1, E1, 10BASE-T,100BASE-TX, POTS,SONET, SDH, DSL,802.11a/b/g/n PHY, ITU-T G.hn PHY

[edit]Comparison with TCP/IP

In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.^[6] RFC 3439contains 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-endTransport 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.^{[citation needed]}

[edit]See also