Error correction is relatively expensive (computationally and in bandwidth).
For example, 10 redundancy bits are required to correct 1 single-bit error in a 1000-bit message. Detection? In contrast, detecting a single bit error requires only a single-bit, no matter how large the message.
The most popular error detection codes are based on polynomial codes or cyclic redundancy codes (CRCs).
Allows us to acknowledge correctly received frames and to discard incorrect ones.
For example, consider parity: A single parity bit is appended to each data block (e.g. each character in ASCII systems) so that the number of 1 bits always adds up to an even (odd) number.
1000000(1) 1111101(0)
The Hamming Distance for parity is 2, and it cannot correct even single-bit errors (but can detect single-bit errors).
As another example, consider a 10-bit code used to represent 4 possible values: ``00000 00000'', ``00000 11111'', ``11111 00000'', and ``11111 11111''. Its Hamming distance is 5, and we can correct 2 single-bit errors:
For instance, ``10111 00010'' becomes ``11111 00000'' by changing only two bits.
However, if the sender transmits ``11111 00000'' and the receiver sees ``00011 00000'', the receiver will not correct the error properly.
Finally, in this example we are guaranteed to catch all 2-bit errors, but we might do better: if ``00111 00111'' contains 4 single-bit errors, we will reconstruct the block correctly.
To understand errors, consider the following:
possible data words are legal. However, by choosing check bits carefully, the resulting codewords will have a large Hamming Distance. The larger the Hamming distance, the better able the code can detect errors. To detect d 1-bit errors requires having a Hamming Distance of at least d+1 bits. Why?
To correct d errors requires 2d+1 bits. Intuitively, after d errors, the garbled messages is still closer to the original message than any other legal codeword.
There are two types of attacks against errors:
In data communication, line noise is a fact of life (e.g., signal attenuation, natural phenomenon such as lightning, and the telephone repairman). Moreover, noise usually occurs as bursts rather than independent, single bit errors. For example, a burst of lightning will affect a set of bits for a short time after the lightning strike.
Detecting and correcting errors requires redundancy -- sending additional information along with the data.
Flow control deals with throttling the speed of the sender to match that of the receiver. Usually, this is a dynamic process, as the receiving speed depends on such changing factors as the load, and availability of buffer space.
One solution is to have the receiver extend credits to the sender. For each credit, the sender may send one frame. Thus, the receiver controls the transmission rate by handing out credits.
Retransmission timers are used to resend frames that don't produce an ACK. When sending a frame, schedule a timer to expire at some time after the ACK should have been returned. If the timer goes off, retransmit the frame.
In some systems, the receiver also returns a negative acknowledgment (NACK) for incorrectly-received frames. This is nothing more than a hint to the sender so that it can retransmit a frame right away without waiting for a timer to expire.
The advantage of encoding violations is that no extra bandwidth is required as in bit-stuffing. The IEEE 802.4 standard uses this approach.
Use reserved characters to indicate the start and end of a frame. For instance, use the two-character sequence DLE STX (Data-Link Escape, Start of TeXt) to signal the beginning of a frame, and the sequence DLE ETX (End of TeXt) to flag the frame's end.
Problem: What happens if the two-character sequence DLE ETX happens to appear in the frame itself?
Solution: Use character stuffing; within the frame, replace every occurrence of DLE with the two-character sequence DLE DLE. The receiver reverses the processes, replacing every occurrence of DLE DLE with a single DLE.
Example: If the frame contained ``A B DLE D E DLE'', the characters transmitted over the channel would be ``DLE STX A B DLE DLE D E DLE DLE DLE ETX''.
Disadvantage: character is the smallest unit that can be operated on; not all architectures are byte oriented.
Problem: What happens if the reserved delimiter happens to appear in the frame itself? If we don't remove it from the data, the receiver will think that the incoming frame is actually two smaller frames!
Solution: Use bit stuffing. Within the frame, replace every occurrence of two consecutive 1's with 110. E.g., append a zero bit after each pair of 1's in the data. This prevents 3 consecutive 1's from ever appearing in the frame.
Likewise, the receiver converts two consecutive 1's followed by a 0 into two 1's, but recognizes the 0111 sequence as the end of the frame.
Example: The frame ``1011101'' would be transmitted over the physical layer as ``0111101101010111''.
Note: When using bit stuffing, locating the start/end of a frame is easy, even when frames are damaged. The receiver simply scans arriving data for the reserved patterns. Moreover, the receiver will resynchronize quickly with the sender as to where frames begin and end, even when bits in the frame get garbled.
The main disadvantage with bit stuffing is the insertion of additional bits into the data stream, wasting bandwidth. How much expansion? The precise amount depends on the frequency in which the reserved patterns appear as user data.
Disadvantage: Receiver loses synchronization when bits become garbled. If the bits in the count become corrupted during transmission, the receiver will think that the frame contains fewer (or more) bits than it actually does. Although checksum will detect the incorrect frames, the receiver will have difficulty resynchronizing to the start of a new frame. This technique is not used anymore, since better techniques are available.
If we don't follow the OSI reference model as gospel, we can imagine providing several alternative service semantics:
Connection state keeps track of sending order and which frames require retransmission. For example, receiver state includes which frames have been received, which ones have not, etc.
When would such a service be appropriate?
Typically, each frame is assigned a unique sequence number, which the receiver returns in an acknowledgment frame to indicate which frame the ACK refers to. The sender must retransmit unacknowledged (e.g., lost or damaged) frames.
This layer interfaces directly to and performs application services for the application processes; it also issues requests to the presentation layer. Note carefully that this layer provides services to user-defined application processes, and not to the end user. For example, it defines a file transfer protocol, but the end user must go through an application process to invoke file transfer. The OSI model does not include human interfaces. The common application services sublayer provides functional elements including the Remote Operations Service Element (comparable to Internet Remote Procedure Call), Association Control, and Transaction Processing (according to the ACID requirements).
Above the common application service sublayer are functions meaningful to user application programs, such as messaging (X.400), directory (X.500), file transfer (FTAM), virtual terminal (VTAM), and batch job manipulation (JTAM). These contrast with user applications that use the services of the application layer, but are not part of the application layer itself.
This 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 the Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded text file 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.
The iSCSI, which implements the Small Computer Systems Interface encapsulated into TCP/IP packets, is a session layer protocol increasingly used in the Storage Area Networks and internally between processors and high-performance storage devices. iSCSI uses TCP for guaranteed delivery, and carries SCSI command descriptor blocks (CDB) as payload to create a virtual SCSI bus between iSCSI initiators and iSCSI targets.
This 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 it was not developed under the OSI Reference Model and does not strictly conform to the OSI definition of the Transport Service, the best known example of a layer 4 protocol is the TCP( Transmission Control Protocol). The transport layer is the layer that converts messages into TCP segments or UDP ( User Datagram Protocol) , SCTP (Stream Control Transmission Protocol), etc. packets.
Of the actual OSI protocols, not merely protocols developed under the model, 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 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer.
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 over an IP network, or end-to-end encryption . While GRE (Generic Routing Encapsulation) 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.
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, to 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.
This 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-multi point media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multi access media, was developed independently of the ISO work,in the IEEE project802. IEEE work assumed sub layering 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 the 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 layers by protocols such as the TCP, but is still used in niches where the 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 Fiber distributed data interface (FDDI) bit stream is not used by the data link layer.
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 the RS-232 do use physical wires to control access to the medium.
The major functions and services performed by the physical layer are:
In 1977, work on a layered model of network architecture, which was to become the OSI model, started in the ANSI (American National Standards Institute) working group on Distributed Systems (DISY). With the DISY work and worldwide input, the ISO (International Organization for standardization) began to develop its OSI networking suite.According to Bachman, the term "OSI" came into use on 12 October 1979. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model) and a set of concrete protocols. The standard documents that describe OSI can be downloaded from ISO .
Parts of OSI have influenced Internet protocol development, but none more than the abstract model itself, 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.
In particular, Internet protocols are deliberately not as rigorously architected as the OSI model, but a common version of TCP/IP model splits it into four layers. The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its End-to-End Layer includes the graceful close function of the OSI Session Layer as well as the Transport Layer. Its Internetwork Layer is equivalent to the OSI Network Layer, while its Interface layer includes the OSI Data Link and Physical Layers. 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.
Protocols enable an entity in one host to interact with a corresponding entity at the same layer in a remote 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 inside the local host.
what is a router ?Routers uses the headers and forwarding tables to determine the best path for forwarding the packets, and they use the protocols such as the ICMP to communicate with each other and configure the best route between any two hosts.
Very little filtering of data is done through routers.
A simple computer network may be constructed from two computers where network adapter (Network Interface Controller ( NIC )) is added to each computer and then connecting them together with a special cable crossover cable. This type of network is useful for transferring information between two computers that are not normally connected to each other by a permanent network connection or for basic home networking applications. Alternatively, a network between two computers can be established without dedicated extra hardware by using a standard connection on both computers, connecting them to each other via a special null modem cable which is cross linked.
Practical networks generally consist of more than two interconnected computers and generally require special devices in addition to the Network Interface Controller that each computer needs to be equipped with. Examples of some of these special devices are hubs, switches and routers.
An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g. a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.
An intranet is a set of interconnected networks, Internet Protocol is used and uses IP-based tools such as web browsers, that is under the control of a single administrative entity. That administrative entity closes the intranet to the rest of the world, and allows only specific users. Most commonly, an intranet is the internal network of a company or other enterprise.
In modern practice, the interconnected networks use the Internet Protocol. There are at least three variants of internetwork, depending on who administers and who participates in them:
Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet itself is not considered to be a part of the intranet or extranet, although the Internet may serve as a portal for access to portions of an extranet.
Global area networks (GAN) specifications are in development by several groups, and there is no common definition. In general, however, a GAN is a model for supporting mobile communications across an arbitrary number of wireless LAN's , satellite coverage areas, etc. The key challenge in mobile communications is "handing off" the user communications from one local coverage area to the next.
IEEE mobility efforts focus on the data link layer and make assumptions about the media.
A WAN is a data communications network that covers a relatively broad geographic area (i.e. one city to another and one country to another country) and that often uses transmission facilities provided by common carriers, such as telephone companies.
We will follow traditional usage and call these machines hosts.The hosts are connected by a communication subnet, or just subnet for short .the hosts are owned by the customers whereas the communication subnet is typically owned and operated by a telephone company or Internet service provider. The job of the subnet is to carry messages from host to host, just as telephone system which carries words from speaker to listener.
in most WAN's , the subnet consists of 2 distinct components:the switching computers are commonly known as routers.
the figure shows how a WAN is useful for companies.
A Metropolitan Area Network is a network that connects two or more Local Area Networks or Campus Area Networks together but does not extend beyond the boundaries of the immediate town, city, or metropolitan area. Multiple routers, switches & hubs are connected to create a MAN.
A personal area network (PAN) is a computer network used for communication among computer devices close to one person. Some examples of devices that may be used in a PAN are printers, fax machines, telephones, or scanners. The reach of a PAN is typically within about 20-30 feet (approximately 4-6 Meters). And this can be used for communication among the individual devices (intrapersonal communication), or for connecting to a higher level network and the Internet (an uplink).
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