how to generate qr code using vb.net Data Communications Basics 14 Introduction to Network Technologies and Performance in Software

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Data Communications Basics 14 Introduction to Network Technologies and Performance
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data moves from a computer into the top of the protocol stack, it gets wrapped in a series of headers and trailers that allow each layer of the stack to do its job A simplified conceptual example of data moving from a computer through an IP stack onto an Ethernet LAN is shown in Figure 21 This describes the basic elements, with many detailed fields left out in order to reduce confusion Data starts on the local computer As it is passed along, moving from the top of the protocol stack down to the network interface card, it is broken into the correct size for the protocol by the network driver The network driver is a small piece of software that communicates between the computer system and its network card As the data progresses down the TCP/IP stack from the top, service information is added at the TCP level In the case of TCP, services are mapped to a logical entity called a port number Following this, the IP layer adds the Network layer addressing information (in this case the IP address) The IP layer then hands the packet down to the Data Link layer, where the media access control (MAC) address or physical address is appended A cyclical redundancy check (CRC) is added to the end of the packet to ensure packet integrity The packet is now fully assembled and ready to be passed to the Physical layer, where it is turned into electrical or optical signals on the physical media In some cases the packet may be further processed by an interconnect In the example, for instance, the completed packet might move to a router to be transported across a wide area network using the frame relay protocol In this case, a frame relay header and trailer would be appended by the sending router, and then stripped off at the receiving end by the receiving router The process that happens at each layer of the protocol stack, which treats anything passed down from above as data and appends appropriate headers and/or trailers to it, is known as encapsulation
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223 Data forwarding functions
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This section describes five key packet forwarding functions and their relationship to the network stack The network equipment that makes use of each function will be discussed later
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Figure 21 Data framing
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Data Communications Basics Data Communications Basics 15
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Figure 22 The function of a repeater
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Figure 23 The function of a bridge
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Repeating Repeating occurs at the physical layer Repeating is used to extend cable distances and to isolate noise As shown in Figure 22, only the Physical layer of the protocol stack is involved in repeating A repeater simply looks at the electrical (or optical) signals on the media, and recreates those signals on a second piece of media The new signals are regenerated and cleaned up to meet the physical specification of the Physical layer protocol All traffic is repeated to all connections No destination decisions are made Bridging Bridging is accomplished at the Data Link layer (Figure 23) It can be used to connect two different physical media, such as the commonly used Ethernet
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Data Communications Basics 16 Introduction to Network Technologies and Performance
LAN cabling Thinnet (10Base2) and twisted-pair (10Base-T) Packets are forwarded from one link to another as needed, based on the Data Link layer address LAN switching also works in this fashion, but at much higher speed Network layer addressing is irrelevant for bridging
Routing Routing (Figure 24) operates at the Network layer; one use of routing is to connect networks that have different Data Link layers Common examples would include connecting a LAN using Ethernet to a FDDI backbone, or connecting a LAN to a WAN Routing can be very complex, but with the complexity comes flexibility and power The most common Network layer protocol used for routing is IP, but Novell s IPX and other protocols also are routed Routing relies on careful configuration in order to operate correctly When configured correctly it provides secure, efficient communications that can scale up to very large networks For example, HewlettPackard maintains a routed network with over 110,000 hosts worldwide Gateways Gateways (Figure 25) are used when two entirely different network stacks need to exchange data Computers can be configured to act as gateways by installing a card for each type of network, along with some appropriate software To connect a TCP/IP Ethernet network to an SNA network would require a gateway due to differences at all levels in the protocol stack Connecting an Ethernet network to a Token-Ring LAN would require only a bridge, provided the upper layers of the protocol stack are the same ATM switching Asynchronous Transfer Mode (ATM), shown in Figure 26, is a Data Link protocol It deserves special mention, however, both for its notoriety and for the way it operates Data is transmitted in small, fixed-size packets (53 bytes long) called cells The small cell size gives ATM the ability to interleave voice, data, and video traffic and deliver deterministic performance End stations have ATM ad-
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