vb.net qr code Figure 1-7 The relationship between the DTE and DCE in a typical network in Software

Print Code-39 in Software Figure 1-7 The relationship between the DTE and DCE in a typical network

Figure 1-7 The relationship between the DTE and DCE in a typical network
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Figure 1-8 The parallel bus ribbon cable in a PC
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Figure 1-9 The parallel bus, this time etched (inside the circle) on the motherboard of the PC
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Finally, we offer simplex, half-duplex, and full-duplex transmission. Simplex transmission means one-way only, like a radio broadcast. Halfduplex transmission means two-way, but only one way at a time, like
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CB radio. Finally, full-duplex means two-way simultaneous transmission, like telephony or two-way data transmission.
Network Transport
The fabric of the network cloud is a rich and unbelievably complex collection of hardware and software that moves customer traffic from an ingress point to an egress point, essentially anywhere in the world. It s a function that we take entirely for granted because it is so ingrained in day-to-day life. But stop for a moment to think about what the network actually does. Not only does it deliver voice and data traffic between end points, but it does so easily and seamlessly, with various levels of service quality, as required, to any point on the globe (and in fact beyond!) in a matter of seconds and with zero human involvement. It is the largest fully automated machine on the planet and represents one of the greatest technological accomplishments of all time. Think about that: I can pick up a handset here in Vermont, dial a handful of numbers, and seconds later a telephone rings in Ouagadougou, Burkina Faso, in North Central Africa. How that happens borders on the miraculous. We will explore it in considerably greater detail later in the book. Transport technologies within the network cloud fall into two categories: fixed transport and switched transport. Fixed transport, sometimes called private line or dedicated facilities, includes such technologies as T1, E1, DS3, SONET, SDH, dedicated optical channels, and microwave. Switched transport technologies include modem-based telephone transport, X.25 packet switching, frame relay, switched Ethernet, and asynchronous transfer mode (ATM). Together with the access technologies described previously and customer premises technologies such as Ethernet, transport technologies offer the infrastructure components required to craft an end-to-end solution for the transport of customer information.
The Many Flavors of Transport
Over the last few years the network has been functionally segmented into a collection of loosely defined regions that define unique service types. These include the local area (sometimes referred to as the premises region), the metropolitan area, and the wide area (sometimes
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First Things First
First Things First
known as the core). Local area networking has historically defined a network that provides services to an office, a building, or even a campus. Metro networks generally provide connectivity within a city, particularly to multiple physical locations of the same company. They are usually deployed across a ring architecture. Wide area networks, often called core transport, provide long-distance transport and are typically deployed using a mesh networking model.
Transport Channels
The physical circuit over which customer traffic is transported in a network is often referred to as a facility. Facilities are characterized by a number of qualities such as distance, quality (signal level, noise, and distortion coefficients), and bandwidth. Distance is an important criterion because it places certain design limitations on the network, making it more expensive as the circuit length increases. Over distance, signals tend to weaken and become noisy, and specialized devices (amplifiers, repeaters, regenerators) are required to periodically clean up the signal quality and maintain the proper level of loudness to ensure intelligibility and recognizability at the receiving end. Quality is related to distance in the sense that they share many of the same affecting factors. Signal level is clearly important, as is noise, both of which were just discussed. Distortion is a slightly different beast and must be dealt with equally as carefully. Noise is a random event in networks caused by lightning, fluorescent lights, electric motors, sunspot activity, and squirrels chewing on wires; it is unpredictable and largely random. Noise, therefore, cannot be anticipated with any degree of accuracy; its effects can only be recovered from. Distortion, on the other hand, is a measurable, predictable characteristic of a transmission channel and is usually frequency-dependent. For example, certain frequencies transmitted over a particular channel will be weakened, or attenuated, more than other frequencies. If we can measure this, then we can condition the channel to equalize the treatment that all frequencies receive as they are transmitted down that channel. This process is indeed known as conditioning, and is part of the higher cost involved in buying a dedicated circuit for data transmission. For example, think about the last time you attended a parade. How did you know the band was approaching You heard the low-frequency tones of the bass drums and tubas long before you saw the band or you heard the higher frequencies of the trumpets and trombones. Higher frequency
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