barcode generator in vb.net Figure 4-19 A collision on a contentionbased LAN. in Software

Maker EAN-13 in Software Figure 4-19 A collision on a contentionbased LAN.

Figure 4-19 A collision on a contentionbased LAN.
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We will discuss techniques for this in the contention world a bit later in this chapter. Contention-based LANs employ CSMA/CD, in which a station observes the following guidelines when attempting to use the shared network. First, it listens to the shared medium to determine whether it is in use or not; that s the Carrier Sense part of the name. If the LAN is available (not already in use), it begins to transmit, but continues to listen while it is transmitting, knowing that another station could also choose to transmit at the same time; that s the Multiple Access part. In the event that a collision is detected, usually indicated by a dramatic increase in the signal power measured on the shared LAN, both stations back off and try again. That s the Collision Detection part. Ethernet is the most common example of a CSMA/CD LAN. Originally released as a 10-Mbps product based on Institute of Electrical and Electronics Engineers (IEEE) standard 802.3, Ethernet rapidly became the most widely deployed LAN technology in the world. As bandwidthhungry applications such as E-Commerce, Enterprise Resource Planning (ERP), and Web access evolved, transport technologies advanced, and bandwidth availability (and capabilities) grew, 10-Mbps Ethernet began to show its age. Today new versions of Ethernet have emerged that offer 100-Mbps (Fast Ethernet) and 1,000-Mbps (Gigabit Ethernet) transport, with plans afoot for even faster versions. Furthermore, in keeping with the demands being placed on LANs by convergence, standards are evolving for LAN-based voice transport that guarantee quality of service for mixed traffic types. Gigabit Ethernet is still in a somewhat nascent stage, but most believe that it will experience a high uptake rate as its popularity climbs. Dataquest predicts that Gigabit Ethernet sales will grow to $2.5 billion by 2002; this is a reasonable number, considering that 2 million ports were sold in 1999 with expectations of hitting a total installed base of 18 million by 2002. Emerging applications certainly make the case for Gigabit Ethernet s bandwidth capability. LAN telephony, server interconnection, and video to the desktop all demand low-latency solutions, and Gigabit Ethernet may be positioned to provide it. A number of vendors have entered the marketplace, including Alcatel, Lucent Technologies, Nortel Networks, and Cisco Systems. The other aspect of the LAN environment that has begun to show weaknesses is the overall topology of the network itself. LANs are broadcast environments, which means that when a station transmits, every station on the LAN segment hears the message (see Figure 4-20). Although this is a simple implementation scheme, it is also
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Figure 4-20 When one station transmits, all stations hear the message. This can result in significant waste of bandwidth.
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wasteful of bandwidth, since stations hear broadcasts that they have no reason to hear. In response to this, a technological evolution has occurred. It was obvious to LAN implementers that the traffic on most LANs was somewhat domain-oriented; that is, it tended to cluster into communities of interest based on the work groups using the LAN. For example, if employees in sales shared a LAN with shipping and order processing, three discernible traffic groupings emerged according to what network architects call the 80:20 Rule. The 80:20 Rule simply states that 80 percent of the traffic that originates in a particular work group tends to stay in that work group, an observation that makes network design distinctly simpler. If the traffic naturally tends to segregate itself into groupings, then the topology of the network could change to reflect those groupings. Thus was born the bridge. Bridges are devices with the responsibility of filtering traffic that has to propagate in the forward direction as well as traffic that does not. For example, if the network described previously were to have a bridge inserted in it (see Figure 4-21), all the employees in each of the three work groups would share a LAN segment, and each segment would be attached to a port on the bridge. When an employee in sales transmits a message to another employee in sales, the bridge is intelligent enough to know that the traffic does not have to be forwarded to the other ports. Similarly, if the sales employee now sends a message to someone in shipping, the bridge recognizes that the sender and receiver are on different segments and thus forwards the message to the appropriate port, using address information in a table that it maintains (the filter/forward database). Bridges operate at layer two of the OSI Model and, as such, are frame switches. Following close on the heels of bridging is a relatively new technique called LAN switching. LAN switching qualifies as bridging on steroids.
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