Practical TCP/IP and Ethernet Networking- P12

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Practical TCP/IP and Ethernet Networking- P12: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter....

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Nội dung Text: Practical TCP/IP and Ethernet Networking- P12

4KZ]UXQOTM L[TJGSKTZGRY node, in a predefined order, and as such it is an example of a point-to-point system. Nodes are arranged in a closed loop, so that the initiating node is the last one to receive a packet. As a physical topology, a ring describes a network in which each node is connected to exactly two other nodes. Information traverses a one-way path, so that a node receives packets from exactly one node and transmits them to exactly one other node. A message packet travels around the ring until it returns to the node that originally sent it. In a ring topology, each node can act as a repeater, boosting the signal before sending it on. Each node checks whether the message packet’s destination node matches its address. When the packet reaches its destination, the destination node accepts the message, then sends it back to the sender, to acknowledge receipt. As you will see later in this chapter, since ring topologies use token passing to control access to the network, the token is returned to sender with the acknowledgment. The sender then releases the token to the next node on the network. If this node has nothing to say, the node passes the token on to the next node, and so on. When the token reaches a node with a packet to send, that node sends its packet. Physical ring networks are rare, because this topology has considerable disadvantages compared to a more practical star- wired ring hybrid, which is described later. Figure 2.17 Ring topology 8OTM ZUVURUM_ GJ\GTZGMKY • A physical ring topology has minimal cable requirements • No wiring center or closet is needed • The message can be automatically acknowledged • Each node can regenerate the signal 8OTM ZUVURUM_ JOYGJ\GTZGMKY • If any node goes down, the entire ring goes down • Diagnosis/troubleshooting (fault isolation) is difficult because communication is only one-way • Adding or removing nodes disrupts the network • There will be a limit on the distance between nodes As well as these three main topologies, some of the more important variations will now be considered. Once again, you should be clear that these are just variations, and should not be considered as topologies in their own right.
6XGIZOIGR :)6/6 GTJ +ZNKXTKZ 4KZ]UXQOTM 5ZNKX Z_VKY UL ZUVURUM_ 9ZGX]OXKJ XOTM ZUVURUM_ A star-wired ring topology, also known as a hub topology, is a hybrid physical topology that combines features of the star and ring topologies. Individual nodes are connected to a central hub, as in a star network. Within the hub, however, the connections are arranged into an internal ring. Thus, the hub constitutes the ring, which must remain intact for the network to function. The hubs, known as multistation access units (MAUs) in IBM token ring network terminology, may be connected to other hubs. In this arrangement, each internal ring is opened and connected to the attached hubs, to create a larger, multi-hub ring. The advantage of using star wiring instead of simple ring wiring is that it is easy to disconnect a faulty node from the internal ring. The IBM data connector is specially designed to close a circuit if an attached node is disconnected physically or electrically. By closing the circuit, the ring remains intact, but with one less node. The IBM token ring networks are the best-known example of a star-wired ring topology. In token ring networks, a secondary ring path can be established and used if part of the primary path goes down. The star-wired ring is illustrated in Figure 2.18. Figure 2.18 Star-wired ring The advantages of a star-wired ring topology include: • Troubleshooting, or fault isolation, is relatively easy • The modular design makes it easy to expand the network, and makes layouts extremely flexible • Individual hubs can be connected to form larger rings • Wiring to the hub is flexible The disadvantages of a star-wired ring topology include: • Configuration and cabling may be complicated because of the extreme flexibility of the arrangement. *OYZXOH[ZKJ YZGX ZUVURUM_ A distributed star topology is a physical topology that consists of two or more hubs, each of which is the center of a star arrangement. A good example of such a topology is an
4KZ]UXQOTM L[TJGSKTZGRY ARCnet network with at least one active hub and one or more active or passive hubs. The 100VG ANYLAN utilizes a similar topology. Figure 2.19 Distributed star topology 3KYN ZUVURUM_ A mesh topology is a physical topology in which there are at least two paths to and from every node. This type of topology is advantageous in hostile environments in which connections are easily broken. If a connection is broken, at least one substitute path is always available. A more restrictive definition requires each node to be connected directly to every other node. Because of the severe connection requirements, such restrictive mesh topologies are feasible only for small networks. Figure 2.20 Mesh network :XKK ZUVURUM_ A tree topology, also known as a distributed bus or a branching tree topology, is a hybrid physical topology that combines features of star and bus topologies. Several buses may be daisy-chained together, and there may be branching at the connections (which will be hubs). The starting end of the tree is known as the root or head end. This type of topology is used in delivering cable television services. The advantages of a tree topology are: • The network is easy to extend by just adding another branch, and that fault isolation is relatively easy The disadvantages include: • If the root goes down, the entire network goes down • If any hub goes down, all branches off that hub go down • Access becomes a problem if the entire network becomes too big
6XGIZOIGR :)6/6 GTJ +ZNKXTKZ 4KZ]UXQOTM Figure 2.21 Tree topology 3KJOG GIIKYY SKZNUJY A common and important method of differentiating between different LAN types is to consider their media access methods. Since there must be some method of determining which node can send a message, this is a critical area that determines the efficiency of the LAN. There are a number of methods, which can be considered, of which the two most common in current LANs are the contention method and the token passing method. You will become familiar with these as part of your study of LANs, although some of the other methods will also be briefly discussed. )UTZKTZOUT Y_YZKSY The basis for a first-come-first-served media accesses method. This operates in a similar manner to polite human communication. We listen before we speak, deferring to anyone who already is speaking. If two of us start to speak at the same time, we recognize that fact and both stop, before starting our messages again a little later. In a contention-based access method, the first node to seek access when the network is idle will be able to transmit. Contention is at the heart of the carrier sense multiple access/collision detection (CSMA/CD) access method used in the IEEE 802.3 and Ethernet V2 networks. The carrier sense component involves a node wishing to transmit a message listening to the transmission media to ensure there is no ‘carrier’ present. In fact, the signaling method used on Ethernet type systems that make use of this method do not use a carrier in its true sense, and the name relates back to the original Aloha project in Hawaii that used radio links for transmission. The length of the channel and the finite propagation delay means that there is still a distinct probability that more than one transmitter will attempt to transmit at the same time, as they both will have heard ‘no carrier’. The collision detection logic ensures that more than one message on the channel simultaneously will be detected and transmission, from both ends, eventually stopped. The system is a probabilistic system, since access to the channel cannot be ascertained in advance. :UQKT VGYYOTM Token passing is a deterministic media access method in which a token is passed from node to node, according to a predefined sequence. A token is a special packet, or frame, consisting of a signal sequence that cannot be mistaken for a message. At any given time, the token can be available or in use. When an available token reaches a node, that node can access the network for a maximum predetermined time, before passing the token on.
4KZ]UXQOTM L[TJGSKTZGRY This deterministic access method guarantees that every node will get access to the network within a given length of time, usually in the order of a few milliseconds. This is in contrast to a probabilistic access method (such as CSMA/CD), in which nodes check for network activity when they want to access the network, and the first node to claim the idle network gets access to it. Because each node gets its turn within a fixed period, deterministic access methods are more efficient on networks that have heavy traffic. With such networks, nodes using probabilistic access methods spend much of their time competing to gain access and relatively little time actually transmitting data over the network. Network architectures that support the token passing access method include token bus, ARCnet, FDDI, and token ring. To transmit, the node first marks the token as ‘in use’, and then transmits a data packet, with the token attached. In a ring topology network, the packet is passed from node to node, until the packet reaches its destination. The recipient acknowledges the packet by sending the message back to the sender, who then sends the token on to the next node in the network. In a bus topology network, the next recipient of a token is not necessarily the node that is nearest to the current token passing node. Rather, the next node is determined by some predefined rule. The actual message is broadcast on to the bus for all nodes to ‘hear’. For example, in an ARCnet or token bus network, the token is passed from a node to the node with the next lower network address. Networks that use token passing generally have some provision for setting the priority with which a node gets the token. Higher-level protocols can specify that a message is important and should receive higher priority. Figure 2.22 Token passing A token ring network requires an active monitor (AM) and one or more standby monitors (SMs). The AM keeps track of the token to make sure it has not been corrupted, lost, or sent to a node that has been disconnected from the network. If any of these things happens, the AM generates a new token, and the network is back in business. The SM makes sure the AM is doing its job and does not break down and get disconnected from the network. If the AM is lost, one of the SMs becomes the new AM, and the network is again in business. These monitoring capabilities make for complex circuitry on network interface cards that use this media access method. 6URROTM Polling refers to a process of checking elements, such as computers or queues, in some defined order, to see whether the polled element needs attention (wants to transmit, contains jobs, and so on). In roll call polling, the polling sequence is based on a list of elements available to the controller. In contrast, in hub polling, each element simply polls the next element in the sequence.