Practical TCP/IP and Ethernet Networking- P11

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Practical TCP/IP and Ethernet Networking- P11: 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- P11

6XGIZOIGR :)6/6 GTJ +ZNKXTKZ 4KZ]UXQOTM • 802.1E System load protocol • 802.1F Guidelines for layer management standards • 802.1G Remote MAC bridges • 802.1I MAC bridges (FDDI supplement) /+++ 2UMOIGR ROTQ IUTZXUR This is the interface between the network layer and the specific network environments at the physical layer. The IEEE has divided the data link layer in the OSI model into two sub layers – the media access MAC sub layer, and the logical link layer LLC. The logical link control protocol is common for all IEEE 802 standard network types. This provides a common interface to the network layer of the protocol stack. The protocol used at this sub layer is based on IBM’s SDLC protocol, and can be used in three modes, or types. These are: • Type 1: Unacknowledged connectionless link service • Type 2: Connection oriented link service • Type 3: Acknowledged connectionless link service, used in real time applications such as manufacturing control /+++ )93')* The carrier sense, multiple accesses with collision detection type LAN is commonly – but strictly speaking incorrectly – known as an Ethernet LAN. Ethernet refers to the original DEC/INTEL/XEROX product known as Version II (or Bluebook) Ethernet. Subsequent to ratification this system has been known as IEEE 802.3. IEEE 802.3 is virtually identical, but not absolutely identical to Bluebook Ethernet, in that they differ in two bytes within the frame. The following chapter will deal with this anomaly. Subsequently, two additional specifications have been approved viz. IEEE 802.3u (100 Mbps or ‘fast’ Ethernet) and IEEE 8023z (1000 Mbps or ‘gigabit’ Ethernet). /+++ :UQKT H[Y The other major access method for a shared medium is the use of a token. This is a type of data frame that a station must possess before it can transmit messages. The stations are connected to a passive bus, although the token logically passes around in a cyclic manner. This standard is the ratification of the Token Bus LAN developed by General Motors for its manufacturing automation protocol (MAP). The media used is usually broadband coax, and speeds vary from 1 Mbps to 10 Mbps. /+++ :UQKT XOTM As in 802.4, data transmission can only occur when a station holds a token. The logical structure of the network wiring is in the form of a ring, and each message must cycle through each station connected to the ring. This standard is the ratified version of the IBM token ring LAN. However, where IBM token ring supports speed of 4 and 16 Mbps, IEEE 802.5 supports 1 and 4 Mbps. The physical media for the token ring can be unshielded twisted pair, coaxial cable or optical fiber. The original specification called for a single ring, which creates a problem if the ring gets broken. A subsequent enhancement of the specification, called IEEE 802.5u, introduces the concept of a dual redundant ring, which enables the system to continue operating in case of a cable break. Work is currently underway on a 100 Mbps token ring specification.
4KZ]UXQOTM L[TJGSKTZGRY /+++ 3KZXUVUROZGT GXKG TKZ]UXQY This committee is responsible for defining the standards for MANs. It has recommended that a system known as distributed queue data bus (DQDB) be utilized as a MAN standard. The DQDB network is sponsored by Telecom Australia and defines the protocol for integrated voice and data on the same medium, within an area up to 15 km in diameter. /+++ (XUGJHGTJ 2'4Y ZKINTOIGR GJ\OYUX_ MXU[V :'- The 802.7 Committee provides technical advice on broadband technique. /+++ ,OHKX UVZOI 2'4Y :'- The fiber optic equivalent of the 802.7 broadband TAG. The committee is attempting to standardize physical compatibility with FDDI and synchronous optical networks (SONET). It is also investigating single mode fiber and multimode fiber architectures. /+++ /TZKMXGZKJ \UOIK GTJ JGZG 2'4Y This committee has recently released a specification for isochronous Ethernet as IEEE 802.9a. It provides a 6.144 Mbps voice service (96 channels at 64 kbps) multiplexed with 10 Mbps data on a single cable. It is designed for multimedia applications. /+++ 9KI[XK 2'4Y Current proposals include two methods to address the lack of security in the original specifications. These are: • A secure data exchange sub layer SDE sitting between the LLC and the MAC sub layer. There will be different SDEs for different systems i.e. military and medical • A secure interoperable LAN system architecture SILS. This will define system standards for secure LAN communications /+++ =OXKRKYY 2'4Y The IEEE802.11 Wireless LAN standard uses the 2.4 GHz band and allows operation to 1 or 2 Mbps. The 802.11b standard also uses the 2.4 GHz band, but allows operation at 11 Mbps. The latest IEEE 802.11a specification use the 5.7 GHz band instead and allows operation at 54 Mbps. /+++ ,GYZ 2'4Y This specification covers the system known as 100VG AnyLAN. Developed by Hewlett- Packard, this system operates on voice grade (CAT3) cable – hence the VG in the name. The AnyLAN indicates that the system can interface with both IEEE 802.3 and IEEE 802.5 networks (by means of a special speed adaptation bridge). 4KZ]UXQ ZUVURUMOKY (XUGJIGYZ GTJ VUOTZZUVUOTZ ZUVURUMOKY The way the nodes are connected to form a network is known as its topology. There are many topologies available but they form two basic types, broadcast and point-to-point. Broadcast topologies are those where the message ripples out from the transmitter to reach all nodes. There is no active regeneration of the signal by the nodes and so signal propagation is independent of the operation of the network electronics. This then limits the size of such networks.
6XGIZOIGR :)6/6 GTJ +ZNKXTKZ 4KZ]UXQOTM Figure 2.13 shows an example of a broadcast topology. Figure 2.13 Broadcast topology In a point-to-point communications network, however, each node is communicating directly with only one node. That node may actively regenerate the signal and pass it on to its nearest neighbor. Such networks have the capability of being made much larger. Figure 2.14 shows some examples of point-to-point topologies. Figure 2.14 Point-to-point topologies 2UMOIGR GTJ VN_YOIGR ZUVURUMOKY A logical topology defines how the elements in the network communicate with each other, and how information is transmitted through a network. The different types of media-access methods, determine how a node gets to transmit information along the network. In a bus topology, information is broadcast, and every node gets the same information within the amount of time it actually takes a signal to cover the entire length of cable. This time interval limits the maximum speed and size for the network. In a ring topology, each node hears from exactly one node and talks to exactly one other node. Information is passed sequentially, in an order determined by a predefined process. A polling or token mechanism is used to determine who has transmission rights, and a node can transmit only when it has this right. A physical topology defines the wiring layout for a network. This specifies how the elements in the network are connected to each other electrically. This arrangement will determine what happens if a node on the network fails. Physical topologies fall into three main categories... bus, star, and ring topology. Combinations of these can be used to form
4KZ]UXQOTM L[TJGSKTZGRY hybrid topologies to overcome weaknesses or restrictions in one or other of these three component topologies. ([Y ZUVURUM_ A bus refers to both a physical and a logical topology. As a physical topology, a bus describes a network in which each node is connected to a common single communication channel or ‘bus’. This bus is sometimes called a backbone, as it provides the spine for the network. Every node can hear each message packet as it goes past. Logically, a passive bus is distinguished by the fact that packets are broadcast and every node gets the message at the same time. Transmitted packets travel in both directions along the bus, and need not go through the individual nodes, as in a point-to-point system. Rather, each node checks the destination address that is included in the message packet to determine whether that packet is intended for the specific node. When the signal reaches the end of the bus, an electrical terminator absorbs the packet energy to keep it from reflecting back again along the bus cable, possibly interfering with other messages already on the bus. Each end of a bus cable must be terminated, so that signals are removed from the bus when they reach the end. In a bus topology, nodes should be far enough apart so that they do not interfere with each other. However, if the backbone bus cable is too long, it may be necessary to boost the signal strength using some form of amplification, or repeater. The maximum length of the bus is limited by the size of the time interval that constitutes ‘simultaneous’ packet reception. Figure 2.15 illustrates the bus topology. Figure 2.15 Bus topology ([Y ZUVURUM_ GJ\GTZGMKY Bus topologies offer the following advantages: • A bus uses relatively little cable compared to other topologies, and arguably has the simplest wiring arrangement • Since nodes are connected by high impedance tappings across a backbone cable, it’s easy to add or remove nodes from a bus. This makes it easy to extend a bus topology • Architectures based on this topology are simple and flexible • The broadcasting of messages is advantageous for one-to-many data transmissions ([Y ZUVURUM_ JOYGJ\GTZGMKY These include the following: • There can be a security problem, since every node may see every message, even those that are not destined for it • Diagnosis/troubleshooting (fault-isolation) can be difficult, since the fault can be anywhere along the bus
6XGIZOIGR :)6/6 GTJ +ZNKXTKZ 4KZ]UXQOTM • There is no automatic acknowledgment of messages, since messages get absorbed at the end of the bus and do not return to the sender • The bus cable can be a bottleneck when network traffic gets heavy. This is because nodes can spend much of their time trying to access the network 9ZGX ZUVURUM_ A star topology is a physical topology in which multiple nodes are connected to a central component, generally known as a hub. The hub of a star usually is just a wiring center; that is, a common termination point for the nodes, with a single connection continuing from the hub. In some cases, the hub may actually be a file server (a central computer that contains a centralized file and control system), with all its nodes attached directly to the server. As a wiring center, a hub may, in turn, be connected to the file server or to another hub. All signals, instructions, and data going to and from each node must pass through the hub to which the node is connected. The telephone system is doubtless the best known example of a star topology, with lines to individual customers coming from a central telephone exchange location. There are not many LAN implementations that use a logical star topology. The low impedance ARCnet networks are probably the best examples. However, you will see that the physical layout of many other LANs look like a star topology even though they are considered to be something else. An example of a star topology is shown in Figure 2.16. Figure 2.16 Star topology 9ZGX ZUVURUM_ GJ\GTZGMKY • Troubleshooting and fault isolation is easy • It is easy to add or remove nodes, and to modify the cable layout • Failure of a single node does not isolate any other node • The inclusion of a central hub allows easier monitoring of traffic for management purposes 9ZGX ZUVURUM_ JOYGJ\GTZGMKY • If the hub fails, the entire network fails. Sometimes a backup central machine is included, to make it possible to deal with such a failure • A star topology requires a lot of cable 8OTM ZUVURUM_ A ring topology is both a logical and a physical topology. As a logical topology, a ring is distinguished by the fact that message packets are transmitted sequentially from node to