Networks: Access Management

Transcription

1 Networks: Access Management Class Notes # 1 Introduction September 8, 2003 A network can be described as two or more computing devices that are interconnected. The devices are, typically, connected to share resources. More specifically, a computer network can perform the following: Use printers attached to other computers. Share files between computers. Execute software that resides on a server. Share a single Internet connection among computers attached to the network. Play multi-user games. Networked computers can save data on a single common system, where this common system can be backed up on periodic cycle (e.g. once per day). Etc. A computer network is often called a LAN (Local Area Network). A LAN is optimized for a moderate size geographic area such as a single office building, warehouse, or campus. Any computer on a LAN can communicate directly with each other and therefore will have higher data rates than a MAN or WAN connection. Figure one illustrates a small LAN consisting of a few nodes, however a LAN is typically confined to a single building. Local Area Network Server Hub Figure 1: A LAN consisting of a few nodes 1

2 A MAN (Metropolitan Area Network) interconnects two or more LANs using public or shared facilities. A MAN is optimized for a larger geographical area ranging from several blocks of building to entire cites. Error rates and delays may be slightly higher than might be obtained on a LAN. A WAN (Wide Area Network) interconnects two or more MANs. A WAN is optimized for very large geographical areas ranging from countries to continents. The data rates, errors, and delays are more significant than LAN or MAN. 2

3 Network Topologies A network topology is a geometric configuration of a network (i.e. different ways computers can be connected to make a network). Networks links can be arranged in several different ways: star topology, bus/tree topology, ring topology, etc. Ring Topology In the ring topology, the network consists of a set of repeaters joined by point-to-point links in a closed loop. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting it, bit by bit, on the other link as fast as it is received, with no buffering at the repeater. The links are unidirectional; that is; data are transmitted in one direction only, and all oriented in the same way. Therefore, data circulate around the ring in one direction (clockwise or counter-clockwise). Data are transmitted in packets inserted onto the ring by the stations. The packets contain source and destination address fields as well as other control information and user data. As a packet circulates, the destination station copies the data into a local buffer. Typically, the packet continues to circulate until it returns to the source station, where it is absorbed, removing it from the ring. Because the ring is constructed as a series of point-to-point links, almost any transmission medium can be used. Twisted pair, because of its low cost, is common. Coaxial cable provides greater capacity. Optical fiber is used to achieve very high data transfer rates. Given below is a summary for transmission media used in a ring network: Table one: Transmission media for Local networks: Ring Transmission media Data rate Distance between Number of repeaters (Mbps) repeaters (km) Unshielded twisted pair Shielded twisted pair Baseband coaxial cable Optical fiber Repeater Repeater Repeater Repeater Figure 2: Ring topology 3

4 Bus and Tree Topology Both bus and tree topologies are characterized by the use of a multipoint medium. With the bus topology, all stations attach, through appropriate interfacing hardware, directly to a linear transmission medium, or bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations, and is absorbed at the endpoints. Figure three illustrates a bus structure. The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the head-end. One or more cables start at the head-end, and each or these may have branches. The branches in turn may have additional branches to allow quite complex layouts. Figure four illustrates a tree structure. Tap Terminating Resistance Figure 3: Bus topology Headend Tap Figure 4: Tree topology 4

5 As with the ring, transmission is in the form of packets containing addresses and user data. Each station monitors the medium and copies packets addressed to itself. Because all stations share a common transmission link, only one station can successfully transmit at a time, and some form of medium access control technique is needed to regulate access. One of the most common transmission media for the bus and tree topology is the twisted pair wiring. Twisted pair is relatively low cost and is typically pre-installed in office buildings. It is the most cost effective choice for single building low traffic requirements. Higher performance requirements can be met by coaxial cable, which provides higher throughput, can support a larger number of devices, and can span greater distances than twisted pair. Optical fiber has even greater capacity than coaxial cable and is starting to be a popular choice. It has been little used so far due to cost and technical limitations. As the cost of optical fiber transmission components continue to drop, this configuration will become increasingly practical. Table two summarizes the transmission media for bus/tree LANs. Table two: Transmission media for local networks: Bus/Tree Transmission media Data rate (Mbps) Range (km) Number of taps Twisted pair 1 10 Less than 2 10 s Baseband coaxial cable 10; 50 with limitations Less than s Broadband coaxial cable 500; 20 per channel Less than s Optical fiber 45 Less than s Star Topology In the star topology, each station is directly connected to a common central switch. The star topology is also employed for implementing a packet broadcasting LAN. Each station attaches to a central node, referred to as the star coupler (e.g. hub or switch), via two point-to-point links, one for transmission in each direction. Only one station may successfully transmit. Therefore, the medium access control techniques used for the packet star topology are the same as for bus and tree. Any form of guided transmission medium can be used with the star topology. Table three summarizes the possible transmission media. Table three: Transmission media for local networks: Star topology Transmission media Data rate Distance from station Number of stations (Mbps) to central switch (km) Unshielded twisted pair (1 Mbps) 10 s 0.1 (10 Mbps) Baseband coaxial cable 70 Less than 1 10 s Optical fiber Less than 1 10 s The star topology is the most common topology used for LAN. If a hub is used as the star coupler, each station will receive a packet from the transmitting station even if it is not the intended recipient. Where as if a switch is used, the packet will only be received 5

6 on the intended station. The switch is more efficient but generally tends to be more expensive than a hub. Star-Ring Topology There are major limitations with the Star topology and with the ring topology. The major disadvantage with the star topology is that if the hub or switch fails, the entire network goes down. The major disadvantage with the ring topology is that if one of the nodes fail or is shut down for maintenance, then the data transmission stops for the network. The ring can also be broken if any cable between the nodes is damaged. A star ring topology overcomes some of the problems associated with the star and ring structure. Also, the star ring topology allows for the construction of a larger local network due to the limitations in the ring and star architecture. Figure four illustrates the star ring network using MAU (Multi-station Access Units). MAU MAU MAU Laser printer Server Figure 5: Star - Ring topology 6

7 Transmission Media for LAN The transmission media connects the nodes of a network together. There are various types of transmission media, each with its own physical characteristics. These characteristics determine the range, quality, and speed of the signal carried. Baseband Coaxial cable The most popular form of baseband bus LAN uses coaxial cable. Most baseband coax systems use a special 50 Ohm cable rather than the standard CATV 75 Ohm cable. This is due to digital signals suffering less intense reflections from the insertion capacitance of taps, and provides better immunity against low-frequency electromagnetic noise. There are trade offs involving data rate, cable length, number of taps, and the electrical characteristics of the transmit and receive components for a baseband coaxial system. For example, the lower the data rate, the longer the cable can be. The Ethernet specification and the original IEEE standard specified the use of 50 Ohm cable with a 0.4 inch diameter, and a data rate of 10 Mbps. With these parameters, the maximum length of the cable is set to 500 meters. The distance between any two taps must be a multiple of 2.5 meters; to ensure that reflections from adjacent taps do not add in phase. In IEEE jargon, the system is referred to as 10BASE5 (10 Mbps, base-band, 500 meter segment length. The lower cost 10BASE2 specification, dubbed cheapnet, was added for personal computer local networks. Table four compares 10BASE5 and 10BASE2 specifications Table 4: IEEE specification for 10 Mbps Baseband coaxial bus LAN Parameter 10BASE5 10BASE2 Data rate 10 Mbps 10 Mbps Maximum segment length 500 m 200 m Network span 2500 m 1000 m Nodes per segment Node spacing 2.5 m 0.5 m Cable diameter 0.4 inches 0.25 inches To extended the length of the network, a repeater may be used. It consists of two transceivers joined together and connected to two different segments of coax cable. A maximum of four repeaters is allowed in the path between any two stations, extending the effective cable length to 2.5 km. Unshielded twisted pair baseband In recent years, there has been increasing interest in the use of twisted pair as a transmission medium for LANs. The most popular approach to the use of unshielded twisted pair for a LAN is a star topology approach. The central element of the star is an active element, referred to as a hub. The hub acts as a repeater and a transmission form any one station is received by all other stations, and if two stations transmit at the same time there will be a collision. Multiple levels of hubs can be cascaded in a hierarchical configuration. There is one header hub(hhub) and one or more intermediate hubs (IHUB). Typically, there is a wiring closet on each floor of an office building, and a hub 7

8 can be placed in each one. Each hub could service the stations on its floor. Figure six illustrates a two level hub configuration. HHub IHub IHub Station Station Station Station Station Figure 6: Two level hierarchy The 10Base-T (10 Mbps, base-band, Twisted pair) is another baseband IEEE specification used in a star configuration. The 10BASE-T specifications are given below: Table 5: 10BASE-T specifications Parameters Specifications Maximum data rate (Mbps) 10 Maximum segment length (m) 100 Maximum repeaters 4 Maximum concentrators (hubs) in sequence 4 Fiber Optic cable Fiber optic cables support higher data rates and the signal can be carried for a long distance before the signal must be retransmitted. Also, optical signals are immune to EMI/RFI (electrical interference) and therefore, do not suffer signal degradation or interference in a noisy environment. The main disadvantage is cost. Fiber optic cables are available in two types: Single mode fiber: a core diameter of 8 micro-meters carrying one signal. Typically, single mode fibers are used to carry signals at long distances. Multimode fiber: a core diameter of micro-meters carrying many signals at once. Typically, used to carry signals for short distances. Multimode fibers without repeaters can carry a signal up to 10 km at a data rate of 100 Mbps and carry a signal up to 1 km at a data rate of 1 Gbps. Single mode fibers can carry a signal over 100 km with data rates of 200 Mbps. 8

9 Optical fiber is typically used in a point-to-point (e.g. star topology) or ring topology. The reason is that the connections and splices of fiber optic cables are difficult to make since: the cable must be cut precisely, the ends have to be polished, and the cable must be precisely aligned to the connector. There are two popular types of fiber optical connectors available: ST connector, and the newer SC connector. Wireless LANs Wireless LANs are becoming increasingly popular but are relatively expensive. There are three main types of wireless LANs: microwave, spread spectrum, and infrared. The microwave wireless LANs operate in the microwave region of the frequency spectrum. The microwave wireless LANs are susceptible to electrical noises. A reasonable data rates can be achieved with a microware wireless LAN with the ability to transmit through walls. The spread spectrum LAN systems can work from distances of 10 meters to 60 meters with a low data rate of 1 Mbps to 2 Mbps. The infrared LAN systems can achieve high data rates (up to 20 Mbps) and are inexpensive. The disadvantage is the short range and line of sight requirement. 9