Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Bus (computing) wikipedia , lookup
Topology (electrical circuits) wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Index of electronics articles wikipedia , lookup
Cellular repeater wikipedia , lookup
MIL-STD-1553 wikipedia , lookup
Telecommunications engineering wikipedia , lookup
Lecture 6 – LAN Topology Objective After attending this lecture, students should have a brief understanding of: three topology of Local Area Network: bus topology, star, and extended star topology – network layout, and signal transmission method Ethernet – architecture, access method Choice of local area network topology Terminology Terms Topology Network Architecture Access Method Meaning The physical layout of a Local Area Network. Describes how devices are connected together, what media is supported and its limitation, and how electrical signal is transmitted over the media Defines the rules and layout of a network Defines the components Defines communication protocol A regulatory mechanism that allows devices on a network to share the networking media equitably and in some cases cost effectively as well. It is a set of predefined rules or protocols that determine how a device gains access or entry to the networking media. 1. Bus topology 1.1 General Characteristics of Bus topology All devices on a LAN are attached independently to a linear networking media known as bus, trunk line, or highway. The bus wire must end in a terminating resistance, or terminator. Example of bus topology – Ethernet W T C W Symbol : C– TW– 1.2. C W Connector Terminator Workstation (including server, workstation, etc.) Signal transmission over the media T A transmitted signal travels in both directions; Each device can check the signal on the media through its network card; Each signal contains the source MAC address, source IP address, destination MAC address, and destination IP address Only the device with matching destination IP address, and destination MAC address can copy the data and pass it to higher layer; Other devices with non-matching IP address, or non-matching destination address ignore the data; A signal, at reaching the end of a bus, is absorbed by the terminator. This prevent the signal bouncing back to the media. Remark: A broadcast signal means every device on a LAN has a matching destination address. 1.3 Only one workstation at a time transmits data Avoidance of signal collision (Carrier Sense) Before a workstation transmits its data to the media, the transceiver (a feature inside a network card, or a physical component) checks whether the media is being used by other devices. Collision impact - When a collision occurs, the data from each device impact - that is, the voltage pulses from each device are both present on the common bus wire at the same time - and thus the data from both devices is damaged. - The network area within which data packets originate and collide is called a collision domain Collision Recovery - On detection of collision, the network card of a network device issues a backoff; enforcing different retransmission delay for each device on the network. 1.4 Advantage & Disadvantages Advantage Simple wiring layout Low installation cost Network still function in case of a node failure. 2. Bus topology Disadvantage High management cost; Long fault diagnosis time; The bus itself is a single point of failure. E.g. When adding new device to a bus topology, the network is required to shut down. 2.1 General Characteristics of Bus topology All devices on a LAN are attached independently to a central point (usually known as HUB). An active hub connects the networking media, and regenerates the electrical signal, thus supporting longer distance connection. A passive hub is simply a device used to connect networking media. E.g. Horizontal cabling W HUB W Symbol : W– W Workstation (including server, workstation, etc.) 2.2. Signal transmission over the network A transmitted signal travels to the central point (HUB) along its own connection line. Each signal contains the source MAC address, source IP address, destination MAC address, and destination IP address Only the device with matching destination IP address, and destination MAC address can copy the data and pass it to higher layer; Address recognition is performed by the hub. Remark: A broadcast signal means every device on a LAN has a matching destination address. 2.3 Advantage & Disadvantages Advantage Easier to design Easy for maintenance – diagnosis is simpler; Greater reliability – any node failure does not affect the rest of a network Disadvantage Higher media installation cost A hub itself is the single point of failure Limited distance, support up to 100 meter between the HUB and a workstation due to the use of UTP cable. Maximum distance between two workstations are 200 meter apart (with HUB sitting in the middle) Maximum coverage area is 200m * 200m 3. Extended Star Topology 3.1 Signal Loss When transmitting over a media, electrical signal could be weakened due to heat loss, radiation, or leakage. Each media has its own limited distance to support data transmission before signal loss. 3.2 Extending a star topology By using repeater, a weakened signal can be amplified. A star topology can provide a larger area coverage by using repeaters. A star topology can be extended by using internetworking devices that do not result in attenuation of the signal. When this occurs, the resulting topology is referred to as an extended star topology. Review Questions 1. Explain why star topology does not require external terminator whilst bus topology needs such components at both ends? 2. Identify in what situation that a bus net breaks down while a star network does not. 3. Which topology would you prefer? 4. 4.1 Ethernet Criteria for a network architecture Criteria How to achieve reliability Provide error detection mechanism. Provide error recovery facility. modularity Relatively small variety mass-produced blocks in a broad assortment of network devices. connectivity Incorporate a variety of hardware and software products. Devices and products can function in an interoperable and unified manner. Ease of use Users need have no concern for or knowledge of the network's structure or implementation modification Evolve and adapt as needs change or expand, or as new technologies emerge. implementation Follow industry networking standards. Allow for the installation of a variety of configurations 4.2. History of Ethernet Time & Place 1960s at the University of Hawaii. In the 1970s the Xerox 1980s IEEE 4.3 Event The access method used by Ethernet was developed. To advance and popularize Ethernet Standardization of Ethernet architecture– 802.3 Ethernet access method CSMA/CD - carrier sense multiple access with collision detection Functions: Transmitting and receiving data packets Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model. Detecting errors within data packets or on the network. Function to be performed at a device Wants to xmit data (Carrier Sense) Media Access Collision detection Action taken after collision Action taken by the device Receiving data by a receiver Data verification at receiver Perform listen before transmit mode. Listen to the media and check whether it is in use (by electrical voltage / pulse). Transmit data if not busy. Sender keeps on listen to the media to ensure no device is using the media at the same time. A collision is detected if signal strength is doubled. Sending devices continue transmission to ensure the collision is detected by all workstations. Each device invokes a backoff algorithm, resulting in different delays at devices attempting to perform transmission. No priority for collided senders when attempting for re-transmission. Only the device whose MAC address and IP address matches the destination MAC address and destination IP address carried by inside Ethernet data frame will copy the data. Validate data packet using frame check sequence (a parity check field inside Ethernet data packet). Ignore data if error found. Pass data to upper layer for further checking/process if no error. No acknowledgement back to sender. Characteristics of Ethernet Ethernet is a broadcast transmission medium All devices on a network can see all data that passes along the networking media, but not processing data. A connectionless network architecture. Also known as a best-effort delivery system. 4.4 Ethernet vs Token Ring Item Access Method Physical layout Data delivery Ethernet CSMA / CD Linear Bus (10Base2 or 10Base5) Star (10BaseT) Opportunistic Token Ring Token Passing Circular wiring Assurance Review Questions 1. Can collisions occur in a Token Ring Network architecture? Why or Why not? 2. Which of the two network architectures listed above could be considered to be deterministic? 3. In which of the two network architectures listed above is a broadcast access method used? 4. What is token passing? 5. Why and how can network architecture determine the type of topology used?