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Network Architectures Prepared by Arvind Sharma

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Network Architectures Prepared by Arvind Sharma NETWORK ARCHITECTURES PREPARED BY ARVIND SHARMA What are Network Architectures? There was a need to find an efficient way to transmit information or data over a topology. Network architecture solved this problem by providing a design specification of the physical layout of connected devices. Network architecture is the combination of network topology, network standards and network protocols that are used to establish a network. It specifies the types of cables (or wireless media) being used (or wireless media being deployed), network cards and connectors being used in a network. It also defines the mechanism through which data can travel on the network and passes to each device. Types of Network Architectures There are three main types of network architectures: Ethernet Token Ring Fibber Distributed Data Interface (FDDI) Ethernet In 1972, Dr. Robert Metcalfe and David Boggs at Xerox invented the most popular local area network architecture named Ethernet. This first Ethernet network was established at Xerox’s Palo Alto Research Centre to interconnect computers and laser printers at a data transfer rate of 2.94 Mbps. In 1980, Digital Equipment Corporation (DEC), Intel and Xerox jointly developed first Ethernet Local Area Network standard called DIX standard. This standard was based on thick co-axial cable and allowed data transfer at 10Mbps. Ethernet uses both bus and star physical topology. Ethernet 10base-2 and 10base-5 networks use a bus physical topology. The most popular Ethernet 10base-T, 100base-T and 1000base-T networks use a star physical topology. Ethernet bus networks connect computers, printers or repeaters etc. through thin (10base-2) or/and thick (10base-5) co-axial cables using BNC (Bayonet Nut Connector) connectors. Thick co-axial cables are generally used as backbone cables. These bus networks broadcast signals in both directions on the backbone cable, so all directly connected devices can receive the signal. There is no need of hub in the bus networks. A special 50 ohms connector called a terminator must be placed at the both ends of the backbone cable as shown in figure 7.1. It is used to prevent signals from reflecting back on the cable and causing interference. Figure 7.1: A view of Ethernet bus network connected using co-axial cables Most common Ethernet networks are star networks. These networks allow computers, printers, bridges, repeaters, switches and routers to be connected to a hub mostly through twisted-pair cables as shown in figure 7.2. However, fibber-optic cable can be used as a backbone cable in the Ethernet star network. Internetworking & Middleware 1 - Network Architecture 1/3 Figure 7.2: A view of Ethernet star network connected through a hub Figure 7.3: A physical layout of the star Ethernet network Internetworking & Middleware 1 - Network Architecture 2/3 Ethernet controls data transfer on the network using CSMA/CD (carrier sense multiple access with collision detection) access method. In this method all computers that wish to transmit data on the network cable have the same priority. Any computer or network device that wants to send data over an Ethernet must listen for silence on the network. If it hears something, it waits for silence before transmitting data. If it hears nothing, it transmits data and continues to listen. It detects a collision by measuring the strength of the data signal. Collision occurs in a collision domain, when two computers attempt to send data at the same time. After the collision, signal will be double of its normal strength and can cause data to be corrupted. A computer close to the collision site (in 10base-2 or 10base-5 networks) detects collision and sends Jam Signal to indicate all other network devices to stops transmission. This Jam Signal can be 32-bit long or more and normally carries all 1’s to signal other network devices that a collision has been occurred. Hub sends jam signal to all computers or devices connected to it through twisted-pair cables in a network using star topology. All computers or network devices immediately stop transmission and wait for sometime (controlled by a chip on their network card) before retransmission. This phenomenon is called Backoff Algorithm and ensures that two computers will not try to transmit again at the same time. A collision domain is defined as a single Ethernet network in which there will be a collision if two or more computers attached to the system transmit at the same time. There will be less chances of collision, if collision domain is small. Hubs and repeaters (will be discussed later in this section) comprise one collision domain. However, switches and bridges create multiple collision domains and reduce chances of collision. Ethernet networks, depending on their specification can operate at 10Mbps (Traditional Ethernet), 100Mbps (Fast Ethernet) or 1000Mbps (Gigabit Ethernet) using baseband transmission. The latest IEEE 802.3 specifications are based on the original Ethernet standard that was designed to use thick co-axial cables in the Local Area Networks. Gigabit Ethernet using IEEE 802.3Z specification can transfer data at 1000Mbps. Each IEEE 802.3 specification prescribes its own cable types and uses the following general format: Signalling Rate (Mbps)-Band (Base or Broad)-Length (meters) or Cable e.g. 100baseT or 10base2. Baseband: It is a characteristic of a network technology where only one type of carrier frequency (voltage) is carried by a cable. Broadband: It is a characteristic of a network technology where multiple carrier frequencies can be sent through a single cable. Broadband systems use modulation techniques to transmit digital data over analogue carrier waves. ADSL and Cable TV are examples of Broadband transmission. Manchester and Differential Manchester coding is used for data transmission in Local Area Networks (LANs). Manchester coding is specially used for data transmission over the co-axial or twisted-pair cables in Ethernet LANs (IEEE 802.3 standard). However, Differential coding is used for data transmission over the twisted-pair cables in Token-ring LANs (will be discussed in the next section). Live animation and more information about Ethernet can be found at the following web sites: http://www.datacottage.com/nch/eoperation.htm http://www.ii.metu.edu.tr/~ion504/demo/lan/html/wiredcsma.html http://www.ii.metu.edu.tr/~ion504/demo/lan/html/medium.html http://www.netbook.cs.purdue.edu/anmtions/anim06_1.htm http://www.netbook.cs.purdue.edu/anmtions/anim06_2.htm http://www.netbook.cs.purdue.edu/anmtions/anim06_5.htm Internetworking & Middleware 1 - Network Architecture 3/3 .
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