Basic Design Issues of an Ethernet Network - Essay Example

ETHERNET/ 802.3 Table of Contents Introduction 2.Basic Design Issues of an Ethernet network 1 Using Hubs/Repeaters 1.2. Hubs/Repeaters Limitations 1 1.3. Using Bridges/Switches 3 1.4. Basic Bridges/Switches Operations 4 1.5. Using Routers 6 3. References 7 Technick n.d. Ethernet Tutorial Part I: Networking Basics, viewed 23 May 2011, http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_networking_ethernet_01. 7 List of Figures Figure 1 Repeater and Hub with different topologies 2 Figure 2 Basic Switch Setup 3 Figure 3 Loop Removal Through STP 6 Figure 4 Typical Router Configuration 7 List of Tables Table 1 Node Limitation in Hubs 5 Till present, Ethernet maintains its position of being the most popular LAN technology due to the balance it provides between the speed and cost, its ease of installation, being most widely accepted in the computer market and its universal support to practically all protocols (Lantronix n.d.). It can typically transmit data rates up to 10 Mbps. The IEEE Ethernet standard 802.3 (‘IEEE Ethernet’ 2011) defines all the configuration rules laying down a network and also defines the networked devices’ interaction with one another. 1. Basic Design Issues of an Ethernet network Due to the benefits of networks, users keep adding and networks keep growing. 1.1. Using Hubs/Repeaters The distance between nodes may increase and data signal’s quality may suffer. So, we use repeaters and hubs (as shown in Figure 1). Repeaters buffer the input signal and send the amplified version of the same signal to only the output port. Hubs are basically broadcasters (multiport repeaters). The signal is repeated to all ports other than the input port. 1.2. Hubs/Repeaters Limitations Ethernet is a shared medium, so, all nodes use the same medium for transmission of data. When two nodes send data at the same instant, the data collides and its validity and integrity is compromised. Data has to be sent again. Although Ethernet uses CSMA/CD (Tanenbaum 2003) for collision yet there are still delays. So, networks are designed to avoid collisions while ensuring efficient utilization of bandwidth, providing good network throughput. (a) Repeater on 10Base2 cable (b) Hub on 10Base-T cable (c) Bus and star topologies, with 10Base2 and 10Base-T cables Figure 1 Repeater and Hub with different topologies In Figure 1, in case two nodes broadcast at the same time, collision occurs. Hubs/repeaters also limit the number of nodes that can be supported (see Table 1 (Technick n.d.)). So networks cannot expand beyond a limit. As at a time, the entire bandwidth can be utilized by only one node, so the overall throughput is limited. Table 1 Node Limitation in Hubs Network Type Max Node per Segment Max Distance per Segment 10BASE-T 2 100m 10BASE2 30 185m 10BASE5 100 500m 10BASE-FL 2 2000m 1.3. Using Bridges/Switches For avoiding collisions, the network traffic needs to be isolated. So the big networks are divided into smaller segments. These segments are connected together through bridges and switches. Bridges and switches (bridges with enhancements) are transparent intelligent devices that facilitate growth of LANs with the same underlying functionality. They can connect a node or a full Ethernet segment (with its own set of switches, hubs and repeaters) of even different LAN technologies on each of their ports (see Figure 2). They map the MAC addresses of nodes to each port. (a) Directly with hosts (b) With other switches Figure 2 Basic Switch Setup For each incoming frame the bridge/switch checks the destination MAC address. If the destination MAC lies on another port, only then it forwards, else it filters the frame thus keeping it within the segment it came from. The collision domain is separated. If there is a collision in one segment, the effect stays in that segment alone. Two nodes in the network can transmit at the same time, so throughput in increased. Switches/bridges save bandwidth. Unlike hubs, they can directly be connected to the sending and receiving nodes, so the node can utilize the complete bandwidth. 1.4. Basic Bridges/Switches Operations Bridges/switches are self learning devices. They learn which MAC address is on which port. They populate their MAC tables on basis of the source MAC address of each incoming packet on a port. In case of an unknown port number for a host, they broadcast on all ports till a response comes from the host after which the bridge/switch adds the port number. They maintain more than one MAC addresses for a port e.g. in case of a hub, they have the addresses of all hosts connected to the hub (Boney 2001). In case the network has number of bridges/switches, several paths between devices could exist which leads to loops. Loops could cause a frame to continue propagating in a network endlessly wasting bandwidth (see Figure 3). In order to avoid loops, Spanning Tree Protocol (STP) was formulated in the IEEE 802.1d specification (Putra 2008; Pulsewan n.d.). It prevents formation of loops in case switches/ bridges are found interconnected through numerous paths. BPDU (Bridge Protocol Data Unit) messages are exchanged with other switches for detecting loops, and removing them by disabling the particular interfaces. The Spanning tree algorithm ensures that only one path exists between two devices (CISCO n.d.). In order to keep track of network changes and for enabling or disabling ports, all switches/bridges in a network exchange data frames called BPDU, every 2 seconds. These BPDUs contain Bridge Identification or the BID made of priority number and MAC address of the switch/bridge. Switch/bridge with the lowest BID becomes the root switch/bridge. If priorities are same, then one with lowest MAC is chosen. All other switches/bridges estimate the shortest path to the root switch/bridge. In case of multiple paths, the lowest cost path is chosen, thereby forming a loop free topology. On attaching a switch/bridge to a network, it passes through a number of phases before actually forwarding data. It will first pass numerous BPDUs around to determine network topology. In case a switch/bridge is causing a loop then it may remain in the blocking state till a TCN (Topology Change Notification) BPDU arrives to the root node from some non-root switch/bridge. The root node then sets a Topology change (TC) flag in the BPDUs it sends out. The TC flag instructs all the switches/bridges to refresh their forwarding tables. So, a switch/bridge has 5 operating states: 1. Listening: processes BPDUs to determine topology 2. Learning: builds MAC forwarding Table 3. Blocking: blocks that port which may be causing a loop. The port can only receive BPDUs. 4. Forwarding: receives and sends data. 5. Disabled: When a switch is operated manually and not part of STP. Figure 3 shows an example of loop formation and how it is removed owing to the STP. (a) With Loop (b) Without Loop Figure 3 Loop Removal Through STP 1.5. Using Routers Routers although work in a similar manner as switches and bridges, yet they filter on basis of logical addresses rather than Ethernet addresses. They follow a protocol. They divide networks into subnets, and allow traffic destined for a particular IP address to flow between segments. Routers are basically used when interconnecting networks which could be public (Internet) or private (LAN/WAN). Figure 4 shows a basic configuration of router. Figure 4 Typical Router Configuration 2. References Boney J, December 2001, Cisco IOS in a Nutshell, OReilly Publishing. CISCO, n.d., Spanning Tree Protocol (STP) / 802.1D, viewed 23 May 2011, http://www.cisco.com/en/US/tech/tk389/tk621/tk5/tsd_technology_support_sub-protocol_home.html IEEE Ethernet 802.3 Working Group, 27 April 2011, viewed 23 May 2011, http://www.ieee802.org/3/. Lantronix, n.d., Ethernet Tutorial - Part I: Networking Basics, viewed 23 May 2011, http://www.lantronix.com/resources/net-tutor-etntba.html. Pulsewan, n.d. What is Transparent Bridging?, viewed 23 May 2011, http://www.pulsewan.com/data101/transparent_bridging_basics.htm Putra H.C, 2008, ‘Spanning Tree Protocol (STP)’, Geeks Documentation, All About Technology Information, web log post, 12 July, viewed on 23 May 2011, http://harrychanputra.blogspot.com/2008/07/spanning-tree-protocol-stp_12.html. Tanenbaum, A.S 2003, Computer Networks, Pearson Prentice Hill. Technick n.d. Ethernet Tutorial Part I: Networking Basics, viewed 23 May 2011, http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=guide_networking_ethernet_01. 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