# 2. LAN Topologies Gilbert Ndjatou Page 1

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1 2. LAN Topologies Two basic categories of network topologies exist, physical topologies and logical topologies. The physical topology of a network is the cabling layout used to link devices. This refers to the layout of cabling, the locations of nodes, and the interconnections between the nodes and the cabling. Logical topology is the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices. A network's logical topology is not necessarily the same as its physical topology. For example, the original twisted pair Ethernet using repeater hubs was a logical bus topology carried on a physical star topology. Some networks are able to dynamically change their logical topology through configuration changes to their routers and switches. Wired LANs are classified in one of the following topologies (page 227): Bus topology Ring topology Star topology, and Mesh topology Each of these topologies has advantages and disadvantages Gilbert Ndjatou Page 1

2 Bus Topology A network in this category consists of a single cable (bus) to which each computer is attached. Each computer attached to the bus can send a signal (message) through the bus that is received by all other computers attached to the bus: If a computer address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches a computer address, the data is accepted. Computers attached to a bus network must coordinate to ensure that only one computer sends a signal at any time. Ring Topology Bus Network Topology A ring topology is a bus topology in a closed loop. Data travels around the ring in one direction. When one node sends data to another, the data passes through each intermediate node on the ring until it reaches its destination. The intermediate nodes repeat (re transmit) the data to keep the signal strong. If one node is unable to re transmit data, it severs communication between the nodes before and after it in the bus. In some networks in this category, each computer is connected to a small device and the cables (buses) connect those devices instead. This approach allows the network to continue to operate even if some of the computers are disconnected or fail. Ring Network Topology 2017 Gilbert Ndjatou Page 2

3 Mesh Topology A network in this category provides a direct connection between each pair of computers. The disadvantage of this approach is the large number of connections that is needed to connect all computers If there are N computers in a network, the number of connections will be N x ( N 1) / 2. Few wired LANs employ this topology because physical connections are expensive. Star Topology Mesh topology In Star topology, every node (computer workstation or any other peripheral) is connected to a central node which may be a hub, a router, or a switch. All traffic that traverses the network passes through the central node. An advantage of the star topology is the simplicity of adding additional nodes. The primary disadvantage of the star topology is that the hub represents a single point of failure. A typical hub is an electronic device that accepts data from the sending computer and delivers it to the appropriate destination. Star network topology 2017 Gilbert Ndjatou Page 3

5 Frames and Byte Stuffing In a packet switching network, a frame corresponds to a packet and consists of the following two parts: The frame header that contains the information used to process the frame such as an address that specifies the intended recipients of the message. The payload that contains the message being sent. Some technologies delineate each frame by sending a short prelude before the frame and a short postlude after the frame. In the ASCII character set, the Start Of Header (SOH) character marks the beginning of a frame and, the End Of Transmission (EOT) character marks the end of a frame. A technique known as byte stuffing is used to make it possible to transmit a message that contains one or more of the character SOH and EOT. With byte stuffing, a third character such as the character ESC is selected such that each of these three character is replaced in the text of the message with a sequence of two characters. For example: Byte to be sent SOH EOT ESC Sequence sent ESC A ESC B ESC C 2017 Gilbert Ndjatou Page 5

6 4. Wired LAN Technology (Ethernet and IEEE 802.3) The Ethernet is the most common transmission technologies in use for wired local area networks. It was commercially introduced by Xerox PARC in 1980, and first standardized by IEEE in 1983 as IEEE Over time, Ethernet has largely replaced competing wired LAN technologies such as token ring, FDDI, ARCNET, and AppleTalk. Over the years, the hardware devices, cabling, and media used with Ethernet have changed dramatically. But newer versions of Ethernet remain backward compatible. Thick Ethernet (page 257) The Thick Ethernet was the original Ethernet. It is also known as 10BASE5 or Thicknet. It uses a thick and stiff coaxial cable up to 500 meters (1,600 ft) in length. Up to 100 electronic devices called transceivers are connected to the Ethernet cable and handle carrier detection, conversion of bits into appropriate voltage for transmission, and conversion of incoming signals to bits Computers with NIC are attached to transceivers by a cable known as Attachment Unit Interface (AUI). The system is difficult to install and maintain Gilbert Ndjatou Page 6

7 Thinnet Ethernet (page 257) The second generation of Ethernet was known as 10BASE2. It is also informally known as Thinwire Ethernet or Thinnet. It integrated transceiver on the Network Interface Card, and used a thinner and more flexible RG-58 coaxial cable that runs from one computer to another. The major disadvantage is that if a user unplugs a segment of the network or relocates wires, or move a computer, the entire network will stop working. With Thicknet and Thinnet, since all communication happens on the same wire, any information sent by one computer is received by all, even if that information is intended for just one destination. The network interface card accept a message only if it is addressed to it and ignores it otherwise. A collision happens when two stations attempt to transmit at the same time: in this case, they corrupt transmitted data and require stations to re-transmit Gilbert Ndjatou Page 7

8 Twisted Pair Ethernet Wiring and Hubs (page 258) The third generation of Ethernet which is used today is informally known as twisted pair Ethernet. With the original version of twisted pair Ethernet, every node (computer workstation or any other peripheral) is connected by a twisted pair of wire to a central node called a hub. Electronic components in the hub emulate a physical cable, making the entire system operate like a conventional Ethernet. Each computer attached to the hub can send a signal (message) through the hub that is received by all other computers attached to the hub: If a computer address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches a computer address, the data is accepted. Computers attached to a hub must coordinate to ensure that only one computer sends a signal at any time. Note that although the physical topology of the twisted pair Ethernet wiring is the star topology, its logical topology is in fact the bus topology. The following table lists the three types of twisted pair Ethernet and the cable used with each: Designation Name Data Rate Cable Used 10Base T Twisted Pair Ethernet 10 Mbps Category 5 100BaseT Fast Ethernet 100 Mbps Category 5E 100BaseT Gigabit Ethernet 1 Gbps Category 6 Notes: Higher speed Ethernet technologies use an electronic device known as a switch instead of a hub. To remain backward compatible, standards for the higher speed versions specify that interfaces should automatically sense the speed at which a connection can operate and slow down to accommodate older devices. A twisted pair Ethernet cable uses RJ45 connectors. There are two types of twisted pair Ethernet cable: straight and crossed: A crossed cable is used to connect two switches: it connects each pin of the RJ45 on one end of the cable to a different pin on the RJ45 attached on the other end. A straight cable is used to connect a computer to a switch: it connects each pin of the RJ45 attached on one end of the cable to the same pin on the RJ45 attached on the other end. Some sophisticated interface hardware can detect an incorrect cable and adapt. However, most hardware will not function correctly if a crossed cable is used when a straight cable is required Gilbert Ndjatou Page 8

9 A 1990s network interface card supporting both coaxial cable-based 10BASE2 and twisted pair-based 10BASE-T The Ethernet Frame Format The header of an Ethernet frame consists of the following: A 48-bit destination address field that contains the address of the intended recipient(s) of the message, which is followed by A 48-bit source address field that contains the address of the computer that sent the frame, and which is followed by A 16-bit type field. Its payload has a variable length of 46 to 1500 bytes. The Cycle Redundancy Check (CRC) is a 4-byte code that follows the payload. It is an error-correcting code that is designed to protect against common types of errors on communication channels. Ethernet frame type field The Ethernet frame type is also referred to as the EtherType. It is used by the sender to indicate which protocol is encapsulated in the payload of the frame: Each such protocol has a code, and When the frame arrives at its destination, the receiver uses that code to determine which software module should be used to process the frame Gilbert Ndjatou Page 9

10 IEEE s Version of the Ethernet (802.3) In 1983, the IEEE developed a standard for Ethernet (referred to as Ethernet) and attempt to redefine the Ethernet frame format. However, most installations still use the original frame format. The major difference between conventional Ethernet and the Ethernet follows: The Ethernet interprets the type field as a packet length, and It adds an extra 8-byte header that contains the packet type. This extra header is referred to as Logical Link Control/Sub-Network Attachment Point (LLC/SNAP) header. The Ethernet reduced the payload maximum size from 1500 bytes to 1492 (1500 8) bytes. To keep the two versions of Ethernet compatible, we can think of the LLC/SNAP (or SNAP) header as the first 8 bytes of the payload and then use the following convention: 1. If bytes of frame contain a numeric value less than 1500 then a. The field is interpreted as a packet length and the standard applies. 2. Otherwise a. The field is a type field and the original Ethernet standard applies Gilbert Ndjatou Page 10

11 Extending a LAN across a Long Distance: Fiber Modems, Repeaters, Bridges, and Switches A typical LAN technology is designed to span a few hundred meters: it works well within a single building or a small campus. One limitation on distance is due to the fact that the hardware is designed to emit a fixed amount of energy, and If the wiring is extended beyond the design limits, stations will not receive a sufficiently strong signal, and errors will occur. The following mechanisms are used to extend a LAN across a long distance: Fiber modem extensions, Repeaters, Bridges, and Switches As a general rule, these extension mechanisms do not increase the strength of signals or extend cables. They insert additional hardware components that can relay signals across longer distances. An Ethernet extender (also network extender or LAN extender) is any device used to extend an Ethernet or network segment beyond its inherent distance limitation which is approximately 100 meters (330 ft) for most common forms of twisted pair Ethernet. These devices employ a variety of transmission technologies and physical media (wireless, copper wire, fiber-optic cable, coaxial cable). Ethernet extenders are the perfect solutions for: point-of-sale terminals, IP cameras, wireless access points, campus environments, warehouses and other industrial installations, remote offices or outposts, and any number of applications that require broadband speeds to be carried over distances exceeding the 328 foot limit of network cable. There are many types of commercial Ethernet extenders available today Gilbert Ndjatou Page 11

13 Notes: 1. A bridge allows simultaneous activities on attached segments: A pair of computers on one segment can communicate at the same time as a pair of computers on another segment. 2. Although vendors no longer sell stand-alone bridge devices, the concept of bridging has been incorporated in other network devices such as modems and wireless routers: Example1: A DSL or cable modem provides a form of bridging that isolates the local network at the subscriber s premise from the network at the ISP: A packet is sent to the ISP only if it is not sent to a local destination on the subscriber s network. Example 2: Many wireless routers incorporate an Ethernet switch that provides bridging functionality as follows: The Ethernet switch in a wireless router has ports (switch ports are labelled local to distinguish them from the Internet port that connects to the ISP s modem) to which computers can be connected. If two computers connected to the switch communicate, the wireless router will send the packets between them without sending them to the ISP network Gilbert Ndjatou Page 13

14 Switching and Layer 2 Switches Switching is a mechanism that forms the basis of modern Ethernets. An Ethernet switch (also called Layer 2 switch) is an electronic device that provides multiple ports to which computers are attached just like a hub: Every node (computer workstation or any other peripheral) is connected by a twisted pair of wire to an Ethernet switch (which is the central node) (refer to Figure 15.6, page 258). It allows computers attached to its ports to send packages to one another. The difference between a hub and an Ethernet switch follows: A hub operates as an analog device that forwards signals from one computer attached to it to all other computers attached to it, whereas, An Ethernet switch is a digital device that forwards an Ethernet frame from one computer to the computer(s) at the destination address in the frame. A hub can be viewed as a communication medium that connects all the computers attached to it (logical bus topology), whereas, An Ethernet switch can be viewed as a bridged LAN with one computer per LAN segment (logical star topology) (refer to Figure 17.6, page 298). An Ethernet switch consists of the following (refer to Figure 17.7, page 299): An intelligent interface attached to each port, and A central fabric that provides simultaneous transfer of packets between pairs of interfaces. An interface contains the following parts: A processor A memory, and The hardware devices needed to transfer packets. It performs the following tasks: accept an incoming packet consult a forwarding table, and send the packet across the fabric to the correct output port. It also accepts packets from the fabric and transmits them out the port Gilbert Ndjatou Page 14

15 Note: Because it has a memory, an interface can buffer arriving packets when an output port is busy. So, if computer 1 and computer 2 want to send packets to computer 3 at the same time, either interface 1 or interface 2 will hold its packet while the other is transmitting. The major advantage of using a switched LAN instead of a hub is parallelism: A hub can only support one transmission at a time whereas An Ethernet switch permits multiple independent transfers to occur at the same time: If a switch has N ports, N/2 independent transfers can occur at the same time. Ethernet switches are available in many sizes: The smallest consists of an inexpensive, stand-alone device that provides four connections which are sufficient to interconnect: A computer, A printer, and Two other devices such as a scanner and a backup disk. Larger switches are used in major corporations to connect tens of thousands of computers and other devices throughout a company. A core Ethernet switch 2017 Gilbert Ndjatou Page 15

16 Ethernet switches are often interconnected to form a single large network. Modern Ethernet implementation: switched connection, collision-free. Each computer communicates only with its own switch, without competition for the cable with others Gilbert Ndjatou Page 16

17 VLAN Switches Virtual Local Area Network switches (VLAN switches) are an extension of Ethernet switches with virtualization. It allows a manger to configure a single switch to emulate multiple, independent switches as follows: It specifies a set of ports on the switch and designates them to be on virtual LAN 1. Virtual LAN 2, virtual LAN 3,... etc are created in the same way. When a computer broadcasts a packet, only the computers on the same virtual LAN as this computer can receive it. A VLAN switch acts like a set of smaller switches. Dividing computers into separate broadcasting domains is important in certain large companies or service providers where it may be important to guarantee that a set of computers can communicate without others receiving the packets and without receiving packets from outsiders. Multiple Ethernet Switches and Shared VLANs IEEE has extended the Ethernet standard to make it possible to configure VLANs that cross multiple Ethernet switches as follows (standard IEEE 802.1Q): Each VLAN is assigned a unique identification number, and An extra 16-bit field (known as VLAN tag) is added to the header of each frame to hold the identification number of the VLAN to which the computer sending the frame belongs. The VLAN tag is inserted by the switch into the frame whenever it receives a frame from a computer. This frame is sent from one switch to another, but Before the frame is delivered to a destination computer by a switch, it removes the VLAN tag from it. When a computer broadcasts a frame, The switch to which this computer is attached does the following: a. deliver a copy of the frame to each of its ports that is part of the VLAN. b. It insert the VLAN tag of the VLAN to which this computer belongs to the frame. c. It then sends the resulting frame across the inter-switch link. The other switches do the following: a. receive the frame, b. extract the VLAN tag from it, and c. deliver a copy of the frame to each connected computer that is part of the VLAN Gilbert Ndjatou Page 17

18 Exercises: Pages 235 & 236: 13.8, 13.10, 13.11, 13.12, 13.13, 13.14, 13.15, Page 263: 15.1, 15.2, 15.3, 15.4, 15.6, 15.7, 15.8, 15.9, 15.10, 15.11, Pages 302 & 303: 17.1, 17.3, 17.4, Gilbert Ndjatou Page 18

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