Computer Communication and Networking Lecture # 4 by Zainab Malik 1

Network Categories Depends on Geographical Area: Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN) Local Area Network: – LAN size is limited to few Kilometers. – Can be as simple as two PC and a printer or it can be extended over a company. – Link devices in an office, campus and in a building etc. 2

LAN (contd…) Devices can be connected in a mesh topology, bus topology, ring topology and star topology. – Mesh Topology: Advantages: Dedicated Point-to-Point Connection. Secure. Easy to find fault. Disadvantages: Difficult to install and reconnect a device. Require amount of cables and I/O ports. 3

LAN (contd…) Bus Topology: Advantages: Lesser number I/O ports per station. Lesser amount of cabling. Disadvantages: Capacity is shared among all stations. Lack of Security. No concurrent Transmission. Installation of a device require entire network to get shutdown. As the length increases, Signal power decreases. 4

Repeaters Distance limitation in local-area networks – Electrical signal becomes weaker as it travels – Imposes a limit on the length of a LAN Repeaters join LANs together – Analog electronic device – Continuously monitors electrical signals on each LAN – Transmits an amplified copy 5

LAN (Contd…) Ring Topology: Advantages: Easy Fault detection. Each device act as a repeater. Disadvantage: Lack of security. Medium capacity is shared among all. Should have a limited Length. To add a device, entire network should be shuttled down. 6

Star Topology: All devices Connected to a Central Device. – Central Devices are Hub Bridges Switches Routers etc. 7

Hub: Joins multiple input lines electrically Designed to hold multiple line cards Do not necessarily amplify the signal An active hub strengthens and regenerates the incoming signals before sending the data on to its destination. Passive hubs do nothing with the signal. Hub internal diagram 8

Limitations of Repeaters and Hubs One large shared link Each bit is sent everywhere So, aggregate throughput is limited E.g., three departments each get 10 Mbps independently … and then connect via a hub and must share 10 Mbps Cannot support multiple LAN technologies Does not buffer or interpret frames So, can’t interconnect between different rates or formats E.g., 10 Mbps Ethernet and 100 Mbps Ethernet Limitations on maximum nodes and distances Shared medium imposes length limits E.g., cannot go beyond 2500 meters on Ethernet 9

Bridges Connects two or more LANs Extracts destination address from the frame Looks up the destination in a table Forwards the frame to the appropriate LAN segment Each segment can carry its own traffic 10

Switches Typically connects individual computers A switch is essentially the same as a bridge … though typically used to connect hosts, not LANs Like bridges, support concurrent communication Host A can talk to C, while B talks to D Switch Internal Diagram 11

Dedicated Access Dedicated access Host has direct connection to the switch rather than a shared LAN connection Completely supports concurrent transmissions Each connection is a bidirectional point-to-point link 12

Advantages Over Hubs/Repeaters Only forwards frames as needed Filters frames to avoid unnecessary load on segments Sends frames only to segments that need to see them Extends the geographic span of the network Separate segments allow longer distances Improves privacy by limiting scope of frames Hosts can “snoop” the traffic traversing their segment… but not all the rest of the traffic Can join segments using different technologies 13

Disadvantages Over Hubs/Repeaters Delay in forwarding frames Bridge/switch must receive and parse the frame … and perform a look-up to decide where to forward Storing and forwarding the packet introduces delay Need to learn where to forward frames Bridge/switch needs to construct a forwarding table Higher cost More complicated devices that cost more money 14

Switch Filtering/Forwarding When switch receives a frame: – index switch table using MAC address if entry found for destination – then{ » if dest on segment from which frame arrived » then drop the frame » else forward the frame on interface indicated – } else flood 15

Moving From Switches to Routers Advantages of switches over routers Plug-and-play Fast filtering and forwarding of frames No pronunciation ambiguity (e.g., “rooter” vs. “rowter”) Disadvantages of switches over routers Large networks require large tables Broadcast storms can cause the network to collapse 16

Routers: Routers contain internal tables of information called routing tables. Routing tables keep track of all known network addresses and possible paths throughout the internetwork, along with cost of reaching each network. Unlike bridges and switches, routers cannot compile routing tables from the information in the data packets they process. This is because the routing table contains more detailed information than is found in a data packet 17

Tree Topology Helps to expend the network. Security issues depend on the central devices used. 18