Introduction to Networking

1. Introduction to Networking Frank Stanley, Jason Fisk, Philip Berg

2. Overview • Definitions • Network Topology • Ethernet • OSI Model • Hubs, Bridges, Switches, and Routers • Network Adaptor • Network Links

3. Definitions • Network – A group of computers connected together in a way that allows information to be exchanged between them. • Node – Anything that is connected to the network. • Segment – Any portion of a network that is separated, by a switch, bridge, or router, from other parts of the network • Backbone – The main cabling of a network that all of the segments connect to. Generally the backbone is capable of carrying more information that the individual segments. • Topology – The way that each node is physically connected to the network.

4. More definitions • Local Area Network (LAN) – A network of computers in the same general physical location. • Wide Area Network (WAN) – A network of computers that are connected over a large area. • Network Interface Card (NIC) – Every computer and most other devices are connected to the network via a NIC. Generally this is an Ethernet card. • Media Access Control (MAC) Address – The physical address of a device, such as a NIC. It has two parts, each 3 bytes long. The first 3 bytes identify the company that made the NIC, the second 3 bytes are the individual serial number for the NIC.

5. Even more definitions • Unicast – A transmission from one node addressed specifically to another node. • Multicast – A node sends a packet addressed to a specific group address. Only members of this group will see the packet. • Broadcast – A node sends out a packet that is intended to be read by all other nodes on the network.

6. Topology • Bus • Ring • Star • Star Bus • Mesh

7. Bus Each node is daisy-chained along the same backbone. Information sent from a node travels down the backbone until it reaches its destination node. Each end of a bus must be terminated with a resistor so that signals do not bounce back. The bus is ok for small networks but if more than one dozen computers are added, performance problems are the result. Also if the backbone fails the entire network will go down.

8. Ring A ring network is a bus that is not terminated, but connected at the ends to make a ring. Each node takes turns sending and receiving information through the use of a token. The token moves from node to node and each node extracts the data meant for it and adds anymore data that it needs to. Only the node with the token is allowed to send data, all other nodes must wait. Like a bus if the backbone fails the entire network will go down.

9. Star In a star network each device is connected to a central hub (which may be a hub or a switch). The hub takes a signal from any node and passes it to all the other nodes in the network. Compared to the bus topology the star requires more cabling, but a failure in a cable will only remove one node. However, if the hub fails the entire network fails.

10. Star Bus The star bus (also known as tree) is the most common network topology in use today. It is a combination of the star and the bus. Nodes in particular areas are connected to hubs and the hubs are connected to a backbone bus. Quite often stars are nested within stars.

11. Mesh The mesh topology involves routing. Unlike other topologies, messages sent out can take any of several possible paths to reach their destination. Some WANS, like the internet, use the mesh topology.

12. Ethernet • Very popular for several decades • Relatively inexpensive • Reasonably fast Why use Ethernet?

13. Traditional Ethernet • Development began in 1972 at Xerox Parc by Bob Metcalfe and D.R. Boggs • Specifications based on this work appeared in IEEE 802.3 in 1980 • 10base5 or “Thicknet” was the first incarnation of ethernet • 10base2 or “Thinnet” followed and was half the thickness of Thicknet • Most common today is 10base-T because of the advantages of Unshielded Twisted Pair (UTP) over coaxial cabling.

14. Ethernet Original drawing of ethernet by Bob Metcalfe, 1976

15. Fast Ethernet • Introduced in the mid 1990’s • Increased the performance of traditional ethernet while avoiding the need to re-cable the existing network • 100Base-T (using UTP) • 100Base-FX (using fiber optic cabling)

16. Gigabit Ethernet • Offers speeds of 1 Gigabit a second • First made to travel over copper or optical cabling • Now is successfully supported on 1000base-T • 1000base-T uses Cat5 cabling like 100base-T but uses extra twisted pairs to achieve gigabit speeds

17. Ethernet in Use • Ethernet functions nearly identically regardless of speed or layout • Devices use a NIC which interfaces with the system bus • Data sent over ethernet exists in the form of frames • Frames consist of two headers and a data section having a combined length of no more than 1518 bytes • The first header which is 6 bytes long is the destination address • The second header which is 6 bytes long is the source address • The standard says that frames are broadcast to all devices and it is up to the NIC to identify which frames are intended for it.

18. Ethernet in Use • When a device wants to transmit it first checks if a transmission is in progress and if not then transmits • Simultaneous transmissions are possible and result in collisions which cause both transmissions to fail and have to be retransmitted • A specialized algorithm is used to determine the proper waiting time for re-transmission • Ethernet cables are limited in reach before signal loss occurs (as short as 100 meters) • Repeaters (such as hubs) are used to connect cables so that they can span large distances

19. Ethernet Wiring • There are two standard formats for wiring T568A and T568B • T568B is the same as T568A except the green and orange pairs are swapped • Either standard format can be used in an ethernet network

20. Ethernet Wiring • Crossover cables are used to connect hub to hub or to connect two machines together directly without a hub

21. OSI Model • Stands for Open Systems Interconnection • Provides an international standard by defining a network framework that consists of seven protocol layers

22. Hubs, Bridges, Switches, and Routers

23. Hubs • Function is simply to replicate data • OSI layer 1 (physical layer) • Dumb (no knowledge of protocols) • Can only connect segments of the same speed • Inexpensive (4-port 10Mbps $15)

24. Hubs • A hub cannot direct data, it can only move it. • So a message from F to C will go to every segment including the one from which it originated. • This results in shared bandwidth for all nodes connected to the hub, and a single collision domain.

25. Hubs • There are three types of hubs. • Passive hubs only retransmit the data. • Active hubs amplify the signal before it is retransmitted. • Intelligent hubs are stackable. They typically also include remote management support via SNMP, and virtual LAN (VLAN) support

26. Bridges • Bridges work at layer 2, the data link layer. • Forwards data between two LANs. • The LANs need not share the same topology or speed. One LAN could use a token ring topology while the other uses a 10baseT ethernet.

27. Bridges • Bridges develop tables so that subsequent messages can be forwarded to the right network. • Table is built by watching which address comes from which network segment. • Uses the bridging table to determine whether to forward the data or to drop (filter) it if it is destined for the same segment from which it originated. LAN A LAN B Bridge

28. Bridges Bridges are also called "store-and-forward" devices because they look at the whole Ethernet packet before making filtering or forwarding decisions. Filtering packets, and regenerating forwarded packets enables bridging technology to split a network into separate collision domains. This allows for greater distances and more repeaters to be used in the total network design.

29. Switches • Work on layer 2 of the OSI model. • Only forwards data to specific network segment. This results in multiple, smaller collision domains, and more efficient bandwidth usage. • Does computations in hardware, so they are very fast.

30. Switches • Like a bridge, a switch will only forward data that is not destined for the same network segment from which it originated. • A switch also builds a table of addresses by watching incoming packets. • There are three basic types of switches. Switch

31. Switches • Cut-through switches read the MAC address as soon as a packet is detected by the switch. After storing the six bytes that make up the address information, they immediately begin sending the packet to the destination node, even as the rest of the packet is coming into the switch.

32. Switches • Store and forward switches will save the entire packet to the buffer and check it for CRC errors or other problems before sending. If the packet has an error, it is discarded. Otherwise, the switch looks up the MAC address and sends the packet on to the destination node. Many switches combine the two methods, using cut-through until a certain error level is reached and then changing over to store and forward. Very few switches are strictly cut-through, since this provides no error correction.

33. Switches • Fragment-free is a less common method. It works like cut-through except that it stores the first 64 bytes of the packet before sending it on. The reason for this is that most errors, and all collisions, occur during the initial 64 bytes of a packet.

34. Routers • Routers operate at layer 3 (network layer) of the OSI model. So they can be used to connect two or more IP networks. • Consists of a computer with at least two network interfaces (plus software). • They communicate with each other and share information that allows them to determine the best route through a complex network of many LANs. • Like switches and bridges, routers also do packet filtering. • Capable of Network Address Translation (NAT).

35. Assign port 458 to client 1 request. Re-format request with routers IP and new port 458. Send new request out When response comes in to port 458, return it to client 1. NAT 458 894 247 331 C1 Router C2 Host or Another Router

36. Routers • One of the tools a router uses to decide where a packet should go is a configuration table. A configuration table is a collection of information, including: • Information on which connections lead to particular groups of addresses • Priorities for connections to be used • Rules for handling both routine and special cases of traffic • A configuration table can be a simple as a half-dozen lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages. • Routers have two main responsibilities.

37. Routers • Optimizing the routing paths. A router uses a routing algorithm (Dykstra) to determine the optimal path to the destination. These algorithms maintain routing tables which contain route information such as destination/next hop association. Routing algorithms has several goals: • Optimality - finding the "best" route depending on the metric weightings used by the specific router (there are several metrics used by routers, each assigns different weights to routing algorithm's parameters such as number of hops, delay) • Simplicity and low overhead - router algorithms must be as efficient as possible with minimum utilization overhead.  An efficient algorithm is particularly important when using routing software on a limited resources computer.

38. Routers • Robustness and stability - routers are located at network junction points, therefore they must be robust with the ability to handle unusual behavior of hardware and software components. • Rapid convergence - routers must agree on the optimal paths over the net in order to avoid loops.  When a network event (computer/router failure, network segment going down etc..)  force changes in the router's routing tables, each router must perform recalculations based on the update messages it received from other routers on the net. • Flexibility - is the ability to adapt accurately and quickly to network changes and events. For example when the optimal routes are no longer accessible or even optimal, due to some change in the network (segments going down, change in bandwidth, router queue size, network delay, and other variables), the routing algorithms should quickly adjust to the new situation and choose the next best route to replace the old ones which are not accessible any more.

39. Routers • Switching - transport of packets over networks. • When computer wants to send a packet over the net, it formats a packet with the router's physical address and the destination address (protocol address) of the target host.  The router searches it's routing tables for the destination host. If there is no entry for the destination host the router usually drops the packet, otherwise (there is an entry for the destination host) it replaces the physical address with the next hop's address and retransmits the packet. The next hop isn't  necessarily the ultimate destination host, it may be another router which performs the same routine again. A packet may "visit" few routers / hosts on its route, each time it's destination physical address changes.

40. Node CPU | | Cache |---------- Network ---- (to Link) |--------------| adaptor Memory | I/O Bus

41. Network Adaptor • It simply interfaces

42. Cross Country/Transcontinental • DS1 (T1) 1.5 Mbps • DS3 (T3) 44.7 Mbps • STS-1 51 Mbps • STS-3 155 Mbps • STS-12 622 Mbps • STS-24 1.24 Gbps • STS-48 2.48 Gbps

43. Last Mile Link: Cable Modem • Unused space • Cable Modem mainly tuner, modulator, demodulator and media access control device. • 30-40Mbps

44. Last Mile Link: DSL • Again unused space. • Voice uses range of 3,400 Hertz. • Range 18,000 feet. • At 6,000 feet 8Mbps. • In practice 2Mbps (one way ticket).

45. Last Meter Link: Bluetooth • Simplicity: Takes the “plug” away from “plug and play.” • Connect anything to anything. • 2.402-2.48Ghz, 10 mill watt. • unidirectional 432.6-Kbps. 20% of that is used for connection maintenance. • Other wireless: 802.11, SWAP, HomeRF, IrD.

46. Last Meter Link: Power line • Works w/existing power line. • 50 Kbps to 350 Kbps, new models claim 14Mbps. • www.intellon.com

47. Open Issues • Active Networking: ABone • Using the Network as storage • Ubiquitous Networking

48. References • Computer Networks by Larry L. Peterson & Bruce S. Davie • http://www.howstuffworks.com • http://compnetworking.about.com/cs/ethernet1/ • http://www.howstuffworks.com/lan-switch1.htm • http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ethernet.htm • http://www.erg.abdn.ac.uk/users/gorry/course/road-map.html • http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/osi.html • http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/bridge.html • http://www.ee.vt.edu/~pushkin/ecpe4614/presentationsSp2000.PDF • http://www.anderson.ucla.edu/resources/acis/network/netinfo/infra.html • http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/switch.html • http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/hub.html • http://www.lantronix.com/learning/tutorials/switching.html • http://www.rad.com/networks/1997/nettut/router.html