Guide to Networking Essentials

1. Guide to Networking Essentials Chapter 2 Network Design Essentials

2. Objectives • Basics of network layout • The standard networking topologies • The variations on standard networking topologies • Hubs and switches • Construct a network layout

3. Basics of Network Layout • The basics of good network design • Analyzing network requirements • Selecting a network topology • Selecting the equipments to fit that topology • Topology refers to the physical layout of its computers, cables, and other resources, and also to how those components communicate with each other • Topology has a significant effect on the network’s performance and growth, and equipment decisions • Terms: topology, layout, diagram, map

4. Physical Topology vs. Logic Topology • Physical topology: the arrangement of cabling • Three physical topologies: bus, ring, star • Logical topology: the path that data travels between computers on a network • Three logic topologies: bus, ring, switching What are the differences of physical topology and logic topology? • The logical topologies (bus, ring, and switching) are usually implemented as a physical star. Let’s see an example …

5. Physical Topology • Three physical topologies: • A bus: consists of a series of computers connected along a single cable segment • A ring: computers connected to form a loop • A star: computers connected via a central concentration point

6. Low fault tolerance Physical Bus Topology • Physical bus topology is the simplest, and once the most common method for connecting computers • All computers are connected via a single cable – backbone Why does physical bus topology become obsolete?

7. Signal Propagation • In any network, computers communicate by sending information across the media as a series of signals • In a physical bus topology, the signals travel along the length of the cable in all directions • The signals continue to travel until … • they weaken enough so as not to be detectable • OR until they encounter a device that absorbs them • This traveling across the medium is called signal propagation • At the end of a cable, the signal bounces back

8. Signal Bounce Is there any problem with signal bouncing?

9. Cable Termination A terminator attached to each end of a cable prevents signals from bouncing. • Clearing the network for new communications.

10. Cable Failure What would happen if a cable break occurs?

11. Logical Bus Topology • A physical bus topology is almost always implemented as a logical bus • Although physical bus is obsolete, logical bus topology is still in use (usually on physical star topology) • When a computer has data to send, it addresses that data, breaks it into manageable chunks, and sends it across the network as electronic signals • All computers on a logical bus receive them • Only the destination computer accepts the data • All users must share the available amount of transmission time,  network performance is reduced Let’s watch simulation 2-1…

12. Passive Topology vs. Active Topology • In an active topology network, computers and other devices regenerate signals and are responsible for moving data through the network • In a passive topology network, computers only listen for data being sent; they are not responsible for moving data from one computer to the next. A logic bus topology belongs to a(n) (active / passive) topology network.

13. Physical Ring Topology • Each computer connects directly to the next computer in a line; • A circle of cable forms a physical ring; • Every computer in a ring is responsible for retransmitting the data  so it is a(n) active / passive topology. Is there any problem with the physical ring topology?

14. Logical Ring Topology Data in a logical ring topology travels from one computer to the next computer until the data reaches its destination • Token passingis one method for sending data around a ring • Token is a small packet, passes around the ring to each computer in turn. • If a computer (sender) has packets to send, it modifies the token, adds address and data, and sends it around the ring. • The receiver returns an acknowledge packet to the sender. • Upon receiving the acknowledge packet, the sender releases the tokens and sends it around the ring.

15. Logical Ring Topology (2) • Logical ring on a physical star – show how it works • Modern logical ring topologies use “smart hubs” that recognize a computer’s failure and remove the computer from the ring automatically • An advantage of the ring topology lies in its capability to share network resources fairly. Why?

16. Physical Star Topology • Computers connected by cable segments to a central device. • Pros • Inherent centralization of resources  easy for administration and trouble shooting • Robust: network still works even any computer or cable segment fails • Cons: • Single point for failure • Requires more cable installation

17. A Logical Bus on Physical Star (star bus) • The center device is a hub. • Once the hub receives signals, it retransmits them down every other cable segment to all other computers attached to the hub.

18. A Logical Ring on Physical Star (star ring) • The center device in a star ring is usually called a concentrator.

19. Switching on Physical Star • A switch takes a signal coming from a device connected and builds a circuit on the fly to forward the signal to the intended destination computer • Superior to other logical topologies because • unlike bus and ring, multiple computers can communicate simultaneously without affecting each other • A dominant logical topology in LAN design

20. Wireless Topologies Wireless networking has a logical and physical topology • Ad hoc topology: two computers can communicate directly with one another; sometimes called a peer-to-peer topology • Infrastructure mode:Use a central device, called an access point (AP), to control communications • Star physical topology because all the signals travel through one central device • Logical bus topology

21. Variations of Physical Topologies • Three typical variations or combinations of physical topologies: • Extended star • Mesh • Combination star and bus • These combinations can be used to get the most from any network

22. Extended Star Topology • Other switches (or hubs) are connected to the central switch’s ports. • Extended star is also referred to as a hierarchical star

23. Mesh Topology • The most fault tolerant and the most expensive topology. n (n -1) / 2 cable segments for a mesh topology with n computers. • Mesh topology is used in a WAN. An example …

25. Combination Star Bus Topology

26. Hubs and Switches • Both hubs and switches can act as the center of a star topology • In everyday use, a hub is “the center of activity” • This definition is appropriate in network usage also • In network usage, there are a number of variations on this central theme • Active hub • Passive hub • Repeating hub (a type of active hub) • Switching hub

27. Active Hubs • Active hub is the most common type of hub today • Regenerate, or repeat, the signals • Require electrical power to run • have eight or more ports • Also called multiport repeaters or repeating hubs • Takes a signal coming in on one port • Cleans the signal (e.g., by filtering out noise) • Strengthens the signal • Sends the regenerated signal out to all other ports • Drawback: require sharing the cable bandwidth among all connected stations

28. Passive Hubs • The signal passes through the passive hub without any amplification or regeneration. • As a junction point and requires no power

29. Switches Switch is a central connecting point in a star topology network • Does more than simply regenerate signals • Looks just like a hub, with several ports for connecting workstations in a star topology • Determines to which port the destination device is connected and forwards the message to that port is able to handle several conversations at one time  providing the full network bandwidth to each device rather than requiring bandwidth sharing Let’s watch simulation 2-2: basic operation of a switch

30. Construct a network layout Steps to evaluate the underlying requirements • Determine how the network will be used which often decides the topology you use • Decide the types of devices for interconnecting computers and sites • The type and usage level of network resources dictates how many servers you need and where to place servers

31. Selecting a Topology • Most new network designs come down to only one choice: How fast should the network be? • The physical topology will certainly be a star, and the logical topology is almost always switching • Ethernet switches are typically used on a LAN, but you might consider other logical topologies for other reasons: • Use of legacy equipment – such as token ring • Network size – using hub-based bus topology • Cost restrictions – using hub instead of switch • Difficulty to run cables – go wireless ?

32. Creating the Layout Network must be documented, and network diagram must be kept up to date. • Some useful questions to be answered before drawing the diagram: • How many client computers will be attached? • How many servers will be attached? • Will there be a connection to the Internet? • How will the building’s physical architecture influence decisions, such as whether to use a wired or wireless topology, or both? • Which topology or topologies will you use?

33. Creating the Layout (cont’)

34. Summary • Basic physical topologies: bus, star, or ring • Physical bus: easy to install but outdated • The logical bus topology is still used, but is almost always implemented as a physical star • Physical ring: connects devices in such a way that the cabling starts and ends with the same computer • Rarely used (except in FDDI) • Logical ring topology typically uses token passing to send data around ring; normally implemented as a star • Physical star: centralized management and higher degree of fault tolerance • Topology of choice in today’s networks

35. Summary (cont’) • Variations on major topologies • Extended star (most widely used) • Mesh (most fault tolerant) • Combination star and bus • Hub: central point of concentration for a star network • Can be active (if it regenerates the signals) or passive • Switch: provides better performance than a hub • Device of choice in corporate star topology networks • Constructing a network layout