WLAN Fundamentals. By Marc-André Léger. DESS, MASc, PHD(candidate)

Transcription

1 WLAN Fundamentals By Marc-André Léger DESS, MASc, PHD(candidate) Fall 2007

2 Save the forest Please do not print out more than one module at a time as it may evolve

3 Module 3: WLAN Infrastructure devices part 2

4 Basic Network Topologies Topology: the geometric configuration This is different than protocols!

5 Four basic topologies Local area networks are interconnected using one of four basic configurations: 1. Bus/tree 2. Star-wired bus 3. Star-wired ring 4. Wireless

6 Basic Network Topologies Logical vs physical design (topologies) 1. Logical design: how the data moves around the network from workstation to workstation 2. Physical design: how the network physically appears if drawn on a sheet of paper (laid out) Names of topologies usually refer to the physical design.

7 Bus/Tree Topology The original topology 1970 s Workstation has a network interface card (NIC) provides a physical connection to a network Data can be transferred using either Baseband: digital signals Broadband: analog signals.

8 Baseband or broadband

9 Baseband The original band of frequencies of a signal before it is modulated for transmission at a higher frequency. A type of data transmission in which digital or analog data is sent over a single unmultiplexed channel, such as an Ethernet LAN. Baseband transmission use TDM to send simultaneous bits of data along the full bandwidth of the transmission channel. Compare with broadband.

10 A type of data transmission in which a single medium (wire) can carry several channels at once. Cable TV, for example, uses broadband transmission. In contrast, baseband transmission allows only one signal at a time. Most communications between computers, including the majority of local-area networks, use baseband

11 What is broadband? The term is used to describe almost any always on, high speed connection to the internet.

12 More on broadband Broadband is a broad bandwidth connection. It allows a large amount of data to travel through a medium at the same time. There are many opinions about what a large amount of data entails. It is all relative. When the internet was in its infancy, 2.4kbps was the fastest dialup speed available. Current 53kbps dialup speeds would seem to be broadband compared to back then. The other consideration is that network traffic is bidirectional. In internet terms, there are download and upload paths for a connection. Many always on, high speed services have lower upload and higher download capacities. In network terms, these are called asymmetrical services. This complicates things somewhat when rating a service as both are significant.

13 Back to topologies

14 Bus/Tree Topology Workstation has a network interface card (NIC) provides a physical connection to a network Attaches to the bus (a coaxial cable) via a tap. NIC is an electronic device that performs the necessary signal conversions and protocols operations so that the workstation can send and receive data on the network. Tap is a passive device Does not alter the signal Does not require electricity to operate

15 Bus/Tree Topology

16 Bus Topology Consists of a single wire to which individual stations are attached Each end has a terminator attached to it Data travels the entire length of the cable Transmission from any stations travels entire medium (both directions) Inexpensive and easy to install Ethernet is the common form of a bus topology system

17 Bus Topology

18 Bus Tree Topology Baseband signals Digital signals 10 Mbps Bidirectional and more outward in both directions from the workstation transmitting. Easy to install and maintain Fewer than 100 workstations Buses can be split and joined, creating trees.

19 Bus Topology

20 Broadband signals Bus/Tree Topology Usually uni-directional and transmit in only one direction. Analogy and FDM for multiple channels (amplification necessary). Because of this, special wiring considerations are necessary. 100 to 1000 workstations over larger distances due to easy amplification Buses can be split and joined, creating trees.

21 Bus/Tree Topology Twice the propagation delay of baseband

22 Bus/Tree Topology

23 Advantages/disadvantages of a bus Difficult to add new devices if no tap exists. No tap existing means cutting into the line As such, this topology is loosing popularity Plenty still around; Ethernet uses this.

24 Star-wired Topologies Stars versus a single line Two types: 1. Star-wired bus (often call the star topology) 2. Star-wired ring

25 Star-wired Bus Topology Logically operates as a bus, but physically looks like a star. Star design is based on hub. All workstations attach to hub. Hub is an unintelligent device that immediately transmits whatever data it receives to all connections Unshielded twisted pair usually used to connect workstation to hub. Hub takes incoming signal and immediately broadcasts it out all connected links. Hubs can be interconnected to extend size of network. Very popular!

26 Star-wired bus physical topology

27 Interconnection of hubs in starwired bus topology

28 Star-wired Bus Topology Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus. Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable. Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium.

29 Star-wired Ring Topology Logically operates as a ring but physically appears as a star. Star-wired ring topology is based on MAU (multistation access unit) which functions similarly to a hub. Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion in one direction. Like hubs, MAUs can be interconnected to increase network size.

30 Ring topology viewed logically

31 NIC Repeaters for Star-wired All data must pass through a NIC repeater Performs basic functions: Ring Bypass: data does not copy to the workstation, used for inactive devices Copy: data from ring copied to workstation Write: data from the workstation copied to the ring Regenerates: recopies data back onto ring

32 Three possible operations of the workstation repeater in a ring topology

33 Physical organization of a ring topology Looks like a star!

34 Multi-station access unit on a ring topology The MAU replaces the hub for this topology

35 Wireless Topology Not really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point. Range varies from 50 to 800 ft with speeds of 2 to 11 Mbps Most wireless LANs include a wired LAN backbone Usually requires a wireless NIC

36 Wireless Topology different forms Newer IEEE and b (Wi-Fi) standard defines various forms of wireless LAN connections. Speeds up to 11 Mbps with b standard. Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

37 Wireless Topology Acceptable transmission ranges broken up into areas: 2. Basic service set that surrounding an access point 3. Extended service set collection of basic service sets Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

38 Wireless Topology Two basic components necessary: the client radio, usually a PC card with an integrated antenna, and the access point (AP), which is an Ethernet port plus a transceiver. The AP acts as a bridge between the wired and wireless networks and can perform basic routing functions. Workstations with client radio cards reside within a basic service set, while multiple basic service sets create an extended service set.

39 Single-cell wireless LAN configuration

40 Multiple-cell wireless LAN configuration

41 Ad-hoc configuration for a wireless LAN

42 Wireless Topology With directional antennae designed for point-to-point transmission (rare), b can work for more than 10 miles. With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet. Distance is inversely proportional to transmission speed - as speed goes up, distance goes down.

43 Wireless Topology In actual tests, 11 Mbps b devices managed 5.5 Mbps To provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. (how good?) What will Bluetooth s impact be on b (Wi-Fi)?

44 Other Wireless Standards IEEE (older 2 Mbps) IEEE b (11 Mbps, 2.4 GHz) Also called Wi-Fi (wireless fidelity) IEEE a (54 Mbps, 5 GHz, in 2002) IEEE g (54 Mbps, 2.4 GHz, in 2002) HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)

45 Other Wireless Standards

46 Summary of topologies Logical vs physical topologies Bus and star-rings - old technologies Still some around You ll probably use 1. Star-wired bus (star) With bus or routers (now much easier to use) 2. Wireless network (Wi-fi)

47 WLAN Performance

48 Standards WLAN Standards The original WLAN Standard. Supports 1 Mbps to 2 Mbps a High speed WLAN standard for 5 Ghz band. Supports 54 Mbps b WLAN standard for 2.4 Ghz band. Supports 11 Mbps e Address quality of service requirements for all IEEE WLAN radio interfaces f Defines inter-access point communications to facilitate multiple vendor-distributed WLAN networks g Establishes an additional modulation technique for 2.4 Ghz band. Intended to provide speeds up to 54 Mbps. Includes much greater security h Defines the spectrum management of the 5 Ghz band for use in Europe and in Asia Pacific i Address the current security weaknesses for both authentication and encryption protocols. The standard encompasses 802.1X, TKIP, and AES protocols.

49 802.11Range and speeds

50 Calculating bandwidth

51 Calculating bandwidth Data being transferred in bits per second Network adds overhead (20%+) Collisions and interferences add delays (CNWA Ch7-p297)

52 Template File type File size Overhead Total Data 2KB per page 20%+ Video 1MB per min ++ 20% Other

53 WLAN bandwidth

54 Template File type File size Overhead Total

55 End of session 3

56 Questions

57 Please note These slides are produced as presentation material for a technical college course, all references, sources and bibliographical information is available in the commentaries section of the PowerPoint presentation and may not be visible to viewers of PDF versions. The course instructor has no pretensions to be the original author of any of the material.