Network Operations & administration CS 4592

Lecture 21

Instructor: Ibrahim Tariq

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IEEE 802 Series of LAN Standards• 802 standards free to

download from http://standards.ieee.org/getieee802

WiMAX

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IEEE 802.11 Wireless LAN - 1• Wireless LANs• IEEE 802.11 standard:– unlicensed frequency spectrum: 900Mhz, 2.4Ghz

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Figure 14.1 Basic service sets (BSSs)

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Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station

BSS’s combined to form distribution system (DS)

IEEE 802.11 Wireless LAN - 2

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Figure 14.2 Extended service sets (ESSs)

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Ad Hoc Networks• Ad hoc network: IEEE 802.11 stations can dynamically

form network without AP• Applications:– “laptop” meeting in conference room, car– interconnection of “personal” devices– battlefield

• IETF MANET (Mobile Ad hoc Networks) working group

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IEEE 802.11 MAC Protocol: CSMA/CA

802.11 CSMA sender:- if sense channel idle for DISF sec. then transmit entire frame (no

collision detection)-if sense channel busy

then binary backoff

802.11 CSMA receiver:if received OK return ACK after SIFS

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Hidden Terminal effect• hidden terminals: A, C cannot hear each other– obstacles, signal attenuation– collisions at B

• goal: avoid collisions at B• CSMA/CA: CSMA with Collision Avoidance

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Hidden Station Problem

• The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.

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Use of Handshaking to Prevent Hidden Station Problem

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Collision Avoidance: RTS-CTS exchange - 1• CSMA/CA: explicit channel

reservation– sender: send short RTS:

request to send– receiver: reply with short

CTS: clear to send• CTS reserves channel for sender,

notifying (possibly hidden) stations

• Avoid hidden station collisions

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Collision Avoidance: RTS-CTS exchange - 2• RTS and CTS short:– collisions less likely, of shorter

duration– end result similar to collision

detection• IEEE 802.11 allows:– CSMA– CSMA/CA: reservations– polling from AP

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Exposed Station Problem

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Use of handshaking in exposed station problem

Data Communication & Networks, Fall 2009

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Interconnecting Devices

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14-2 BLUETOOTH

Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneously.

ArchitectureBluetooth LayersBaseband LayerL2CAP

Topics discussed in this section:

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Figure 14.19 Piconet

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Figure 14.20 Scatternet

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Figure 14.21 Bluetooth layers

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Interconnecting Devices

• How to get more users attached to a LAN?

• How to extend a single LAN?

• How to connect different LANs?

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Interconnecting Devices (cnt’d)

• Repeater• Hub• Bridge• Switch• Router

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Five Categories of Network Devices

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Repeater

• works at the Physical layer– Regenerates received bits before it sends them

out• connects different half-duplex network

segments• either extends the number of users or the

total span (by improving the quality of the transmitted signal)

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A Repeater Connects two segments of LAN

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Function of Repeater

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A repeater is a regenerator, not an amplifier.

Note

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Hub• multi-port repeater (physical hardware device)• provides physical star topology• no intelligence• no separations of collision domains

– all the hosts compete for the shared bandwidth

HUB

• An Ethernet hub, active hub, network hub, repeater hub or hub is a device for connecting multiple Ethernet devices together and making them act as a single network segment. A hub works at the physical layer (layer 1) of the OSI model

• The device is a form of multiport repeater.

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Bridge• works at layer 2 (requires software)• connects two networks of the same type

– LAN to LAN (example: WLAN to Fast Ethernet) • forwards data (1 packet @ the time) depending on the

destination address in the data packet (not the IP address, but the physical (MAC) address that is unique for every Network Interface Card (NIC))

• all computers are in the same sub-network• packet filtering• separates collision domains – larger network spans• a stand alone device or a PC with the special NIC and the

accompanied software

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Bridge (cnt’d)

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A bridge has a table used in filtering decisions.

Note

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A bridge does not change the physical (MAC) addresses in a frame.

Note

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Switch

• basically a multi-port bridge• provides a better network performance

– forwards more than a single packet at a time• separates collision domains – larger total

network span• bandwidth not shared

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Switch (cnt’d)

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Router

• connects different sub-networks• Layer 3 (Network layer) device• forwarding based on IP addresses not on MAC

addresses• more expensive than a switch (requires CPU)• Layer 3 switches (only work with IP packets)

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An Example

a simple internet