Local Area Networks

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IT 548
Telecommunications & Networking
Local Area Networks

Hood College Dept. of Computer Science Spring 2013

Layer 2: The Datalink Layer



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Datalink layer provides a reliable communications link between devices. Breaks a data stream into chunks called frames, or cells. Three key functions: • error detection • error correction • flow control





In LANs DL can be broken down into two sub-layers: • media access control (MAC)‫‏‬ • logical link control (LLC). • flow and error control

Datalink Layer Addressing

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• Frame transmitted

Datalink Layer Addressing

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• Frame received • Note MAC addresses

LAN Architecture Model
• A network’s architecture consists of:
• Access methodology • Network Logical topology • Network Physical topology

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No single architecture is best in all circumstances.

Access Methodology-CSMA/CD
1 4

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2

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3

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Access Methodology-Token Passing

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CSMA/CD vs. Token Passing

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• CSMA/CD becomes less efficient at high bandwidth demand.

Network Topology
• Logical topology
• Sequential • Broadcast

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• Physical topology
• Ring • Bus • Star

LAN Components
• • • • A central wiring concentrator Media Network Interface Cards Network interface card drivers.

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A local area network, regardless of network architecture, requires the following components:

LAN Technology Architecture

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NOS? What is that?

LAN Technology Choices & Implications

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NIC Technology Analysis Grid

Ethernet
• Traditional Ethernet can be defined as follows:
• Access methodology: CSMA/CD • Logical topology: Broadcast • Physical Topology: Historically—bus, currently—star

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Ethernet Frame Layout

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• •

IEEE 802.3 is commonly referred to as “Ethernet”. There are differences as noted here.

LAN Architecture

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• Typical Fast Ethernet Implementation

Gigabit Ethernet (aka 1000BaseX)‫‏‬
• An upgrade to Fast Ethernet, standardized as IEEE 802.3z : •
• •

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1000BaseSX: uses short wavelength laser multimode fiber optic media, primarily for horizontal building cabling. 1000BaseLX: uses long wavelength laser single mode fiber optic media, primarily for high-speed campus backbone applications. 1000BaseTX: uses four pair of cat 5 UTP with a maximum distance of 100 m.

Wireless LANs
• IEEE 802.11 standard • CSMA/CA at MAC layer • 802.11 frames similar to Ethernet frames

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Wireless LANs – 802.11b

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• 11 Mbps theoretical, 4 Mbps practical • 2.4 Ghz band – subject to interference from common electronic equipment • Shared access – sensitive to number of simultaneous users • Commonly available, inexpensive • Range is measured in 100’s of feet, lower indoors.

Wireless LANs – 802.11g
• • • • • Interoperates with, similar to 802.11b 54 Mbps theoretical Same band Similar range Also very common, inexpensive

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Wireless LANs

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• Wireless access points can provide for client access or provide a bridge

Wireless LANs

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• A wireless client will access the stronger channel

Wireless LANs /Overlaps

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• Care must be taken in wireless LAN designs

Wireless LANs

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• Care must be taken in wireless LAN designs

LAN Interconnection Hardware

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• Many stand-alone hubs may be cascaded

LAN Interconnection Hardware

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Hub functional comparison

Network Management

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• SNMP is used to manage network devices

LAN Interconnection Hardware

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• Shared media – a “party line”
N.B.: This is NOT a switch

• Fixed bandwidth shared by all stations

LAN Interconnection Hardware

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• Multiple, simultaneous connections at the same rate

Switching
• A DL process
• Making forwarding decisions based on the contents of layer #2 frame addresses

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• Switches
• Transparent devices • Receives every frame broadcast on a port • Checks source address of each frame it receives and adds that source...
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