Local Area Networks

1
Local Area Networks
2
LAN characteristics

• confined within geographical area
• relatively high data rate
• under single management

stations
stations
LAN Ingredients...

• Topology
• bus, star, ring
• Transmission medium
• twisted pair, coaxial, fiber optics, wireless
• Medium access techniques
• protocol to coordinate the sharing of media

station
router
server
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Topology

• Topology defines how nodes/stations are connected
• LAN typical topology
• bus/tree all nodes connected to a common medium
• star all nodes are joined to a central nodes
• ring nodes form a ring by point-to-point links
  to adjacent neighbors

ring
star
bus
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Topology Bus
C
D
B
A
D
B and C
D
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
5
Topology Ring
A
A

• nodes function as a repeater
• only destination copies frame to it, all other
  nodes have to discarded the frame
• Unidirectional link

B
C
B
C
B transmits frame addressed to A
C ignores frame
A
A
B
C
B
C
A copies frame
C absorbs returning frame
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Topology Star

• Each station attaches to the central node
• Two point-to-point links for tranmitting and
  receiving
• Two alternatives operation
• broadcasting
• switching

shared hub broadcast packet to every ports but
only destination copies frame
A
C
B
A
B
C
switch hub retransmitts packet to the
destination only
C transmitts frame addressed to A
A
B
C
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Transmission media

• Twisted pair
• Cat 3 -- voice grade UTP, data rate up to 10Mbps
• Cat 5 -- data grade UTP, data rate 100 Mbps
• Coaxial cable
• baseband coaxial support data rates 20 Mbps at
  distance of up to 2 km
• Fiber optics
• support gigabit data rate and longer distances

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Medium Access Control

• Protocol for controlling access to transmission
  medium
• Defined as part of Data Link layer
• The protocol performs
• perform functions related to medium access (MAC
  sublayer)
• concerned with the transmission of a link-level
  between two nodes (LLC sublayer)

Network
LLC
Logical Link Control sublayer
Data Link
MAC
Medium Access Control sublayer
Physical
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LLC Sublayer Functions

• Provides a compatibility interface, irrespective
  of the MAC layer used.
• Service Access Points (SAPs) interface sublayers
  to upper-layer functions
• Optional connection, flow control, and sequencing
  services

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MAC frame format
MAC Frame (802.3)
SAPs define a protocol for upper layer
LLC PDU
DSAP Destination Service Access Point SSAP
Source Service Access Point Control Command and
response functions Data LLC SDU
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MAC Address
0000.0c12. 3456
ROM
RAM

• MAC address is burned into ROM on a network
  interface card

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MAC Strategies

• Where?
• centralized a designated station has an
  authurity to grant access to the network
• distributed stations can dynamically determined
  transmission order
• How?
• Sychronous dedicated specific capacity to a
  conection
• Asynchronous dynamic allocate capacity (3
  categories available)
• Round-robin
• Reservation
• Contention

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MAC techniques

• Contention
• Medium is free for all
• A node senses the free medium and occupies it
• Example Ethernet, IEEE 802.3
• Round robin
• Give every body a turn
• Inefficient for lightly loaded network
• Example Token Ring/IEEE 802.5, Token Bus/IEEE
  802.4, FDDI
• Reservation
• schedule a time slot like TDM
• Example DQDB

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Overview of LAN Standard

• 802.1 Overview
• 802.2 Logical Link Control
• 802.3 CSMA/CD (Ethernet)
• 802.4 Token Bus
• 802.5 Token Ring
• 802.6 Distributed Queue Dual BUS --MAN
  standard
• FDDI Fiber Distributed Data Interface

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LAN/WAN layers comparison

• Separate physical and data link layer for LAN
  and WAN

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Ethenet/802.3 Operation

• Every node can receive a transmission by all
  other nodes
• need addressing scheme to identify a destination
• only destination copies frame to it, all other
  nodes have to discarded the frame

C
B
C
A
B
A
terminator
C finds the bus is free
C transmits frame addressed to A
C
C
B
B
A
A
A copies frame
B ignores frame
signal is absorbed by the terminators
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Token Ring/802.5 Operation
T
T 0
T 0
A
A
T
T 1
A
T
Data

• Token Ring LANs continuously pass a token or a
  Token Ring frame

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FDDI
FDDI Dual Ring
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LAN Frame Format Summary
802.2 LLC
FDDI
FDDI uses 4-bits symbols
802.5
802.3
Ethernet