CS412 Introduction to Computer Networking

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CS412 Introduction to Computer Networking
Telecommunication

• Local Area Networks

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Topics

• LANs - IEEE Project 802
• Ethernet
• Data Link Layer Switching

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Figure 12-1
LAN Compared with the OSI Model
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Figure 12-2
Project 802
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Ethernet

• Ethernet Cabling
• Manchester Encoding
• Ethernet MAC Sublayer Protocol
• Binary Exponential Backoff Algorithm
• Switched Ethernet
• Fast Ethernet
• Gigabit Ethernet
• IEEE 802.2 Logical Link Control
• Retrospective on Ethernet

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802.3 and Ethernet

• 802.3
• 1-Persistent CSMA/CD LAN, 1 - 10 Mbps
• Ethernet
• A specific product that almost implements 802.3
• Cabling (baseband)
• 10Base5 (thick Ethernet)
• 10Base2 (thin Ethernet)
• 10Base-T (twisted pair)
• 10Base-F (fiber optics)

XBaseY Channel capacity Cable type
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Ethernet Cabling

• The most common kinds of Ethernet cabling.

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Ethernet Cabling (2)

• Three kinds of Ethernet cabling.
• (a) 10Base5, (b) 10Base2, (c) 10Base-T.

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Ethernet Cabling (3)

• Cable topologies.
• (a) Linear (b) Spine (c) Tree (d) Segmented

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Manchester Encoding

• Why Manchester encoding?
• Differentiating 0 bit or idle
• Synchronization
• Encoding scheme
• Each bit period is divided into 2 equal intervals
• Each bit period has a transition in the middle

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Manchester Encoding

• Manchester encoding
• Bit 1 high-low, bit 0 low-high
• Differential Manchester encoding
• Bit 1 no transition at the start of interval
• Bit 0 transition at the start of interval

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Ethernet MAC Sublayer Protocol

• Frame formats.
• (a) DIX Ethernet, (b) IEEE 802.3.
• Preamble 10101010 for synchronization
• Start of frame 10101011

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Ethernet MAC Sublayer Protocol

• Addresses
• Ethernet uses 6 bytes
• Support
• Unicast address begins with 0
• Multicasting 1 group number
• Broadcasting all 1s

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Figure 12-6
Collision in CSMA/CD
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Ethernet MAC Sublayer Protocol (2)
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Ethernet MAC Sublayer Protocol

• Minimum frame size 64 bytes
• Why?
• frame_size bits/channel_capacity bps gt 2? s
• In 10-Mbps Ethernet, 2? 50 ?s, therefore
• frame_size gt 50 ?s x 10 Mbps 500 bits,
• rounded up to 512 bits 64 bytes
• As the network speed goes up
• ? minimum frame length must go up or
• maximum cable length must come down

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Binary Exponential Backoff Algorithm

• Wait time t time slots after a collision
• t a random number between 0 and 2i - 1 after i
  collisions
• t 1024, for i 10,...,16
• when i gt 16, reset i 0
• Low delay for light load
• Reasonable delay for high load

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Switched Ethernet

• A simple example of switched Ethernet
• If all ports on a card wired together, each card
  becomes an on-card LAN and forms one collision
  domain.
• If buffer used, one port is a collision domain
  and no collision will occur.

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Figure 12-14
An Ethernet Network Using A Hub
One collision domain
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Figure 12-15
An Ethernet Network Using a Switch
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Fast Ethernet

• The original fast Ethernet cabling.

100Base-T hubs and switches 100Base-F switches
only, with one cable one collision domain
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Gigabit Ethernet

• Configurations
• (a) A two-station Ethernet.
• (b) A multistation Ethernet.

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Gigabit Ethernet (2)

• Gigabit Ethernet cabling.

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IEEE 802.2 Logical Link Control

• LLC
• (a) Position of LLC. (b) Protocol formats.

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Logical Link Control

• LLC forms the upper half of data link layer (MAC
  is below LLC)
• Purposes
• Provides error control and flow control
• Hides differences between 802 networks by
  providing a single format and interface to
  network layer
• Services
• Unreliable datagram
• Acknowledged datagram
• Reliable connection-oriented service

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Logical Link Control

• Sender
• Network layer passes packet to LLC using LLC
  access primitives
• LLC sublayer adds LLC header
• Source and destination access points
• Control sequence and acknowledgement numbers
• 802.x frame payload field
• (LLC header packet)
• Frame is transmitted
• Receiver
• Reversed process

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Restrospective on Ethernet

• Has been 20 years
• Simple and flexible
• Reliable
• Cheap
• Easy to maintain
• Works easily with TCP/IP
• Both IP and Ethernet are connectionless
• Evolution no software change required
• Speed higher and higher
• Hubs, switches

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Data Link Layer Switching

• Bridges from 802.x to 802.y
• Local Internetworking
• Spanning Tree Bridges
• Remote Bridges
• Interconnection Devices
• Repeaters, hubs, bridges, switches, cut-through
  switches, routers, gateways

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Data Link Layer Switching

• Multiple LANs connected by a backbone to handle a
  total load higher than the capacity of a single
  LAN.

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Bridges from 802.x to 802.y

• Operation of a LAN bridge from 802.11 to 802.3.

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Bridges from 802.x to 802.y (2)

• General Problems
• Different data formats
• Different data rates
• Different maximum frame length

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Local Internetworking

• A configuration with four LANs and two bridges.

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Transparent Bridges

• Transparency
• Plug and play
• Operates in Promiscuous Mode
• Accepting every frame transmitted on all LANs to
  which it is attached
• Decides
• Discard or forward
• If forward, to which LAN?
• Look up a huge destination address hash table

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Transparent Bridges

• Hash Table
• Initially empty
• Flooding algorithm
• Backward learning algorithm
• Arrival time noted for dynamic topology
• Scanned periodically to remove old entries
• Routing procedure for an incoming frame
• If dest LAN src LAN then discard
• If dest LAN ! src LAN then forward
• If dest LAN unknown then use flooding

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Figure 16.6 Learning bridge
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Spanning Tree Bridges

• To increase reliability
• Two or more bridges between 2 LANs
• Problem looping

F3
F4
with unknown destination
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Spanning Tree Bridges

• Solution to looping Spanning tree bridges
• LAN ? vertex
• Bridge ? edge(s)

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Remote Bridges

• Connects LANs at remote sites
• Approach
• Putting bridges on each LAN
• Connecting bridges point-to-point
• Point-to-point link considered as a hostless LAN

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Interconnection Devices

• Repeaters, hubs, bridges, switches, cut-through
  switches, routers, gateway
• Issues bandwidth and collision domain

• (a) Which device is in which layer. (b)
  Frames, packets, and headers.

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Interconnection Devices

• (a) A hub. (b) A bridge. (c) a switch.

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Figure 16.2 Repeater
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Figure 16.3 Function of a repeater
A repeater is not an amplifier an amplifier
does not regenerate signals.
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Figure 16.4 Hubs
A hub is a multiport repeater.
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Figure 16.5 Bridge
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Figure 14.16 A network with and without a
bridge
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Figure 14.17 Collision domains in a nonbridged
and bridged network
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Figure 21-16
Switch
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Figure 14.18 Switched Ethernet
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Cut-Through Switch

• As soon as the destination header field has been
  received, the frame can be forwarded.
• Faster (shorter delay)
• No more store-and-forward?
• Bad frames propagation

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Figure 21-10
A Router in the OSI Model