Computer Networks

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Computer Networks

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Contents

• IEEE Standards
• Data Link Layer
• Physical Layer
• Standard Ethernet
• Changes in the Standard
• Fast Ethernet
• Gigabit Ethernet
• Assignment 11

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IEEE Standards

• LAN
• A computer network designed for a limited
  geographic area
• Today, most LANs are also links in a WAN or the
  Internet
• The most dominant technology Ethernet
• IEEE Standard for LAN Project 802

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IEEE Standards

• Data Link Layer
• Logical Link Control (LLC)
• in charge of flow control, error control, framing
  (partly)
• Framing (HDLC frame vs. LLC/MAC frame)
• The purpose of LLC is to provide flow and error
  control for the upper layer protocols that
  actually demand these service
• Upper layer protocols such as IP need the service
  of LLC?
• When is the service of LLC required? ? LAN(s) in
  an isolated system

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IEEE Standards

• Data Link Layer
• Media Access Control (MAC)
• Physical Layer
• dependent on the implementation and type of
  physical media used
• IEEE defines detailed specifications for each LAN
  implementation
• e.g., 10Base5, 10BaseT, ...
• Ethernet evolution through four generations

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Contents

• IEEE Standards
• Standard Ethernet
• MAC Sublayer
• Physical Layer
• Changes in the Standard
• Fast Ethernet
• Gigabit Ethernet
• Assignment 11

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MAC Sublayer

• Functions
• Governs the operation of the access method
• Frames data received from the upper layer and
  passes to the physical layer
• Frame format
• 802.3 MAC frame
• Ethernet V2 frame

1500B
PDU Type
1500B
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MAC Sublayer

• Frame Format
• Preamble (7B)
• for synchronization of receivers H/W with the
  incoming signal
• bit pattern 10101010..
• Added at the physical layer (not formal part of
  the frame)
• SFD (1B 10101011) ? indicates a last chance for
  synchronization
• Destination/Source address (6B)
• serial number on the NIC (unique)
• Broadcast address (only for DA) all 6 bytes set
  to 1
• Length/Type (2B)
• lt 1518 (802.3) length field ? length of data
  field
• gt 1536 (V2) the type of the PDU packet
  encapsulated in the frame
• Data (46-1500B)
• CRC (4B) CRC-32

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MAC Sublayer

• Frame length minimum and maximum
• The maximum length (1518B) historical reasons
• by reducing size of buffer, for preventing
  monopolizing the medium
• The minimum length (64B) for CSMA/CD
• If there is a collision before the physical layer
  sends a whole frame, it must be heard by all
  stations
• 64 bytes for 10Mbps Ethernet (collision domain
  2500m)
• If the upper-layer packet is less than 46 bytes,
  padding is added to make up the difference

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MAC Sublayer

• Addressing
• 6-byte physical address on network interface card
  (NIC)
• Source address
• unicast (only one station)
• Destination address
• Unicast 1-to-1
• Multicast 1-to-many
• Broadcast 1-to-all (FFFFFFFFFFFF)

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MAC Sublayer

• Addressing
• Ex.1 Define the type of the following destination
  addresses
• a. 4A301021101A ? unicast (01001010)
• b. 47201B2E08EE ? multicast (01000111)
• c. FFFFFFFFFFFF ? broadcast
• Ex.2 Show how the address 47201B2E08EE is
  sent out on line
• The address is sent left-to-right, byte by byte
• For each byte, it is sent right-to-left, bit by
  bit

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MAC Sublayer

• Access method CSMA/CD
• Standard Ethernet uses 1-persistent CSMA/CD
• Slot time (bits or sec)
• the amount of time a device waits after a
  collision before retransmitting
• round-trip time time required to send the jam
  sequence
• 512 bits (10Mbps) ? 51.2µs
• For CSMA/CD, slot time transmission time of
  minimum frame
• Slot time and Collision
• In case of minimum-size frames (512 bits)
• slot time frame transmission time ? No problem
  in CSMA/CD
• In case of frames larger than the minimum size
• after the sends the first 512 bits, it is
  guaranteed that collision will not occur during
  the transmission of this frame

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MAC Sublayer

• Access method CSMA/CD
• Slot time and Maximum network length
• if we neglect jam signal transmission time in
  slot time,
• transmission time 2propagation time
• L / R 2d / V ? d LV / 2R V(L/R)/2 ? dmax
  V(Lmin/R)/2
• MaxLength PropagationSpeed Slot time / 2
• (2108)(51.210-6/2)
  5120m
• By considering the delay times in repeaters and
  interfaces, and the time required to send the jam
  sequence
• MaxLength 2500m

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Physical Layer

• Categories of standard Ethernet

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Physical Layer

• Encoding and Decoding

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Physical Layer

• 10Base5 Thick Ethernet (the first
  implementation)
• Uses RG-8 cable too stiff to bend (diameter
  1cm)
• Bus topology with an external transceiver
• Maximum cable length 500m (can be extended up to
  2500m)
• Transceiver for transmitting, receiving, and
  detecting collisions

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Physical Layer

• 10Base2 Thin Ethernet (Cheapernet)
• Uses BNC connecter RG-58 cable easy to bend
  (diameter 0.5cm)
• Bus topology with an internet or external
  transceiver
• Maximum cable length 185m (short range, small
  capacity)

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Physical Layer

• 10Base-T Twisted-Pair Ethernet
• Uses RJ-45 connecter and 4 UTP (CAT 3/5) cables
• Physical star topology (hub) with an internet or
  external transceiver
• Maximum cable length 100m (hub to station)

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Physical Layer

• 10Base-F Fiber Ethernet
• Uses fiber-optic cables and a hub
• Star topology with an external transceiver
  (fiber-optic MAU)
• Maximum cable length 2000m (hub to station)

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Physical Layer

• Summary

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Contents

• IEEE Standards
• Standard Ethernet
• Changes in the Standard
• Bridged Ethernet
• Switched Ethernet
• Full-Duplex Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Assignment 11

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Bridged Ethernet

• The division of a LAN by bridges
• Raise the BW and separate collision domains
• Raising the bandwidth
• In an unbridged Ethernet network, the stations
  share the BW of the network
• Ex. When two stations have a lot of frames to send

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Bridged Ethernet

• Raising the bandwidth
• A bridge divides the network into two or more
  networks
• Bridges interprets the destination address and
  blocks intra-segment traffic
• Bandwidthwise, each network is independent when
  we consider intra-segment traffic only (refer to
  Ch.16)
• Further division of the network, more BW for each
  segment

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Bridged Ethernet

• Separating collision domain
• The collision domains for an unbridged and a
  bridged network
• Smaller collision domains, less collision
  probability

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Switched Ethernet

• Idea Extension of a bridged Ethernet
• Multi-port bridge ? N-port switch (N of
  stations)
• The BW is shared only between the station and the
  switch (5Mbps)
• Layer-2 switch N-port bridge with additional
  sophistication for faster handling of the packets

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Full-Duplex Ethernet

• Motivation
• Communication of 10Base5 and 10Base2 half-duplex
• Features
• Increases the capacity of each domain from 10 to
  20 Mbps
• Instead of using one link between the station and
  the switch, two links are used one to transmit
  and one to receive
• No need for CSMA/CD ? no collision
• Each link a point-to-point dedicated path
• ? no need for carrier sensing nor collision
  detection ? can be off

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Contents

• IEEE Standards
• Standard Ethernet
• Changes in the Standard
• Fast Ethernet
• Motivation
• MAC Sublayer
• Physical Layer
• Gigabit Ethernet
• Assignment 11

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Motivation

• Goals
• Upgrade the data rate to 100 Mbps
• Make it compatible with Standard Ethernet
• Keep the same 48-bit address
• Keep the same frame format
• Keep the same minimum and maximum frame length
• ? IEEE 802.3u Fast Ethernet

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MAC Sublayer

• Features
• Access Method
• Half duplex CSMA/CD (collision domain 250m)
• Full duplex no CSMA/CD
• The implementations keep CSMA/CD for backward
  compatibility
• Minimum and maximum frame size same as those of
  Ethernet
• Autonegotation added for optimal communication
• Allows two devices to negotiate the mode or data
  rate of operation
• In order to allow imcompatible devices to connect
  one another
• In order to allow one device to have multiple
  capabilities
• In order to allow a station to check a hubs
  capabilities
• Data rate, full/half duplex,

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Physical Layer

• Topology
• Implementations

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Physical Layer

• Encoding

BW100MHz
25Msps
BW10MHz
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Physical Layer

• Summary

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Contents

• IEEE Standards
• Standard Ethernet
• Changes in the Standard
• Fast Ethernet
• Gigabit Ethernet
• Motivation
• MAC Sublayer
• Physical Layer
• Ten-Gigabit Ethernet, Metro Optical Ethernet
• Assignment 11

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Motivation

• Goals
• Upgrade the data rate to 1Gbps
• Make it compatible with Standard or Fast Ethernet
• Keep the same 48-bit address
• Keep the same frame format
• Keep the same minimum and maximum frame length
• To support autonegotiation as defined in Fast
  Ethernet
• Usage
• Backbone, high-speed links
• ? IEEE 802.3z Gigabit Ethernet

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MAC Sublayer

• Access method
• Full-duplex Mode
• No collision ? CSMA/CD is not used
• the maximum length of the cable is determined by
  the signal attenuation in the cable
• Half-duplex Mode
• Traditional
• Slot time for Gigabit Ethernet 5.12 bit
  0.512µs
• ? Collision domain 25m ? too short

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MAC Sublayer

• Access method
• Half-duplex
• Carrier Extension
• the minimum length of a frame 512 bytes (8
  times longer)
• very inefficient for short frames
• Frame bursting
• Multiple frames are sent ? look like one frame
• Padding is added between the frames so that the
  channel is not idle

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Physical Layer

• Topology

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Physical Layer

• Implementation
• Encoding (BW of CAT6 250MHz)

5-levels each (2,1,0,-1,-2) (125Msps
(100Base-TX))
BW100MHz
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Physical Layer

• Summary

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10-Gigabit Ethernet

• Goals
• Upgrade the data rate to 10Gbps
• Make it compatible with Standard, Fast, and
  Gigabit Ethernet
• Keep the same 48-bit address
• Keep the same frame format
• Keep the same minimum and maximum frame length
• Allow the interconnection of existing LANs into a
  metropolitan are network (MAN) or a wide are
  network (WAN)
• Make Ethernet compatible with technologies such
  as Frame Relay and ATM
• Usage
• Backbone, high-speed links
• ? IEEE 802.3ae Gigabit Ethernet

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10-Gigabit Ethernet

• MAC Sublayer
• operates only in full-duplex mode
• No need for contention, No CSMA/CD
• Physical Layer
• designed for using fiber-optic cable over long
  distances
• Implementations

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Assignments 11

• Exercises
• 13, 15, 16, 17, 18, 19, 20
• Due Date
• The next week