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8/13/2019 3 Underlying Technologies.pptx
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Underlying TechnologiesComputer Networks
Tutun JuhanaTelecommunication EngineeringSchool of Electrical Engineering & Informatics
Institut Teknologi Bandung
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Objectives
• To briefly discuss the technology of dominant wiredLANs, Ethernet, including traditional, fast, gigabit, and
ten-gigabit Ethernet
• To briefly discuss the technology of wireless WANs,
including IEEE 802.11 LANs, and Bluetooth• To briefly discuss the technology of point-to-point WANs
including 56K modems, DSL, cable modem, T-lines, and
SONET
• To briefly discuss the technology of switched WANsincluding X.25, Frame Relay, and ATM
• To discuss the need and use of connecting devices such
as repeaters (hubs), bridges (two-layer switches), and
routers (three-layer switches)2
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WIRED LOCAL AREA NETWORKS
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• A local area network (LAN) is a computer
network that is designed for a limited
geographic area such as a building or a
campus
• Most LANs today are also linked to a wide
area network (WAN) or the Internet
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IEEE Standard Project 802, designed toregulate the manufacturing and
interconnectivity between different LANs
FDDIFiber Distributed Data Interface
Token Ring
Token Bus
ATM LAN
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IEEE Standards
• Project 802 is a way of specifying
functions of the physical layer and the data
link layer of major LAN protocols
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The Ethernet
• Robert Melancton
"Bob" Metcalfe
(born April 7,1946)
is an electricalengineer from the
United States who
co-inventedEthernet
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Frame Format
„Length” used byIEEE standard to
define the number of
bytes in the data field
“Type” used by
original Ethernet todefine upper-layer
protocol using the
frame
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Frame Length
Reason for the maximum length restriction:
1. To reduce the size of the buffer (memory was very expensive when Ethernet was
designed)
2. It prevents one station from monopolizing the shared medium
• The minimum length restriction is required for the correct operation of CSMA/CD
• If the upper-layer packet is less than 46 bytes, padding is added to make up the
difference
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Addressing
• Each station on an Ethernet network has its ownnetwork interface card (NIC)
• The NIC provides the station with a 6-byte
physical address10
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• Ethernet Address
– The address normally is referred to as the
data link address, physical address, or MAC
address
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Example:
• 00-14-22 OUI for Dell
• 00-04-DC for Nortel
• 00-40-96 for Cisco
• 00-30-BD for Belkin
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• The address is sent left-to-right, byte by byte
• For each byte, it is sent right-to-left, bit by bit
• Example
– Show how the address 47:20:1B:2E:08:EE is sent outon line
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Unicast, Multicast, and Broadcast Addresses
• A source address is always a unicast
address
– the frame comes from only one station
• The destination address can be unicast ,
multicast , or broadcast
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• The broadcast address is a special case of
the multicast address; the recipients are allthe stations on the LAN
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• Define the type of the following destination
addresses
– 4A:30:10:21:10:1A
– 47:20:1B:2E:08:EE
– FF:FF:FF:FF:FF:FF
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Ethernet Evolution
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STANDARD ETHERNET
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Access Method: CSMA/CD
• The IEEE 802.3 standard defines carrier
sense multiple access with collision
detection (CSMA/CD) as the access
method for traditional Ethernet
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• Stations on a traditional Ethernet can beconnected together using a physical bus or star
topology , but the logical topology is always a bus
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Physical star, logically bus topologyPhysical bus topology
• The medium (channel) is shared between stations and only one station at
a time can use it
• All stations receive a frame sent by a station (broadcasting)
• The real destination keeps the frame while the rest drop it
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• How can we be
sure that two
stations are not
using the medium atthe same time?
• If they do, their
frames will collidewith each other
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• To minimize the chance of collision and,
therefore, increase the performance, the
CSMA method was developed
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Space/time model of a collision in CSMA
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Collision of the first bit in CSMA/CD
• A transmits for the duration t4 - t1; C transmits for the duration t3 - t2
for the protocol to work, the length of any frame divided by the bitrate in this protocol must be more than either of these durations
• Before sending the last bit of the frame, the sending station must
detect a collision, if any, and abort the transmission because, once
the entire frame is sent, station does not keep a copy of the frame and
does not monitor the line for collision detection 26
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Minimum Frame Size
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The worst collision on a shared bushttp://cnp3book.info.ucl.ac.be/lan/lan/
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• The frame transmission time T fr must be at
least two times the maximum propagation
time T p
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• Example
– In the standard Ethernet, if the maximum
propagation time is 25.6 μs, what is the
minimum size of the frame?
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CSMA/CD flow diagram
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The station transmits
and receives
continuously and
simultaneously
(using two different
ports)
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Implementation
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FAST ETHERNET
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• IEEE created Fast Ethernet under the name 802.3u• Fast Ethernet is backward-compatible with Standard
Ethernet, but 10 times faster (100 Mbps)
• The goals of Fast Ethernet:
1. Upgrade the data rate to 100 Mbps2. Make it compatible with Standard Ethernet
3. Keep the same 48-bit address.
4. Keep the same frame format.
5. Keep the same minimum and maximum frame lengths
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MAC Sublayer
• Keep only the star topology – There are two choices: half duplex and full duplex
– In the half-duplex approach, the stations are
connected via a hub
– in the full-duplex approach, the connection is madevia a switch with buffers at each port
• The access method is the same (CSMA/CD) for
the half-duplex approach
• For full-duplex there is no need for CSMA/CD
– The implementations keep CSMA/CD for backward
compatibility with Standard Ethernet
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Autonegotiation
• Autonegotiation allows twodevices to negotiate the mode
or data rate of operation
• It was designed particularly for
the following purposes:
– To allow incompatible devices to
connect to one another
– To allow one device to have
multiple capabilities
– To allow a station to check a
hub‟s capabilities.
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Implementation
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GIGABIT ETHERNET
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MAC Sublayer
• A main consideration: keep the MACsublayer untouched To achieve a data
rate of 1 Gbps, this was no longer possible
• Gigabit Ethernet has two distinctiveapproaches for medium access
1. Half-duplex
2. Full-duplex• Almost all implementations of Gigabit
Ethernet follow the full-duplex approach
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Full-Duplex Mode
• There is a central switch connected to allcomputers or other switches
– Each switch has buffers for each input port in which
data are stored until they are transmitted
• There is no collision in this mode CSMA/CDis not used
• The maximum length of the cable is determined
by the signal attenuation in the cable, not by the
collision detection process
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Half-Duplex Mode
• A switch can be replaced by a hub a
collision might occur CSMA/CD is used
the maximum length of the network is
totally dependent on the minimum framesize
• Three solutions have been defined:
1. Traditional
2. Carrier extension
3. Frame bursting
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• Traditional approach
– Keep the minimum frame length 512 bits
– The maximum network length only 25 m
(because the length of a bit is 1/100 shorterthan in standard Ethernet)
• It may not even be long enough to connect the
computers in one single office
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Carrier Extension is very inefficient if we
have a series of short frames to send (each
frame carries redundant data)
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• Frame Bursting – To improve efficiency, frame bursting was proposed
– Instead of adding an extension to each frame,
multiple frames are sent
• To make these multiple frames look like one frame, paddingis added between the frames (96 bits) so that the channel is
not idle The method deceives other stations into thinking
that a very large frame has been transmitted
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512 bytes
ExtensionFrame Frame Frame Frame
Frame burst
Maximum frame burst is 8192 bytes
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Gigabit Ethernet Implementation
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TEN-GIGABIT ETHERNET
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• Ten-Gigabit Ethernet standard : IEEE802.3ae• The goals :
1. Upgrade the data rate to 10 Gbps.
2. Make it compatible with Standard, Fast, and Gigabit Ethernet.
3. Use the same 48-bit address.4. Use the same frame format.
5. Keep the same minimum and maximum frame lengths.
6. Allow the interconnection of existing LANs into a metropolitan
area network (MAN) or a wide area network (WAN)
7. Make Ethernet compatible with technologies such as Frame
Relay and ATM.
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Implementation
• Ten-Gigabit Ethernet operates only in fullduplex mode no need for contention
CSMA/CD is not used
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