1. Chapter 4 Ethernet Technology
Computer Networking From LANs to WANs: Hardware, Software, and Security
Chapter 4
Ethernet Technology
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2. CENGAGE LEARNING: COMPUTER NETWORKING FROM LANS TO WANS
Objectives
Describe the format of an Ethernet frame and the interframe gap
Explain the basic operation of collision detection
Compare the features of the different 10-Mbps Ethernet, Fast Ethernet, Gigabit Ethernet, and 10 Gigabit Ethernet technologies
Discuss the principles of wireless Ethernet
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3. The Ethernet Frame Format
Figure 4-1 IEEE Ethernet frame
Comparison to Ethernet II
Identical preamble, no SFD, 2-byte Type filed in place of Length field, Data field longer than 1500 bytes
Bits transmitted LSB to MSB (most significant byte)
Minimum, maximum sizes: 72 bytes, 1526 bytes
including the bytes used for Preamble and SFD (some book does not include preamble and SFD)
Any received frame not within this size will be considered as corrupted and will be discarded.
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Ethernet Frame
All Ethernet standards use the same frame structure.
Ethernet has a header and a trailer.
There are three versions of Ethernet frame:
DIX
IEEE (Original)
IEEE (Revised 1997
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5. Interframe Gap
Figure 4-2 Interframe gap separates each Ethernet frame
Ethernet devices must allow a minimum idle period between transmission of Ethernet frames.
This idle period is call Interframe Gap
This idle time gives the network devices a chance to stabilize and give time for other network component to process the frame
10-Mbps Ethernet: 9.6 microseconds interframe gap
Corresponds to 96 bit times
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6. Interframe Gap When frame size is larger:
More available bandwidth utilized
96-byte interframe gap becomes less significant
Table 4-2 Effect of frame size on bandwidth utilization
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7. Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
Technique used to share access to the available bandwidth
Refer figure:
The network consist of 5 segments connected by four repeaters
A and B is connected at the farthest end of the network.
There may be many stations connected to each segment and all competing for bandwidth using the same CMSA/CD method.
All this stations operate in a collision domain.
Figure base5 Ethernet network
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8. CENGAGE LEARNING: COMPUTER NETWORKING FROM LANS TO WANS
CSMA/CD
Collision domain
Portion of LAN (or entire LAN)
Two or more transmitting stations interfere with each other
Refer figure:
If "Computer A" send a data signal to "Computer X" and "Computer B" send a data signal to "Computer Y", at same instance, a collision will happen.
The more the number of devices, the more the chances of collision
More collisions will result low quality of network
Switch – to divide a big collision domain to many small collision domains
Collision cannot happen between two devices which are connected to different ports of a Switch.
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9. CENGAGE LEARNING: COMPUTER NETWORKING FROM LANS TO WANS
CSMA/CD – How it works?
CSMA
If the medium is idle, transmit (immediately).
If the medium is busy, continue to listen until the channel is sensed idle.
If two or more stations are waiting to transmit, a collision is unavoidable. CD
If a collision is detected, transmit a jamming signal and then stop transmission.
Jam signal - to ensure all stations are notified the frame transmission failed due to a collision.
After sending jamming signal, wait a random amount of time (repeat from step 1)
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10. CSMA/CD No Collision During Frame Station waits an idle period
Equal to (or longer than) the interframe gap
Station begins transmitting frame one bit at a time
Electronic signal represents each bit traveled
Limited speed within thickwire coax
10.8 microseconds worst case to travel 2500 meters from station A to B
Time based on cable speed coefficient
Signal absorbed at coaxial segment endpoint
By terminating resistor
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11. CSMA/CD A Collision Occurs During the Frame
Ethernet transceivers detect signal, energy distortions
Stations output a jam sequence
Begin random waiting period before retransmitting
Station must still be transmitting to detect collisions
Round-trip time
Important to collision detection
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12. CENGAGE LEARNING: COMPUTER NETWORKING FROM LANS TO WANS
Figure 4-5 Collision example
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13. Ethernet Technologies
10 Mbps Ethernet
100 Mbps Ethernet
1000 Mbps Ethernet (Gigabit Ethernet)
10 Gigabit Ethernet
Wireless Ethernet
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14. Ethernet Technologies
The first Ethernet was developed back in 1970s.
Started with 10 Mbps Ethernet.
Since then, it has evolved to meet the increased demand for high-speed LANs.
Today, the fastest Ethernet can achieve data rate up to 10 Gbps.
Also used in WANs (wide area networks) and MANs (metropolitan area networks).
There are various Ethernet standards to support different data rate, transmission media and connector specification.
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15. Ethernet Technologies
To ensure compatibility between the various standards, Ethernet uses the same frame structure.
As a result, the different Ethernet standards can be used together in a single LAN.
Reasons for Ethernet success:
Simplicity and ease of maintenance
Ability to incorporate new technologies
Reliability
Low cost of installation and upgrade.
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16. CENGAGE LEARNING: COMPUTER NETWORKING FROM LANS TO WANS
Historic Ethernet
The original Ethernet design uses a shared bus topology.
There were two standards:
10BASE2 (Thinnet) – uses thin coaxial cable.
10BASE5 (Thicknet) – uses thick coaxial cable.
These early Ethernet standards were deployed in low bandwidth (10 Mbps) LANs.
Due to the use of bus topology, collision may happen.
Therefore, access to the media was managed by CSMA, and later CSMA/CD.
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Historic Ethernet
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Legacy Ethernet
This generation of Ethernet is characterized by the use of UTP cable and physical star topology.
Hosts are connected to a hub using UTP cable.
This Ethernet standard is called 10BASE-T.
However, the logical topology is still a bus.
A hub broadcast incoming frame to all outgoing ports.
Only one station can transmit at a time (half-duplex).
Collision may happen.
Access to the media is managed using CSMA/CD.
Later, the use of a hub is replaced with a switch.
This minimizes the possibility of collisions and increases the performance and reliability of Ethernet.
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Legacy Ethernet
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Current Ethernet
This Ethernet standard has a data rate of 100 Mbps.
The standard is called 100BASE-T (Fast Ethernet).
The use of hubs has been replaced with switches.
This leads to point-to-point logical topology.
A switch can forward incoming frame only to the port that leads to the receiver.
This minimizes the possibility of collisions.
Eliminates the necessity to perform CSMA/CD.
It also enables the hosts to have the full bandwidth of the media.
Later switch implementation also support full-duplex communications.
*NOTES : FULL DUPLEX = REFERS TO THE TRANSMISSION OF DATA IN TWO DIRECTIONS SIMULTANEOUSLY
HALF DUPLEX = TRANSMISSION OF SEND AND RECEIVE, BUT ONLY ONE-WAY AT A TIME
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Current Ethernet
Topology
physical: Star
Logical: Point-to-point
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22. Moving to 1 Gbps and Beyond
Ethernet nowadays can operate at the rate of 1 Gbps (1000 Mbps) or even 10 Gbps (10000 Mbps).
These are called Gigabit Ethernet and 10-Gigabit Ethernet respectively.
This higher speed is necessary to properly support Voice over IP (VoIP) and multimedia applications.
Switch is still used as the connecting device.
However, the switch must have full-duplex capability.
In addition to UTP cable, fiber optic cable can also be used.
Fiber optic cable will allow for greater distance.
This enables Ethernet to be used in WANs and MANs.
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23. Moving to 1 Gbps and Beyond
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Wireless Ethernet
Ethernet over radio frequency (RF) or Infrared (IR)
Covered by IEEE standard
Wireless Ethernet network components
One or more fixed stations (base stations)
Service multiple mobile stations
Implementation details:
Same frame formats for Ethernet and Token-ring
CSMA/CA utilized
1-Mbps, 2-Mbps, 11-Mbps, 54Mbps supported
Faster speeds becoming available
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Wireless Ethernet
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Differs from CSMA/CD
Wireless transceiver cannot listen for other transmissions while transmitting
Stations attempt to avoid collisions by using random backoff delays
Delay transmission when the network is busy
Handshaking sequence used between communicating stations
*Notes:
A random backoff - minimizes the probability that the same nodes will collide again
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Wireless Ethernet
Two types of wireless LAN
RF-based
Signals propagate through objects (walls)
ISM band used for transmission
Uses spread spectrum technologies frequency hopping and direct sequence
IR-based
Diffused IR bounces signals off walls, ceilings, floors
Data rate limited by the multipath effect
Point-to-point IR
Uses line-of-sight IR lasers, faster data rate than diffused IR, works over larger distances (up to 1 mile)
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27. CENGAGE LEARNING: COMPUTER NETWORKING FROM LANS TO WANS
Summary
Ethernet described by standard
Two Ethernet frame types: version 1 and version 2
Interframe idle time helps regulate transmissions
Collision domain
Only one computer, station transmits data at a time
CSMA/CD: allows bandwidth sharing
Various Ethernet varieties exist
Ethernet media type has its own characteristics
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