Ethernet

Chapter 13
Wired LANs: Ethernet

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

13-1 IEEE STANDARDS
Ethernet: It is a LAN protocol that is used in Bus and Star
topologies and implements CSMA/CD as the medium access
method

Original (traditional) Ethernet developed in 1980 by
three companies: Digital, Intel, Xerox (DIX).
In 1985, the Computer Society of the IEEE started a
project, called Project 802, to set standards to enable
intercommunication among equipment from a variety of
manufacturers.
 Current version is called IEEE Ethernet
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Ethernet

 Ethernet Frame format

FCS

FCS

Frame formats. (a) DIX Ethernet , (b) IEEE 802.3.

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Figure 13.4 802.3 MAC frame

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IEEE Ethernet


In IEEE 802.3 Ethernet Data link layer is split into two sublayers:


Bottom part: MAC




The frame is called IEEE 802.3
Handles framing, MAC addressing, Medium Access control
Specific implementation for each LAN p rotocol






Implemented in hardware

Top part: LLC (Logical Link Control)




The subframe is called IEEE 802.2
Provides error and flow control if needed
It makes the MAC sublayer transparent





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Defines CSMA/CD as the access method for Ethernet LANs and Token
passing method for Token Ring.

Allows interconnectivity between different LANs data link layers

Used to multiplex multiple network layer protocols in the data link layer frame
Implemented in software

Figure 13.1 IEEE standard for LANs

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Ethernet Provides Unreliable, connectionless Service




Ethernet data link layer protocol provides
connectionless service to the network layer

No handshaking between sending and receiving
adapter.
Ethernet protocol provides Unreliable service to the
network layer :
 Receiving adapter doesn’t send ACK or NAK to
sending adapter
 This means stream of datagrams passed to network
layer can have gaps (missing data)


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Gaps will be filled if application is using reliable transport
layer protocol
 Otherwise, application will see the gaps

Ethernet Frame

Preamble:

8 bytes with pattern 10101010 used to synchronize receiver, sender clock rates.

In IEEE 802.3, eighth byte is start of frame (10101011)

Addresses: 6 bytes (explained latter)

Type (DIX)

Indicates the type of the Network layer protocol being carried in the payload
(data) field, mostly IP but others may be supported such as IP (0800), Novell IPX
(8137) and AppleTalk (809B), ARP (0806) )

Allow multiple network layer protocols to be supported on a single machine
(multiplexing)

Its value starts at 0600h (=1536 in decimal)

Length (IEEE 802.3): number of bytes in the data field.

Maximum 1500 bytes (= 05DCh)

CRC: checked at receiver, if error is detected, the frame is discarded





CRC-32

Data: carries data encapsulated from the upper-layer protocols
Pad: Zeros are added to the data field to make the minimum data length = 46 bytes

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Ethernet address
 Six bytes = 48 bits
 Flat address not hierarchical
 Burned into the NIC ROM
 First three bytes from left specify the vendor.
Cisco 00-00-0C, 3Com 02-60-8C and the last 24 bit
should be created uniquely by the company
 Destination Address can be:
 Unicast: second digit from left is even (one
recipient)
 Multicast: Second digit from left is odd (group
of stations to receive the frame – conferencing
applications)
 Broadcast (ALL ones) (all stations receive the
frame)
 Source address is always Unicast

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Note

The least significant bit of the first byte
defines the type of address.
If the bit is 0, the address is unicast;
otherwise, it is multicast.

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Note

The broadcast destination address is a
special case of the multicast address in
which all bits are 1s.

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Figure 13.7 Unicast and multicast addresses

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Example 13.2
Show how the address 47:20:1B:2E:08:EE is sent out on
line.
Solution
The address is sent left-to-right, byte by byte; for each
byte, it is sent right-to-left, bit by bit, as shown below:

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Example 13.1
Define the type of the following destination addresses:
a. 4A:30:10:21:10:1A
b. 47:20:1B:2E:08:EE
c. FF:FF:FF:FF:FF:FF
Solution
To find the type of the address, we need to look at the
second hexadecimal digit from the left. If it is even, the
address is unicast. If it is odd, the address is multicast. If
all digits are F’s, the address is broadcast. Therefore, we
have the following:
a. This is a unicast address because A in binary is 1010.
b. This is a multicast address because 7 in binary is 0111.
c. This is a broadcast address because all digits are F’s.
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Figure 13.5 Minimum and maximum lengths

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Note

Frame length:
Minimum: 64 bytes (512 bits)
Maximum: 1518 bytes (12,144 bits)

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Figure 13.3 Ethernet evolution through four generations

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Categories of traditional Ethernet

•<data rate><Signaling method><Max segment length or cable type>

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IEEE 802.3 Cable Types
Name

Cable Max. Max Cable
Segment
Length

Nodes
/segment

10Base5

thick coax

500 meters

100

10Base2

thin coax

185 meters

30

10BaseT

twisted pair

100 meters

1

10BaseF

Fiber Optic

2Km

1

Toplogy

Bus

Bus

Star

Star
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Figure 13.10 10Base5 implementation

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Connection of stations to the medium using 10Base2

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10BaseT
• Uses


twisted pair Cat3 cable

Star-wire topology

• A hub functions as a repeater with additional functions
• Fewer cable problems, easier to troubleshoot than coax
• Cable length at most 100 meters

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Figure 13.12 10Base-T implementation

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Figure 13.13 10Base-F implementation

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13.4 Fast Ethernet



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100 Mbps transmission rate
same frame format, media access, and collision
detection rules as 10 Mbps Ethernet
can combine 10 Mbps Ethernet and Fast Ethernet
on same network using a switch
media: twisted pair (CAT 5) or fiber optic cable
(no coax)
Star-wire topology
 Similar to 10BASE-T
CAT 3
CAT 5

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Figure 13.19 Fast Ethernet topology

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Figure 13.20 Fast Ethernet implementations

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Full Duplex Operation



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Traditional Ethernet is half duplex

Either transmit or receive but not both simultaneously
With full-duplex, station can transmit and receive data simultaneously
With full duplex, Throughput (actual transmission rate) is doubled.


10-Mbps Ethernet in full-duplex mode, theoretical transfer rate
becomes 20 Mbps

100-Mbps Ethernet in full-duplex mode, theoretical transfer rate
becomes 200 Mbps

Changes that should be made with any computer in order to operate
in Full-Duplex Mode
1)
Attached stations must have full-duplex NIC cards
2)
Must use two pairs of wire one pair for transmitting from host to
switch (inbound) and the other pair for transmitting from switch to
host (outbound)
3)
Must use a switch as a central device not a hub
4)
Devices must be connected point-to-point (dedicated) to the switch

Each station constitutes separate collision domain

CSMA/CD algorithm no longer needed (no collision)

No limit on the segment length

Same 802.3 MAC frame format used
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Figure 13.18 Full-duplex switched Ethernet

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Figure 13.17 Switched Ethernet

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13.5 Gigabit Ethernet


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Speed 1Gpbs
Minimum frame length is 512 bytes
Operates in full/half duplex modes
mostly full duplex

Note

In the full-duplex mode of Gigabit
Ethernet, there is no collision;
the maximum length of the cable is
determined by the signal attenuation
in the cable.

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Figure 13.22 Topologies of Gigabit Ethernet

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Figure 13.23 Gigabit Ethernet implementations

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10Gbps Ethernet


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Maximum link distances cover 300 m to 40 km
Full-duplex mode only
No CSMA/CD
Uses optical fiber only

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Ethernet

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