CECS 406 Network Engineering

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					       CECS 474 Computer Network Interoperability
                   CHAPTER 15
                     Wired LAN Topology: The Ethernet

Versions of Ethernet

Newer versions of Ethernet have managed to stay compatible with older
versions (i.e., the are backwards compatible).

Newer Ethernet Versions:
    • Increase the transmission speed
    • Keep the same Ethernet frame format
          - a fixed length header
          - a variable-length payload
          - a fixed length CRC

Dr. Tracy Bradley Maples (Spring 2011)   1
Speeds of LANs vs. Computers

Often LANs operate at a rate so fast that the computer's CPU cannot
process bits at network speed.

Network Interface Hardware

Question: How can a computer attach to a network that sends and
          receives bits faster that the CPU can handle?

Answer: Special-purpose hardware connects the computer to the
        network and handles all the details of packet transmission
        and reception.

The special-purpose hardware is known as a network adaptor card or
network interface card (NIC).

NICs appear to be I/O devices to the computer.

NICs handle:
            –   MAC address recognition
            –   CRC computation
            –   frame recognition
            –   sending/receiving frames

Dr. Tracy Bradley Maples (Spring 2011)   2
         NICs are built for a specific network technology.

Dr. Tracy Bradley Maples (Spring 2011)   3
Ethernet Wiring

There are three schemes of Ethernet Wiring
         They correspond to three generations Ethernet use
         But all use the same frame format

Original Ethernet Wiring (First Generation)

         Used heavy coaxial cable
         Formal name 10Base5
         Informally called thicknet

Dr. Tracy Bradley Maples (Spring 2011)   4
First Generation Ethernet Wiring (Cont'd)

In 10Base5 wiring, a connection multiplexor can be used to let multiple
computers attach to a network through a single transceiver.

The multiplexor delivers exactly the same electrical signals as a
transceiver, so a computer does not know whether it is attached to a
transceiver or a multiplexor.

Dr. Tracy Bradley Maples (Spring 2011)   5
Second Generation Ethernet Wiring

         Used thinner coaxial cable
         Formal name 10Base2
         Informally called thinnet

Modern Ethernet Wiring

         Uses a hub
         Formal name 10Base-T
         Informally called twisted pair Ethernet
Dr. Tracy Bradley Maples (Spring 2011)   6
Example: Ethernet Wiring in an Office

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The Ethernet Topology Paradox

         Original Ethernet used bus topology
         Modern Ethernet uses star topology

In fact, modern Ethernet is…
         Physical star
         Logical bus
         Called star-shaped bus

Higher Speed Ethernets

Fast Ethernet
         Operates at 100 Mbps
         Formally 100Base-T
         Two wiring standards
         10/ 100 Ethernet devices available

Gigabit Ethernet
         Operates at 1000 Mbps (1 Gbps)
         Slightly more expensive

Dr. Tracy Bradley Maples (Spring 2011)   8
Ethernet Generations

Dr. Tracy Bradley Maples (Spring 2011)   9

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