Ethernet Protocol - Data Link Layer; Figure 9 Mixed Ethernet Media - Control Technology Ethernet TCP/IP Adapter Module CTI 2572 Technical Overview

Mixed Media
Multiple media types may be used on
the same network. In larger
installations it is very common for
distributed 10baseT hubs to be
interconnected via a fiber optic hub.
The fiber optic "backbone" provides
extended distance and additional noise
immunity while the 10BaseT cabling
provides a simple connection to nodes
within a local area.
The flexibility of cabling schemes and
interconnection options is one of the
strengths of the Ethernet physical
layer. You may wish to consult your
network vendor for additional details
on the numerous options available.

3.3 Ethernet Protocol - Data Link Layer

Media Access
As previously presented, Ethernet uses a CSMA/CD (Carrier Sense Multiple Access with
Collision Detection) as a media access scheme. When a station has a message to transmit, it
monitors the line for a carrier signal (which indicates that another station is transmitting). If
there is no carrier signal, the station transmits the data. It is possible that more than one station
will attempt to transmit at the same time; when this occurs the signals will "collide". When
transmitting Ethernet stations detect a collision, each waits a random amount of time, then re-
transmits. Should a second collision occur, the stations back off exponentially until the
transmissions are successfully completed. In actual practice, 3 or more collisions are a rare
occurrence. The fact that stations delay a random amount of time when a collision occurs results
in the network being "non-deterministic." (response time cannot be precisely calculated). As a
practical matter, the Ethernet media access scheme is elegantly simple and performs well with
reasonable network loading.
Ethernet Version 2 and IEEE 802.3 Packet
Ethernet Version 2 and IEEE 802.3 transmit data as "packets" which can vary from 64 to 1518
bytes in length. Each packet contains the source address, destination addresses, the application
data, and a checksum. At this point the Ethernet II and 802.3 specifications diverge.
CTI 2572 Technical Overview Manual
Fiber optic
Backbone
10 BaseT Hub
Workstation Workstation
PLC
Workstation

Figure 9 Mixed Ethernet Media

Fiber optic
Fiber Optic Hub
Backbone
Fiber optic
Backbone
10BaseT Hub
Workstation
10 BaseT Hub
PLC PLC
Workstation
PLC
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