Specifications

Appendix C Operating Environment Installation and Operation Manual
C-12 Ethernet Transmission Technology FCD-E1LC Ver. 1.0
pin assignment has also been standardized. However, because of the need to use
separate transmit and receive pairs, two types of port pin assignments have
developed: station ports and hub ports (the difference is that the transmit and
receive pins in the connector have been interchanged).
This permits to interconnect connectors of different types by a cable wired
pin-to-pin (straight cable). However, when it is necessary to interconnect ports
of same type, a crossed cable (a cable wired to interconnect the transmit pair at
one end to the receive pair at the other end) must be used (see other
alternatives on page
C-13 ).
Interfaces operating on twisted pairs are designated in accordance with data
rate: 10Base-T (10 Mbps) or 100Base-TX (100 Mbps, where X is the number of
pairs). Interfaces that support both rates are identified as 10/100BaseT.
Ethernet Communication Protocol
Today, Ethernet is used as a generic term for a LAN transmission technology that
uses Carrier Sense and Multiple Access with Collision Detection (CSMA/CD) to
enable the transmission of short bursts of data (called
frames ) between two or
more stations (
nodes ). The simplest way to visualize the transmission technology
is to use a bus analogy, where the bus runs between all the users.
Therefore, all the users have permanent access to the full bandwidth of the
transmission medium but can only use it for short times, by transmitting short
data bursts. Each data burst has a fixed structure, called a frame. The frame
structure is explained below. The connection point of each user to the
transmission media is called a node. For identification purposes, each LAN node is
assigned a unique number, called address.
Media Access Method
Media access is performed by means of the carrier sense, multiple access
protocol (CSMA) with collision detection (CD), defined by IEEE Standard 802.3.
The protocol defines three basic steps:
• A node that wants to transmit checks that the LAN is free. If another node is
already transmitting, the node waits until the LAN is free.
• When the LAN is free, the node starts transmission and sends its frame. Each
node has equal access rights, therefore the first node that starts transmitting
is the one that seizes the LAN.
• When two nodes start transmitting at the same instant, a collision occurs. In
this case, the transmitting nodes will continue to transmit for some time, in
order to ensure that all transmitting nodes detected the collision (this is
called “jamming”). After the jamming period, all transmitting nodes stop the
transmission and wait for a random period of time before trying again.
The delay times are a function of collision numbers and random time delay,
therefore there is a good chance that an additional collision between these
nodes will be avoided, and the nodes will be able to transmit their messages.
The basic procedure described above has been developed for half-duplex
communication, because it declares a collision whenever data is received during a
local transmission. However, when using twisted pairs, separate pairs are used