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A-4 APPENDIX / BASIC TCP/IP DESCRIPTION
hardware does not understand the software-
maintained IP addresses. IP uses ARP to translate the
32-bit IP address to a physical address that matches
the addressing scheme of the underlying hardware
(for Ethernet, the 48-bit MAC address).
4.0 THE TRANSPORT LAYER
There are two primary transport layer protocols:
Transmission Control Prot
ocol (TCP) and User Datagram Protocol (UDP). They
provide end-to-end communication services for
applications.
4.1 UDP
This is a minimal service over IP, adding only optional
checksumming of data and multiplexing by port
number. UDP is often used by applications that need
multicast or broadcast delivery, services not offered by
TCP. Like IP, UDP is connectionless and works with
datagrams.
4.2 TCP
TCP is a connection-oriented transport service; it
provides end-to-end reliability, resequencing, and flow
control. TCP enables two hosts to establish a
connection and exchange streams of data, which are
treated in bytes. The delivery of data in the proper
order is guaranteed.
TCP can detect errors or lost data and can trigger
retransmission until the data is received, complete and
without errors.
5.0 The Application Layer
There are many applications available in the TCP/IP
suite of protocols. Some of the most useful ones are
for sending mail (SMTP), transferring files (FTP), and
displaying web pages (HTTP).
Another important application layer protocol is the
Domain Name System (DNS). Domain names are
significant because they guide users to where they
want to go on the Internet.
5.1 DNS
The Domain Name System is a distributed database of
domain name and IP address bindings. A domain
name is simply an alphanumeric character string
separated into segments by periods. It represents a
specific and unique place in the “domain name
space.” DNS makes it possible for us to use
identifiers such as zworld.com to refer to an IP
address on the Internet. Name servers contain
information on some segment of the DNS and make
that information available to clients who are called
resolvers.
6.0 ETHERNET FRAME
6.1 Ethernet Frame
The term Ethernet generally refers to a standard
published in 1982 by Digital Equipment Corp., Intel
Corp., and Xerox Corp. It is the predominant form of
local area network technology used with TCP/IP
today. It uses an access method called CSMA/CD,
which stands for Carrier Sense, Multiple Access with
Collision Detection.
The Ethernet frame length ranges from 64 bytes to
1518 bytes. The Ethernet frame uses 48-bit (6-byte)
destination and source addresses. These are what
we call hardware addresses.
The next 2 bytes is the Ethernet type field which
identifies the type of data that follows. In the
Ethernet frame the data immediately follows the type
field. The data length ranges from 46 bytes to 1500
bytes. The minimum data frames for Ethernet is 46
bytes.
The CRC field is a cyclic redundancy check (a
checksum) that detects errors in the rest of the
frame. (This is also called the FCS or frame check
sequence.) The CRC field is 4 bytes.
Figure A.4 Ethernet Frame (RFC 894)
6.2 Encapsulation
When an application sends data using TCP, the data
is sent down the protocol stack, through each layer,
until it is sent as a stream of bits across the network.
Each layer adds information to the data by
prepending headers (and sometimes adding trailer
information) to the data that it receives. Figure 4.2
shows this process. The unit of data that TCP sends
to IP is called a TCP segment. The unit of data that
IP sends to the network interface is called an IP
datagram. The stream of bits that flows across the
Ethernet is called a frame.
Ethernet Encapsulation (RFC 894):
46-1500 bytes
destination
addr
source
addr
type
CRC
data
6
6
2
46-1500
4
Summary of Contents for DS Series
Page 5: ...1 2 INTRODUCTION END OF INTRODUCTION SECTION ...
Page 8: ...HARDWARE INTERFACE 2 3 Figure 2 3 Figure 2 2 Figure 2 1 ...
Page 13: ...2 8 HARDWARE INTERFACE END OF HARDWARE INTERFACE ...
Page 19: ...3 6 MODBUS RTV PROTOCAL DESCRIPTION END OF MODBUS RTU PROTOCAL DESCRIPTION ...
Page 23: ...5 2 MODBUS FUNCTION FORMATS END OF MODBUS FUNCTION FORMATS ...
Page 29: ...6 6 PHOENIX AC DRIVE FUNCTION FORMATS END OF PHOENIX AC DRIVE FUNCTION FORMATS ...
Page 31: ...7 2 EXCEPTION RESPONSE END OF EXCEPTION RESPONSE ...
Page 37: ...9 2 PARAMETER CONVERSION END OF PARAMETER CONVERSION ...
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