Communication Protocols

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Protocol

A protocol is a standard adopted, which tells the way in which the bits of a frame must be sent from a device (or controller or port or processor) to another device or system

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A protocol defines how are the frame bits:

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1) sent Synchronously or Iso- synchronously or asynchronously and at what rate(s)?

2) preceded by the header bits?

3) How the receiving device address communicated so that only destined device activates and receives the bits?

[Needed when several devices addressed though a common line (bus)]

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4) How can the transmitting device address defined so that receiving device comes to know the source when receiving data from several sources?

5) How the frame-length defined so that receiving device know the frame-size in advance?

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6) Frame-content specifications Arethe sent frame bits specify the control or device configuring or command or data?

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7) Are there succeeding to frame the trailing bits so that receiving device can check the errors, if any in

reception before it detects end of the frame ?

A protocol may also define:

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8) Frame bits minimum and maximum length permitted per frame

9) Line supply and impedances and line-Connectors specifications

Specified protocol at an embedded system port or communication device

IO port bits are sent according to a specified protocol, which is to be followed when communicating with another device through an IO port or channel

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Protocols

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• HDLC, Frame Relay, for synchronous communication

• For asynchronous transmission from a device port RS232C, UART, X.25, ATM, DSL and ADSL

• For networking the physical devices in telecommunication and computer networks Ethernet and token ring protocols used in LAN networks

Protocols in embedded network devices

• For Bridges and routers• Internet appliances application protocols

and Web protocols ─HTTP (hyper text transfer protocol), HTTPS (hyper text transfer protocol Secure Socket Layer), SMTP (Simple Mail Transfer Protocol), POP3 (Post office Protocol version 3), ESMTP (Extended SMTP),

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File transfer, Boot Protocols in Embedded

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• TELNET (Tele network),• FTP (file transfer protocol),• DNS (domain network server),• IMAP 4 (Internet Message Exchange

Application Protocol) and• Bootp (Bootstrap protocol).

Wireless Protocols in embedded devices network

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• Embedded wireless appliances uses wireless protocols

• ─ WLAN 802.11, 802.16, Bluetooth, ZigBee, WiFi, WiMax,

Exemplary Protocol – HDLC

• HDLC (High-level Data Link Control) is a standard protocol for the data link network.

• For synchronous communication between two data link layers on a network.

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The contents of an HDLC frame are shown in the following table:

The contents of an HDLC frame

Flag Address Control Information FCS Flag

8 bits 8 or more bits 8 or 16 bits

Variable length, 0 or more bits

16 or 32 bits 8 bits

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There are two formats Standard HDLC and Extended HDLC for 28 and 216 destination devices or systems, respectively .

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Formats of bits in a HDLC frame

Sequence of bits in a HDLC frame

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• FramestartCompulsory-

signalingFlag bits

flagatstart

bits;are

(01111110)

• Address bitsfor

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destinationCompulsory 8 bits in Standard HDLC16 bits in extended format

• Control bits Case 1: • Information Frame

First bit 0, next 3-bits N(S), next

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bit P/F and last 3-bits N(R) instandard format

N(R) :sequence number of frame received earlier from a

device to which this HDLC frame is being sent

N(S) :sequence number of frame sendP/F :Polling means to detect

through an acknowledgement

• Control bits Case 2:Supervisory Frame; First two bits

(10), next 2-

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bits RR or RNR or REJ or SREJ, next bit P/F and last 3-bits

N(R) in standard format. RR- A message in control bits in case 2,

which conveys ‘Receiver Ready’RNR - Receiver Not ReadyREJ – Reject (Sent when a message rejects). SREJ – ‘Selectively Reject’ Frame received out-of-sequence, repeat suggested. REJ and SREJ are negative Acknowledgement NACK

• Control

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bits Case 3: Un-numberedFrame; First two bits (11), next 2-bits^M,next remaining

bit P/F andlast bits for

M. [8-bits

3-bitare

immaterial after M bits inextended format]M- For command or Response from the transmitter- Reset, Disconnect or a set of defined mopde type

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• Data bits; Compulsory; m frame bits transmitted; Each bit is at the serial line for time T or, each frame is atthe line for time m.T.

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(Frame

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Check Sequence)• FCS bits; Compulsory; 16 bits in

and 32 in

standard format extended format

• Frame

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End flag bits; Compulsory;Flag bits at end = (01111110)