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LECTURE
YMODEM
YMODEM is similar to X-MODEM with only the following major differences:
o Two CANs o ITU-T CRC o Multiple fi
MODEM
col res of
BLAST
Blocked Asynchro XMODEM Full Duplex ow Control Data and Binary Files
Designed at Columbia University o Most Widely used Asynchronous Protocol o File Transfer protocol is similar in operation to XMODEM, with sender waiting for an NAK before it starts TX o Kermit allows the transmission of control characters as Text
Synchronous Protocols
Speed of synchronous TX makes it a better choice over Asynchronous T for LAN, MAN and WAN technology
Classes of Synchronous Protocols
Synchronous Protocols can be divided into two main classes:
o 1024-Byte data unit to abort Transmission -16 for Error Checking les can be sent simultaneously
Z
- Newer Proto
- Combines featu
o o More powerful than
nous Transmission
o o Sliding Window Fl o Allows transfer of
K ERMIT
o
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Character – Oriented Bit – Oriented Protoc
Oriented Protocols
Also called Byte- Oriented Protocol pret a transmission frame or packet as a succession of characters,
of an existing character encoding system
t – Oriented Protocols protocols interpret a transmission frame or packet as a succession of ade meaningful by their placement in the frame formation can be one or multiple bits depending on the information pattern
o Character –Oriented Protocols are not as efficient as bit – oriented protocols and are seldom use They are ea d and employ the same logic as bit-oriented protocols tudy will provide the basis for studying the other data link layer protocols ation is inserted in the data frame as separate g data frames this info is in the form of code words taken from
iented protocol
cation (BSC)
BSC FRAMES
- BSC protocol divides a transmission into frames
CONTROL FRAMES
Protocols ols
Character –
o o These protocols inter each usually composed of one byte o All control information is in the form
B i These individual bits, m Control in embodied in the
d o sy to comprehen o Their s o In all data link protocols, control inform control frames or as addition to existin o existing chara
In character oriented protocols, cter sets such as ASCII o IBN’s BSC is the best known character or
Binary Synchronous Communi
o Developed by IBM in 1964 o Usable in both point-to-point and multiple communications o It supports half-duplex TX using stop-and-wait ARQ flow control o Does not support full duplex TX or sliding window protocol
If a frame is used strictly for control purposes, it is called a Control fram Control frames are used to exchange information b/w communicating devices for ontrol the flow, to request error correction
ATA FRAMES
o o
e
example, to establish the connection, to c etc
D
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The probability of an error in the block of text increases with the length of the frame o The more bits in a frame, the more are the chances of an error o For this reason, text in a message is often divided b/w several blocks o Each block starts with STX and ends with ITB, intermediate text block except the last one o Last block ends with ETX character o After the ETX has been reached, and the last BCC checked, the receiver sends a single ACK for the entire frame The figure includes only 2 blocks but actual frames can have more than two o o on of a single message o nsmission, the ETX
o
Multi
Fram
ontrol Frames information
o o
o Some messages may be too long to fit in a frame Several frames can carry continuati To let the Rx know that the end of frame is not the end of tra character in all the frames but the last one is replaced by an End of Transmission block (ETB) The receiver must acknowledge each frame separately
es
C
o A control frame is used by one device to send commands to or to get from another device A control frame contains control characters but no data It carries information specific to the functioning of the data link layer itself
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g Data Transmission
Control frames serve 3 purposes:
– Establishing Connections
– Maintaining Flow and Error Control durin
– Terminating Connection
D
A
Synchronous Protocols
Reading Sections
Section11.1, 11.
“ Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
Summary
ata Link Protocols
synchronous Protocols
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