Chapter 6 Transmission of Digital Data Interfaces and Modems - CBU

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7 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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Chapter11 Homework



15, 16, 20

Data Link Layer





High
-
Level Data Link Control
(HDLC)


HDLC is based on IBM's
Synchronous

Data Link Control
(SDLC) protocol, which is widely used
by IBM's large customer base in
mainframe

computer environments.

High
-
Level Data Link Control
(HDLC)



A group of
protocols

for transmitting
data

between
network

nodes.


Data is organized into a
frame
and sent across a network to its
destination



The successful arrival of data is verified.



The HDLC protocol also manages the flow at which data is sent.

Types of Stations Defined By
(HDLC)




Primary

stations






Secondary stations





Combined stations

Primary Stations


Control the operation of other stations or links.


Primary

polls the secondaries in a predetermined order,
and secondaries can then transmit if they have outgoing
data.


Primary
sets up

and
tears down

links and manages the
link while it is operational.


Frames issued by primary stations are called
commands
.



Primary

generates commands and interprets responses



Responsible for initiating error recovery procedures, and
handling the flow of transmitting data to and from the
primary

Secondary Stations



Controlled by a primary



Frames issued by secondary stations are called
responses
.



It responds to requests from a primary station.



Needs permission

from primary to send information.



May initiate its own transmission


when it runs
out of buffer

space, at which point it
sends RNR
(receiver not ready)

to the primary
station.


When the buffer space is cleared, it sends RR
(receiver ready).


Combined Stations




Has the features of primary



and secondary stations.




It can issue both command and



response frames.

HDLC

Link Configuration


Refers to relations between primary
and secondary stations

HDLC

Link Configuration


Unbalanced configuration
(master/slave)


Consists of one primary and one or more secondary


Supports full
-
duplex and half
-
duplex transmission.


Balanced configuration


Consists of two or more combined stations


Supports full
-
duplex and half
-
duplex transmission.


Symmetrical configuration


Each physical station Consists of two logical stations (one
primary and one secondary)


Separate line links the primary aspect of the physical station to
secondary aspect of the physical station and vice versa.


One primary and one or more secondary


Supports full
-
duplex and half
-
duplex transmission.



Two or more combined stations


Supports full
-
duplex and half
-
duplex
transmission.


Each physical station Consists of two logical stations (one primary
and one secondary)


Separate line links the primary aspect of the physical station to
secondary aspect of the physical station and vice versa.

HDLC Data Transfer Modes


Normal response mode (NRM)


Used with unbalanced configuration


Primary may initiate data transfer to
secondary


Secondary may only transmit data in
response to a primary command.



The link may be point
-
to
-
point or
multipoint. In the latter case only one
primary station is allowed.


Asynchronous response mode (ARM)


Used with unbalanced configuration


Primary has the line responsibility, initialization, error control,
and logical disconnection.



It allows a secondary station to initiate a transmission without
receiving permission from the primary station
when the line is
idle.



This mode is normally used with point
-
to
-
point configurations
and full duplex links.

HDLC Data Transfer Modes

HDLC Data Transfer Modes


Asynchronous balanced mode (ABM)


Used with balanced configuration



Either combined station may initiate transmission without
permission from the other combined station.



ABM, is used mainly on full duplex point
-
to
-
point links for
computer to computer communications and for connections
between a computer and a packed switched data network.



Each station has an equal status and performs the role of both
primary and secondary functions.


This mode is used in the protocol set known as X.25.


HDLC


HDLC uses synchronous transmission




HDLC frame format is discussed next.



Flag Fields


Flag:


Unique pattern of 01111110 used
to indicate the beginning and end
of a fame.

Address Fields


Address:


Identifies the address of the secondary
station that transmitted or is to receive
data.


Not necessary for PPP links but, is
included anyway.

Control Field


HDLC defines
three types

of frames,
each with different control field
format.


Information frames (I
-
frames)



Supervisory frames (S
-
frames)


Unnumbered frames (U
-
frames)


Control Field


Information frames (I
-
frames)


Carry the data to be transmitted for the
user


Error and flow control data, using ARQ
(automatic repeat request) are
piggybacked on an I
-
frame.

Control Field


Supervisory frames (S
-
frames)



Carry the control information (flow and
error control info of data link layer)



Provide ARQ mechanism when
piggybacking is not used.

Control Field


Unnumbered frames (U
-
frames)



Used for link management control
information.



A U frame can be used to initialize
secondaries.