NETWORKING CONCEPTS. HDLC The most important data link control protocol is HDLC The HDLC protocol...
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MODULE I NETWORKING CONCEPTS
NETWORKING CONCEPTS. HDLC The most important data link control protocol is HDLC The HDLC protocol embeds information in a data frame that allows devices
HDLC The most important data link control protocol is HDLC The
HDLC protocol embeds information in a data frame that allows
devices to control data flow and correct errors. frame Basic
charscteristics HDLC defines 3 types of stations 2 link
configurations 3 data transfer modes of operation
Slide 3
Station Types Primary station: Has the responsibility of
controlling the operation of data flow the link. Handles error
recovery Frames issued by the primary station are called
commands
Slide 4
HDLC Secondary station: Operates under the control of the
primary station. Frames issued by a secondary are called responses.
The primary maintains a separate logical link with each secondary
station on the line. Combined station: Combines the features of
primary and secondary. A combined station may issue both commands
and responses.
Slide 5
HDLC HDLC Link Configurations Unbalanced configuration:
Consists of one primary and one or more secondary stations supports
both full-duplex and half-duplex transmission. Balanced
configuration: Consists of two combined stations supports both
full-duplex and half-duplex transmission.
Slide 6
HDLC HDLC Data Transfer Modes Normal response mode (NRM): Used
with an unbalanced configuration. The primary may initiate data
transfer to a secondary, but a secondary may only transmit data in
response to a command from the primary. Asynchronous balanced mode
(ABM): Used with a balanced configuration. Either combined station
may initiate transmission without receiving permission from the
other combined station.
Slide 7
HDLC HDLC Data Transfer Modes Asynchronous response mode (ARM):
Used with an unbalanced configuration. The secondary may initiate
transmission without explicit permission of the primary. The
primary still retains responsibility for the line, including
initialization, error recovery, and disconnection.
Slide 8
Slide 9
Slide 10
HDLC HDLC Frame Structure All transmissions are in the form of
frames HDLC defines three types of frames: information frames
(I-frames), supervisory frames (S-frames), and unnumbered frames
(U-frames). Each type of frame serves as an envelope for the
transmission of a different type of message.
Slide 11
I-frames are used to transport user data and control
information relating to user data (piggybacking). Additionally,
flow and error control data, using the ARQ mechanism, are
piggybacked on an information frame S-frames are used only to
transport control information. U-frames are reserved for system
management. Information carried by U-frames is intended for
managing the link itself. The first one or two bits of the control
field serves to identify the frame type. The remaining bit
positions are organized into subfields
Slide 12
Slide 13
HDLC HDLC Frame Structure Diagram The flag, address, and
control fields that precede the information field are known as a
header. The FCS and flag fields following the data field are
referred to as a trailer.
Slide 14
HDLC Flag Fields Flag fields delimit the frame at both ends
with the unique pattern 01111110. Bit stuffing used to avoid
confusion with data containing 01111110
Slide 15
HDLC Flag Fields Bit Stuffing 0 inserted after every sequence
of five 1s If receiver detects five 1s it checks next bit If 0, it
is deleted If 1 and seventh bit is 0, accept as flag
Slide 16
HDLC With the use of bit stuffing, arbitrary bit patterns can
be inserted into the data field of the frame. This property is
known as data transparency. Address Field The second field of an
HDLC frame contains the address of the secondary station. If a
primary station created the frame, it contains a to address. If a
secondary creates the frame, it contains a from address. An address
field can be 1 byte or several bytes long, depending on the needs
of the network
Slide 17
HDLC Control Field HDLC defines three types of frames, each
with a different control field format.
Slide 18
HDLC
Slide 19
Control Field for I-Frames The subfields in the control field
are used to define these functions. The first bit defines the type.
If the first bit of the control field is 0, this means the frame is
an I-frame. The next 3 bits, called N(S), define the sequence
number of the frame.
Slide 20
The last 3 bits, called N(R), correspond to the acknowledgment
number when piggybacking is used. The single bit between N(S) and
N(R) is called the P/F bit. The P/F field is a single bit with a
dual purpose. It has meaning only when it is set (bit = 1) and can
mean poll or final. It means poll when the frame is sent by a
primary station to a secondary (when the address field contains the
address of the receiver). It means final when the frame is sent by
a secondary to a primary (when the address field contains the
address of the sender).
Slide 21
Control Fieldfor S-Frame Supervisory frames are used for flow
and error control whenever piggybacking is impossible S-frames do
not have information fields. If the first 2 bits of the control
field is 10, this means the frame is an S-frame. The last 3 bits,
called N(R), corresponds to the acknowledgment number (ACK) or
negative acknowledgment number (NAK) depending on the type of
S-frame. The 2 bits called code is used to define the type of
S-frame itself With 2 bits, we can have four types of S-frames
Slide 22
Receive ready (RR). If the value of the code subfield is 00, it
is an RR S-frame Receive not ready (RNR). If the value of the code
subfield is 10, it is an RNR S-frame Reject (REJ). If the value of
the code subfield is 01, it is a REJ S-frame. Selective reject
(SREJ). If the value of the code subfield is 11, it is an SREJ
S-frame
Slide 23
Control Field for U-Frames Unnumbered frames are used to
exchange session management and control infonnation between
connected devices. Unlike S-frames, U-frames contain an information
field, but one used for system management information, not user
data. As with S-frames, however, much of the information carried by
U-frames is contained in codes included in the control field.
Slide 24
HDLC
Slide 25
Control Fieldfor S-Frames Supervisory frames are used for flow
and error control whenever piggybacking is impossible. S-frames do
not have information fields. If the first 2 bits of the control
field is 10, this means the frame is an S-frame. The last 3 bits,
called N(R), corresponds to the acknowledgment number (ACK) or
negative acknowledgment number (NAK) depending on the type of
S-frame. The 2 bits called code is used to define the type of
S-frame itself
Slide 26
HDLC Information Field The information field is present only in
I-frames and some U-frames. The field can contain any sequence of
bits The length of the information field is variable
Slide 27
HDLC Frame Check Sequence Field The frame check sequence (FCS)
is an error detecting code calculated from the remaining bits of
the frame, exclusive of flags. The normal code is the 16-bit An
optional 32-bit FCS
Slide 28
HDLC Operation
Slide 29
HDLC Operation HDLC operation consists of the exchange of I-
frames, S-frames, and U-frames between two stations Involves 3
phases First, one side or another initializes the data link so that
frames may be exchanged After initialization, the two sides
exchange user data and the control information to exercise flow and
error control. Finally, one of the two sides signals the
termination of the operation.
Slide 30
HDLC Initialization initialization request by issuing one of
the six set mode commands. This command serves three purposes: 1.
It signals the other side that initialization is requested. 2. It
specifies which of the three modes (NRM,ABM,ARM) is requested. 3.
It specifies whether 3- or 7-bit sequence numbers are to be used.
If the other side accepts this request, then the HDLC module on
that end transmits an unnumbered acknowledged (UA) frame back to
the initiating side.
Slide 31
HDLC If the request is rejected, then a disconnected mode (DM)
frame is sent. Data Transfer When the initialization has been
requested and accepted, then a logical connection is established
Both sides may begin to send user data in Iframes, starting with
sequence number 0. The N(S) and N(R) fields of the I-frame are
sequence numbers that support flow control and error control An
HDLC module sending a sequence of I-frames will number them
sequentially and place the sequence number in N(S)
Slide 32
HDLC N(R) is the acknowledgment for I-frames received; it
enables the HDLC module to indicate which number I-frame it expects
to receive next. The receive ready (RR) frame acknowledges the last
I-frame received by indicating the next I-frame expected. Receive
not ready (RNR) acknowledges an I-frame, as with RR, but also asks
the peer entity to suspend transmission of I-frames
Slide 33
HDLC When the entity that issued RNR is again ready, it sends
an RR. REJ indicates that the last I-frame received has been
rejected and that retransmission of all I- frames beginning with
number N(R) is required. Selective reject (SREJ) is used to request
retransmission of just a single frame. Disconnect HDLC issues a
disconnect by sending a disconnect (DISC) frame. The remote entity
must accept the disconnect by replying with a UA
Slide 34
Slide 35
LINK ACCESS PROTOCOL-BALANCED (LAPB) X.25 is a network
interface defined for accessing packet-switched public networks. It
is commonly used for interconnecting LANs. The X.25 defines the
lowest three layers of the OSI Reference Model Physical layer, Data
link Layer and Network Layer. LAPB is a bit-oriented synchronous
protocol that provides complete data transparency in a full-duplex
point-to-point operation. It is a data link layer protocol used to
manage communication between data terminal equipment (DTE) and the
data circuit-terminating equipment (DCE) devices.
Slide 36
Data terminal equipment devices are end systems that
communicate across the X.25 network. They are usually terminals,
personal computers, or network hosts, and are located on the
premises of individual subscribers. DCE devices are communications
devices, such as modems and packet switches
Slide 37
Slide 38
LAPB has been derived from HDLC and shares the same frame
format, frame types, and field functions as HDLC. It differs from
HDLC in the representation of address field. The address field can
contain only one of two fixed (DTE or DCE) addresses. LAPB is
restricted to the ABM transfer mode and is appropriate only for
combined stations
Slide 39
LINKED ACCESS PROTOCOL - D CHANNEL (LAPD) ISDN emerged as an
alternative to traditional dialup networking. It is a set of
communication standards for simultaneous digital transmission of
voice, video, data, and other network services over the traditional
circuits of the public switched telephone networks. It provides a
single, common interface with which to access digital
communications services that are required by varying devices, while
remaining transparent to the user. ISDN standards are constructed
using the Open System Interconnection seven-layer reference
model.
Slide 40
Linked Access Protocol (D Channel) is a Layer 2 (data link)
protocol. D channel is the data or signalling channel which is used
for communications (or "signalling") between switching equipment in
the ISDN network and the ISDN equipment at your site. The LAPD
handle the handshaking (commands and responses), signalling, and
control for all of the voice and data calls that are setup through
the ISDN D channel.
Slide 41
LAPD works in the Asynchronous Balanced Mode (ABM). This mode
is totally balanced (i.e., no master/slave relationship). Each
station may initialize, supervise, recover from errors, and send
frames at any time. The objective of LAPD is to provide a secure,
error-free connection between two end-points so as to reliably
transport Layer 3 messages. The control field of LAPD frame is
identical to HDLC, but the address field differs.
Slide 42
Slide 43
The first Address-field byte contains the service access point
identifier (SAPI), which identifies the portal at which LAPD
services are provided to Layer 3. The C/R bit indicates whether the
frame contains a command or a response. The Terminal Endpoint
Identifier (TEI) field identifies either a single terminal or
multiple terminals compatible with the ISDN network. Example:
Telephones, personal computers.