DLC SERVICES

Unit- 2
 DLC SERVICES
❑ Data link control functions include framing and flow and
error control.
Framing
❑ Physical layer : bit synchronization to ensure that the sender
and receiver use the same bit durations and timing
❑ Frames can be of fixed or variable size.
❑ Fixed-size framing: no need for defining the boundaries of
the frames; the size itself can be used as a delimiter eg: atm
wan
❑ Variable-size framing: need to define the end of one frame
and the beginning of the next.
❑ Two approaches : a character-oriented approach and a
bit-oriented approach.
Character-Oriented Framing
(byte-oriented)
❑ Data to be carried are 8-bit characters from a coding
system such as ASCII.
❑ The header has source and destination addresses and
other control information, and trailer, has error detection
redundant bits, are also multiples of 8 bits.
❑ To separate one frame from the next, an 8-bit (1-byte)
flag is added at the beginning and the end of a frame.
❑ The flag, composed of protocol-dependent special
characters, signals the start or end of a frame
 Character-Oriented Framing
❑ When any character used for the flag could also be part
of the information.
❑ To fix this, a byte-stuffing strategy is used
❑ Byte stuffing (or character stuffing): a special byte is added
to the data section of the frame when there is a character
with the same pattern as the flag.
❑ The data section is stuffed with an extra byte. This byte is
usually called the escape character (ESC) and has a
predefined bit pattern.
❑ Receiver removes the esc character, and treats the next
character as data, not as a delimiting flag.
Byte stuffing (Character stuffing)
Bit-Oriented Framing
❑ The data section of a frame is a sequence of bits to be
interpreted by the upper layer as text, graphic, audio,
video, and so on.
❑ In addition to headers (and possible trailers), we need
a delimiter to separate one frame from the other.
❑ Most protocols use a special 8-bit pattern flag,
01111110, as the delimiter to define the beginning and
the end of the frame
 Bit stuffing
❑ If a 0 and 5 consecutive 1 bits are encountered, an extra 0
is added.
❑ The extra bit is added after one 0 followed by five 1s
regardless of the value of the next bit.
Flow Control

❑ Solution: use two buffers; one at the sending data-link

layer and the other at the receiving data-link layer.
❑ The flow control communication can occur by sending
signals from the consumer to the producer.
❑ When the buffer of the receiving data-link layer is full, it
informs the sending data-link layer to stop pushing frames.
Error Control
❑ The underlying technology at physical layer is not fully
reliable, data-link layer implements error control. Two
methods.
❑ CRC is added to the frame header by the sender and
checked by the receiver.
❑ if the frame is corrupted, it is silently discarded; if it is not
corrupted, the packet is delivered to the network layer.
This method is used mostly in wired LANs →Ethernet.
❑ if the frame is corrupted, it is silently discarded; if it is not
corrupted, an acknowledgment is sent (for the purpose
of both flow and error control)to the sender.
Connectionless and Connection-
Oriented
Connectionless protocol
❑ Frames are sent from one node to the next without any
relationship between the frames;
❑ Each frame is independent. →It means that there is no
connection between frames.
❑ The frames are not numbered and there is no sense of
ordering.
❑ Most of the data-link protocols for LANs are connectionless
protocols.
Connection-Oriented Protocol

❑ A logical connection should first be established

between the two nodes (setup phase).
❑ After all frames are transmitted ,the logical
connection is terminated
❑ The frames are numbered and sent in order.
❑ If they are not received in order, the receiver needs to
wait until all frames belonging to the
❑ Same set are received and then deliver them in order
to the network layer.
❑ Rare in wired lans, in some point-to-point protocols,
some wireless lans, and some wans.
DATA-LINK LAYER PROTOCOLS
❑ Simple, Stop-and-Wait, Go-Back-N, and Selective-
Repeat
Simple Protocol
❑ neither has flow nor error control.
Stop-and-Wait Protocol
❑ Uses both flow and error control.
❑ The sender sends one frame at a time and waits for an
acknowledgment before sending the next one.
❑ To detect corrupted frames, add a CRC to the frame.
❑ If its CRC is incorrect at the receiver , the frame is
corrupted and silently discarded.
❑ The silence of the receiver is a signal for the sender that
a frame was either corrupted or lost.
❑ Every time the sender sends a frame, it starts a timer. If
an acknowledgment arrives before the timer expires,
the timer is stopped and the sender sends the next
frame (if it has one to send).
Stop-and-Wait Protocol
❑ If the timer expires, the sender resends the previous
frame, assuming that the frame was either lost or
corrupted.
❑ The sender needs to keep a copy of the frame until its
acknowledgment arrives.
❑ When the corresponding acknowledgment arrives, the
sender discards the copy and sends the next frame if it
is ready.
FSM for the Stop-and-Wait protocol
Stop-and-Wait protocol
Sender States
❑ The sender is initially in the ready state, but it can move
between the ready and blocking state.
❑ Ready State: sender only waiting for a packet from the
network layer.
❑ If a packet comes from the network layer, the sender
creates a frame, saves a copy of the frame, starts the
only timer and sends the frame.
❑ The sender then moves to the blocking state.
 Stop-and-Wait protocol
❑ Blocking State: When the sender is in this state, three
events can occur:
a. If a time-out occurs, the sender resends the saved copy
of the frame and restarts the timer.
b. If a corrupted ACK arrives, it is discarded.
c. If an error-free ACK arrives, the sender stops the timer
and discards the saved copy of the frame →moves to the
ready state.
Receiver
❑ Always in the ready state. Two events may occur:
a. If an error-free frame arrives, the message in the frame
is delivered to the network layer and an ACK is sent.
b. If a corrupted frame arrives, the frame is discarded.
Sequence and Acknowledgment
Numbers

❑ Duplicate packets, corrupted packets, need to be

avoided.
❑ Need to add sequence numbers to the data frames and
acknowledgment numbers to the ack frames.
❑ Sequence numbers are 0, 1, 0, 1, 0, 1, . . . ; the
acknowledgment numbers can also be 1, 0, 1, 0, 1, 0, …
❑ The sequence numbers start with 0, the acknowledgment
numbers start with 1.
❑ An acknowledgment number always defines the
sequence number of the next frame to receive
Flow diagram
Piggybacking

❑ The two protocols are designed for unidirectional

communication,
❑ To make the communication more efficient, the data in
one direction is piggybacked with the acknowledgment
in the other direction.
High-level Data Link Control (HDLC)

❑ HDLC is a bit-oriented protocol for communication over

point-to-point and multipoint links.
❑ It implements the Stop-and-Wait protocol
Configurations and Transfer Modes
❑ Two common transfer modes that can be used in different
configurations:
❑ Normal response mode (NRM) and asynchronous
balanced mode (ABM).
❑ In NRM, the station configuration is unbalanced. We
have one primary station and multiple secondary
stations.
❑ A primary station can send commands; a secondary
station can only respond.
❑ The NRM is used for both point-to-point and multipoint
links,
❑ In ABM, the configuration is balanced. The link is point-
to-point, and each station can function as a primary
and a secondary (acting as peers),
Framing
❑ 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.
❑ Iframes are used to data-link user data and control
information relating to user data (piggybacking).
❑ 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
Normal response mode

Asynchronous balanced mode

HDLC frames
❑ Each frame in HDLC may contain up to six fields
❑ Beginning flag field, an address field, a control field, an
information field, a frame check sequence (FCS) field,
and an ending flag field.
❑ In multiple-frame transmissions, the ending flag of one
frame can serve as the beginning flag of the next
frame.
HDLC frames
❑ Flag field: has synchronization pattern 01111110,
identifies both the beginning and the end of a frame.
❑ Address field: Has the address of the secondary
station.
❑ If a primary station created the frame, it contains a to
address.
❑ If a secondary station creates the frame, it contains a
from address. Length: varying.
❑ Control field: one or two bytes used for flow and error
control.
❑ The control field determines the type of frame and defines
its functionality

❑ Information field: the user’s data or management

information. Its length can vary
❑ FCS field: frame check sequence -HDLC error detection
field. contain either a 2- or 4-byte CRC.
 Control Field for I-Frames

❑ designed to carry user data / flow- and error-control

information (piggybacking).
❑ first bit defines the type. 0, →is an I-frame, next 3 bits → N(S),
sequence number
❑ last 3 bits, called N(R)→acknowledgment number when
piggybacking is used.
❑ single bit between N(S) and N(R) is called the P/F bit.
❑ when it is set (bit = 1) and mean poll or final.
❑ means poll when the frame is sent by a primary station to a
secondary
❑ means final when the frame is sent by a secondary to a
primary
 Control Field for S-Frames
❑ Supervisory frames are used for flow and error control
whenever piggybacking is either impossible or
inappropriate.
❑ do not have information fields.
❑ If the first 2 bits of the control field are 10, this means
the frame is an S-frame.
❑ The last 3 bits, called N(R) → the acknowledgment
number (ACK) or negative acknowledgment number
(NAK),
❑ The 2 bits called code are used to define the type of
S-frame itself.
S-frames
With 2 bits, we can have four types of S-frames
❑ Receive ready (RR): 00, it is an RR S-frame.
❑ frame acknowledges the receipt of a safe and sound
frame or group of frames- the value of the N(R) field
defines the acknowledgment number.
❑ Receive not ready (RNR): 10, RNR S-frame.
❑ is an RR frame with additional function – acknowledges
the receipt of a frame or group of frames,
❑ announces that the receiver is busy and cannot
receive more frames.
❑ It acts as a kind of congestion-control mechanism by
asking the sender to slow down. The value of N(R) is
the acknowledgment number.
S-frames
❑ Reject (REJ): 01, a NAK frame
❑ That can be used in go-back-n ARQ to improve the
efficiency of the process by
❑ Informing the sender, before the sender timer
expires, that the last frame is lost or damaged. The
value of N(R) is the negative acknowledgment
number.
❑ Selective reject (SREJ): 11
❑ This is a NAK frame used in Selective Repeat ARQ.
❑ The value of N(R) is the negative acknowledgment
number.
 Control Field for U-Frames
❑ Unnumbered frames are used to exchange session
management and control information
❑ U-frames contain an information field, but one used for
system management information, not user data.
❑ U-frame codes are divided into two sections: a 2-bit
prefix before the P/ F bit and a 3-bit suffix after the P/F
bit.
❑ These two segments (5 bits) can be used to create up
to 32 different types of U-frames.
Connection and disconnection
POINT-TO-POINT PROTOCOL (PPP)
Services
❑ PPP defines the format of the frame, also defines how
two can negotiate
❑ Designed to accept payloads from several network
layers (not only IP).
❑ Authentication is also provided in the protocol
❑ Provides network address configuration.
❑ Does not provide flow control; a crc field is used to
detect errors.
Framing
❑ Flag: frame starts & ends with a 1-byte flag with 01111110
❑ Address: set to 11111111 (broadcast address).
❑ Control: 00000011 ;Error control is limited to error detection.
❑ Protocol: type of data in data field:
❑ Payload field: user data / other information ; byte-stuffed ;
escape byte is 01111101
❑ The data field :maximum of 1500 bytes ; padding is
needed if the size is less than the maximum
❑ FCS: Frame Check Sequence (FCS) is a 2-byte or 4-byte
standard CRC.
Transition Phases

❑ A PPP connection goes through phases shown as a

FSM,
❑ Starts with the dead state: when there is no active
carrier (at the physical layer) and the line is quiet.
❑ When one of the two nodes starts the communication,
the connection goes into the establish state; options
are negotiated between the two parties.
❑ If the two parties agree that they need authentication,
then the system needs to do authentication.
❑ The link-control protocol packets, are used for this
purpose.
Transition Phases
Transition Phases
❑ Data transfer takes place in the open state;
❑ The connection remains in this state until one of the
endpoints wants to terminate the connection.
❑ In this case, the system goes to the terminate state.
❑ The system remains in this state until the carrier
(physical-layer signal) is dropped, which moves the
system to the dead state again.
 Multiplexing
❑ PPP uses another set of protocols to establish link,
authenticate and carry the network-layer data.
❑ Link Control Protocol (LCP), two Authentication Protocols
(APs), and Network Control Protocols (NCPs).
❑ a PPP packet can carry data from one of these protocols in
its data field,
❑ Data may also come from several different network layers
Link Control Protocol
❑ LCP is responsible for establishing, maintaining,
configuring, and terminating links.
❑ Provides negotiation mechanisms to set options between
the two endpoints - reach an agreement before the link
can be established.
❑ All LCP packets are carried in the payload field of the
PPP frame with the protocol field set to C021 in
hexadecimal
 Link Control Protocol
❑ The code field defines the type of LCP packet. There are 11
types of packets
Link Control Protocol
❑ 3 categories of packets.
❑ 1st category, has first 4 packet types: used for link
configuration during the establish phase.
❑ 2nd category, comprising packet types 5 and 6, is used for
link termination ; last 5 packets are used for link
monitoring and debugging.
❑ The ID field holds a value that matches a request with a
reply. One endpoint inserts a value in this field, which will
be copied into the reply packet.
❑ The length field defines the length of LCP packet.
❑ The information field contains information, such as
options, needed for some LCP packets.
Link Control Protocol
❑ Information field is divided into three fields: option type,
option length, and option data

Authentication Protocols
❑ Authentication means validating the identity of a user who
needs to access a set of resources.
❑ PPP has created two protocols for authentication: Password
Authentication Protocol and Challenge Handshake
Authentication Protocol.
Password Authentication Protocol (PAP)
❑ PAP performs a two-step process:
a. The user who wants to access a system sends an
authentication identification and a password.
b. The system checks the validity of the identification
and password and either accepts or denies
connection.
❑ When a PPP frame is carrying any PAP packets, the
value of the protocol field is 0xC023.
❑ The three PAP packets are authenticate-request,
authenticate-ack, and authenticate-nak.
PAP packets encapsulated in a PPP
frame
Challenge Handshake Authentication
Protocol (CHAP)
❑ CHAP is a three-way handshaking authentication
protocol that provides greater security than PAP.
❑ the password is kept secret; it is never sent online.
a. The system sends the user a challenge packet
containing a challenge value
b. The user applies a predefined function that takes
the challenge value and the user’s own password
and creates a result. The user sends the result in the
response packet to the system.
c. The system does the same. It applies the same
function to the password of the user (known to the
system) and the challenge value to create a result.
CHAP packets encapsulated in a PPP
frame
Challenge Handshake Authentication
Protocol
❑ CHAP packets are encapsulated in the PPP frame with
the protocol value C223
❑ There are four CHAP packets: challenge, response,
success, and failure.
❑ The first packet is used by the system to send the
challenge value.
❑ The second is used by the user to return the result of
the calculation.
❑ The third is used by the system to allow access to the
system.
❑ The fourth is used by the system to deny access to the
system.
Network Control Protocols

❑ PPP is a multiple-network-layer protocol- can carry a

network-layer data packet from protocols defined by
the Internet, OSI, Xerox, DECnet, AppleTalk, Novel, and
so on.
❑ To do this, PPP has defined a specific Network Control
Protocol for each network protocol.
❑ IPCP (Internet Protocol Control Protocol) configures the
link for carrying IP data packets. Xerox CP does the
same for the Xerox protocol data packets,
 Internet Protocol Control Protocol - IPCP
❑ Configures the link used to carry IP packets in the internet.
❑ The value of the protocol field in hexadecimal is 8021.

❑ IPCP defines 7
packets, distinguished
by their code values
Summary
❑ Data link control deals with the design and procedures
for node-to-node communication
❑ Byte-oriented and bit-oriented.
❑ Flow control means creating a balance
❑ Error control :corrupted frames - discarded;
uncorrupted frames are accepted with or without
sending acknowledgments.
❑ DLC protocol can be either connectionless or
connection-oriented
❑ High-level data link control (HDLC)
❑ Point-to-point protocol (PPP)
Test your Understanding
❑ Why flags are needed when we use variable-size
frames?

❑ What is piggybacking and list its benefit?

❑ Compare and contrast HDLC with PPP