Transcript Data Link Control

Chapter 11 Data Link Control
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Framing
Flow and Error Control
Protocols
Noiseless Channels
Noisy Channels
HDLC
Point-to-Point Protocol
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Framing
• Data link layer needs to pack bits into frames, so that each
frame is distinguishable from another
• Separate a message from one source to a destination, or
from other messages to other destinations, by adding a
sender address and a destination address
• Fixed-size framing: ATM (Chapter 18)
• Variable-size framing
– Need a way to define the end of the frame and the
beginning of the next
– Character-oriented approach and bit-oriented approach
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Character-Oriented Protocols
• Frame structure
• Byte stuffing: process of adding 1 extra byte whenever there is a flag
or escape character in the text
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Bit-Oriented Protocols
• Frame structure
• Bit stuffing: process of adding one extra 0 whenever five consecutive
1s follow a 0 in the data
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Flow and Error Control
• Data link control = flow control + error control
• Flow control refers to a set of procedures used to restrict the amount of
data that the sender can send before waiting for acknowledgement
• Error control in the data link layer is based on automatic repeat
request (ARQ), which is the retransmission of data
• ACK, NAK(Negative ACK), Piggybacking (ACKs and NAKs in data
frames)
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Noiseless Channels: Simplest Protocol
• Simplest protocol with no flow or error control
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Simplest Protocol
• Sender-site algorithm
• Receiver-site algorithm
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Stop-and-Wait Protocol
• Simple tokens of ACK and flow control added
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Stop-and-Wait Protocol
• Sender-site algorithm
• Receiver-site algorithm
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Stop-and-Wait Protocol: Example
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Noisy Channels: Stop-and-Wait ARQ
• Stop-and-wait Automatic Repeat Request (ARQ)
• Error correction in Stop-and-Wait ARQ is done by keeping
a copy of the sent frame and retransmitting of the frame
when the timer expires
• In Stop-and-Wait ARQ, we use sequence numbers to
number the frames. The sequence numbers are based on
modulo-2 arithmetic
• Acknowledgment number always announces in modulo-2
arithmetic the sequence number of the next frame
expected.
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Stop-and-Wait ARQ
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Stop-and-Wait ARQ
• Sender-site algorithm
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Stop-and-Wait ARQ
• Receiver-site algorithm
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Stop-and-Wait ARQ: Example
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Go-Back-N ARQ
• Pipelining improves the efficiency of the transmission
• In the Go-Back-N Protocol, the sequence numbers are modulo 2m,
where m is the size of the sequence number field in bits
• The send window is an abstract concept defining an imaginary box of
size 2m − 1 with three variables: Sf, Sn, and Ssize
• The send window can slide one or more slots when a valid
acknowledgment arrives.
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Go-Back-N ARQ
• Receive window for Go-Back-N ARQ
• The receive window is an abstract concept defining an imaginary box
of size 1 with one single variable Rn. The window slides when a
correct frame has arrived; sliding occurs one slot at a time.
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Go-Back-N ARQ
• Sliding windows, Timers, ACK, Resending a frame
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Go-Back-N ARQ: Send Window Size
• In Go-Back-N ARQ, the size of the send window must be less than 2m;
the size of the receiver window is always 1
• Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the
size of the send window is 1
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Go-Back-N ARQ: Sender Algorithm
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Go-Back-N ARQ: Receiver Algorithm
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Go-Back-N ARQ: Example 1
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Go-Back-N ARQ: Example 2
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Selective Repeat ARQ
• Sender window size
• Receive window size
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Selective Repeat ARQ
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Selective Repeat ARQ: Window Size
• The size of the sender and receiver window must be at most one-half
of 2m
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Selective Repeat ARQ: Sender-Site Algorithm
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Selective Repeat ARQ: Receiver-Site Algorithm
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Selective Repeat ARQ: Example
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Piggybacking
• To improve the efficiency of the bidirectional protocols
• Piggybacking in Go-Back-N ARQ
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HDLC
• High-level Data Link Control
• Two common transfer mode: normal response mode (NRM) and
asynchronous balanced mode (ABM)
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HDLC: Frames
• I(information)-frames, S(supervisory)-frames, U(unnumbered frame)frames
• Flag field: 01111110 to identify both the beginning and the end of a
frame and serve as synchronization pattern for receiver
• FCS field: 2- or 4-byte ITU-T CRC for error detection
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HDLC: Frames
• Control Field: 1- or 2-byte segment of the frame used for flow and
error control
• Determine the type of frame and define its functionality
• Control field for I-frame: P/F (poll/final bit for primary/secondary)
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HDLC: Frames
• Control field for S-frame
• Receive ready (RR), Receive not ready (RNR), Reject (REJ) Selective
reject (SREJ)
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HDLC: Frames
• Control field for U-frame
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HDLC: Example 1
• Connection and disconnection
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HDLC: Example 2
• Piggybacking without error
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HDLC: Example 3
• Piggybacking with error
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HDLC: Bit Stuffing and Unstuffing
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Point-to-Point Protocol: PPP
• One of the most common protocols for point-to-point access
• Many Internet users who need to connect their home computer to the
server of an Internet service provider use PPP
• A point-to-point link protocol is required to control and manage the
transfer of data
• PPP defines/provides
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the format of the frame to be exchanged between devices
how two devices negotiate the establishment of the link and the exchange of data
how network layer data are encapsulated in the data link frame
how two devices can authenticate each other
multiple network layer services
connection over multiple links
Network address configuration
• But, several services are missing for simplicity
– no flow control, simple error control (detection and discard), no sophisticate
addressing for multipoint configuration
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PPP Frame
• Flag: 01111110 the same as HDLC, but it treated as a byte because of
PPP is a byte-oriented protocol
• Address: 11111111 (broadcast address)
• Control: No need because PPP has no flow control and limited error
control
• PPP is a byte-oriented protocol using byte stuffing with the escape byte
01111101
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PPP: Transition States
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PPP: Multiplexing
• PPP uses another set of other protocols to establish the link, authenticate
the parties, and carry the network layer data
• Three sets of protocols defined for powerful PPP: LCP, two APs, several
NCPs
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LCP: Encapsulated in a Frame
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LCP: Common Options
• Options are inserted in the information field of the
configuration packets
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Authentication
• Authentication means validating the identity of a user who
needs to access
• PPP is designed for use over dial-up links
 User authentication is necessary
• PPP has two protocols for authentication
– Password Authentication Protocol (PAP)
– Challenge Handshake Authentication Protocol
(CHAP)
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Password Authentication Protocol (PAP)
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Challenge Handshake Authentication
Protocol (CHAP)
• Three-way hand-shaking authentication protocol with greater security
than PAP
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Network Control Protocol: NCP
• PPP is a multiple-network layer protocol.
• It can carry a network data packet from protocols defined by the
Internet, OSI, Xerox, DECnet, AppleTalk, Novel
• IPCP (IP Control Protocol)
– Configures the link used to carry IP packets in the Internet
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IPCP Packet
IP Datagram in a PPP frame
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Multiple PPP
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Example (1)
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Example (2)
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