Download The Data Link Layer

Chapter 3
The Data Link Layer
3.1 Data Link Layer Design Issues
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Services Provided to the Network Layer
Framing
Error Control
Flow Control
Functions of the Data Link Layer
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Provide service interface to the network layer
Dealing with transmission errors
Regulating data flow
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Slow receivers not swamped by fast senders
Relationship between packets and frames
Fig.3-1 Relationship between packets and frames.
3.1.1 Services Provided to Network Layer
The actual transmission follows the path of Fig. 3-2(b), but it is easier to think in terms
of two data link layer processes communicating using a data link protocol. For this
reason, we will implicitly use the model of Fig. 3-2(a) throughout this chapter.
Fig.3-2 (a) Virtual communication.
(b) Actual communication.
Services Provided to Network Layer
The data link layer can be designed to offer various
services. Three reasonable possibilities that are
commonly provided are
1. Unacknowledged connectionless service.
2. Acknowledged connectionless service.
3. Acknowledged connection-oriented service.
Services Provided to Network Layer
Fig.3-3 Placement of the data link protocol.
3.1.2 Framing
To provide service to the network layer, the data link layer must use the service
provided to it by the physical layer. What the physical layer does is accept
a raw bit stream and attempt to deliver it to the destination. This bit stream
is not guaranteed to be error free. The number of bits received may be less
than, equal to, or more than the number of bits transmitted, and they may
have different values. It is up to the data link layer to detect and, if
necessary, correct errors.
The usual approach is for the data link layer to break the bit stream up into
discrete frames and compute the checksum for each frame. (Checksum
algorithms will be discussed later in this chapter.) When a frame arrives at
the destination, the checksum is recomputed. If the newly-computed
checksum is different from the one contained in the frame, the data link
layer knows that an error has occurred and takes steps to deal with it (e.g.,
discarding the bad frame and possibly also sending back an error report).
Breaking the bit stream up into frames is difficult. In this section we will
look at four methods:
1.
Character count.
2.
Flag bytes with byte stuffing.
3.
Starting and ending flags, with bit stuffing.
4.
Physical layer coding violations.
Character count
Fig.3-4 A character stream. (a) Without errors. (b) With one error.
Flag bytes with byte stuffing
Fig.3-5 (a) A frame delimited by flag bytes.
(b) Four examples of byte sequences before and after stuffing.
Starting and ending flags,
with Bit stuffing
Each frame begins and ends with a special bit pattern, 01111110.
Whenever the sender's data link layer encounters five consecutive 1s
in the data, it automatically stuffs a 0 bit into the outgoing bit stream.
When the receiver sees five consecutive incoming 1 bits, followed by
a 0 bit, it automatically destuffs (i.e., deletes) the 0 bit.
Fig.3-6 (a) The original data.
(b) The data as they appear on the line.
(c) The data as they are stored in receiver’s memory after destuffing.
3.1.3 Error Control
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Feedback
Receiver sents positive or negative
acknowledgements to sender
• Timers
To dealt with frame or the acknowledgement
lost
• Sequence Numbers
To prevent receiver passing the same frame
to the network layer more than once
3.1.4 Flow Control
Prevent a sender to transmit frames faster than the receiver
can accept them.
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feedback-based flow control
rate-based flow control
In the first one, feedback-based flow control, the receiver sends
back information to the sender giving it permission to send
more data or at least telling the sender how the receiver is
doing. In the second one, rate-based flow control, the
protocol has a built-in mechanism that limits the rate at
which senders may transmit data, without using feedback
from the receiver.
3.2 Error Detection and Correction
• Error-Correcting Codes
Include enough redundant information along with each
block of data sent, to enable the receiver to deduce
what the transmitted data must have been.
Used on unreliable channels , such as wireless links.
• Error-Detecting Codes
Include only enough redundancy to allow the receiver to
deduce that an error occurred, but not which error,
and have it request a retransmission.
Used on unreliable channels , such as fiber.
Error Detection and Correction
Normally, a frame consists of m data bits and r
redundant bits. The total length n = m + r referred
to as an n-bit codeword.
The number of bit positions in which two codewords
differ is called the Hamming distance .
To detect d errors, you need a distance d + 1 code .To
correct d errors, you need a distance 2d + 1 code.
Parity code
matrix code
Hamming code
Polynomial code, also known as a CRC (Cyclic
Redundancy Check)
3.2.1 Error-Correcting Codes
Design a code with m message bits and r check bits that will
allow all single errors to be corrected. The total length n=m+r
There are n illegal codewords, so we need n + 1 bit patterns to
point out n errors and no error.
n+1 ≤2r
m + r + 1 ≤2r
For example, if m=7 then r≥ 4
The bits that are powers of 2 (1, 2, 4, 8, 16, etc.) are check bits.
The rest (3, 5, 6, 7, 9, etc.) are filled up with the m data bits.
Each check bit forces the parity of some collection of bits. To
see which check bits the data bit in position k contributes to,
rewrite k as a sum of powers of 2.
11=1+2+8 Bit 11 is checked by bits 1, 2, and 8
Error-Correcting Codes
When a codeword arrives, the receiver initializes a
counter to zero. It then examines each check bit, k (k
= 1, 2, 4, 8, ...), to see if it has the correct parity. If not,
the receiver adds k to the counter. If the counter is
zero after all the check bits have been examined (i.e.,
if they were all correct), the codeword is accepted as
valid. If the counter is nonzero, it contains the number
of the incorrect bit. For example, if check bits 1, 2,
and 8 are in error, the inverted bit is 11, because it is
the only one checked by bits 1, 2, and 8.
An Example of Hamming Code
Fig.3-7 Use of a Hamming code to correct burst errors.
3.2.2 Error-Detecting Codes
CRC are based upon treating bit strings as
representations of polynomials with coefficients of
0 and 1 only.
For example, 110001 has 6 bits and thus
represents a six-term polynomial with coefficients
1, 1, 0, 0, 0, and 1: x5 + x4 + x0.
The sender and receiver must agree upon a
generator polynomial, G(x), degree of r(r bits
checksum).
CRC
The algorithm for computing the checksum is as follows:
1. Let r be the degree of G(x). Append r zero bits to the
low-order end of the frame so it now contains m + r bits
and corresponds to the polynomial xrM(x).
2. Divide the bit string corresponding to G(x) into the bit
string corresponding to xrM(x), using modulo 2 division.
3. Subtract the remainder (which is always r or fewer bits)
from the bit string corresponding to xrM(x) using modulo 2
subtraction. The result is the checksummed frame to be
transmitted. Call its polynomial T(x).
Figure 3-8.
Calculation of the
polynomial code
checksum.
This figure illustrates the calculation for a
frame 1101011011 using the generator
G(x) = x4 + x + 1.
3.3 Elementary Data Link Protocols
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An Unrestricted Simplex Protocol
A Simplex Stop-and-Wait Protocol
A Simplex Protocol for a Noisy Channel
Protocol Definitions
Continued 
Fig.3-9 Some definitions needed in the protocols to
follow. These are located in the file protocol.h.
Protocol
Definitions
(ctd.)
Some definitions
needed in the
protocols to follow.
These are located in
the file protocol.h.
3.3.1
Unrestricted
Simplex
Protocol
3.3.2
Simplex
Stop-andWait
Protocol
3.3.3 A Simplex Protocol
for a Noisy Channel
A positive
acknowledgement
with retransmission
protocol.
Continued 
A Simplex Protocol for a Noisy Channel (ctd.)
A positive acknowledgement with retransmission protocol.
3.4 Sliding Window Protocols
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piggybacking
sending window
receiving window
A One-Bit Sliding Window Protocol
A Protocol Using Go Back N
A Protocol Using Selective Repeat
piggybacking
When a data frame arrives, instead of immediately sending a separate
control frame, the receiver restrains itself and waits until the
network layer passes it the next packet. The acknowledgement is
attached to the outgoing data frame (using the ack field in the frame
header). The technique of temporarily delaying outgoing
acknowledgements so that they can be hooked onto the next
outgoing data frame is known as piggybacking.
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piggybacking immediately
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Waiting for a moment and piggybacking
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Sending a separate acknowledgement frame
Sliding Window
The essence of all sliding window protocols is that at any instant of time,
the sender maintains a set of sequence numbers corresponding to
frames it is permitted to send. These frames are said to fall within
the sending window. Similarly, the receiver also maintains a
receiving window corresponding to the set of frames it is permitted
to accept.
The frames with sequence numbers within the sender's window can be
sent. When an acknowledgement comes in, the sliding window is
advanced by one.
The receiving data link layer's window corresponds to the frames it may
accept. Any frame falling outside the window is discarded. When a
frame whose sequence number is equal to the lower edge of the
window is received, it is passed to the network layer, an
acknowledgement is generated, and the window is rotated by one.
3.4.1 A One-Bit Sliding Window Protocol
A sliding window protocol with a maximum window size of 1 is called a one-bit
sliding window protocol. Such a protocol uses stop-and-wait since the sender
transmits a frame and waits for its acknowledgement before sending the next one.
Fig.3-13 A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received.
3.4.2 A Protocol Using Go Back N
The sender is be able to continuously transmit
frames before blocking. When a frame is in error
or lost, the sender have to retransmit the frame
and the frames transmitted after that frame.
3.4.3 A Protocol Using Selective Repeat
The sender is be able to continuously transmit
frames before blocking. When a frame is in error
or lost, the sender only retransmit the frame in
error.
Example of Sliding Windows
Fig.3-16 Pipelining and error recovery. Effect on an error when
(a) Receiver’s window size is 1.
(b) Receiver’s window size is large.
A Sliding Window Protocol Using Selective Repeat
Fig.3-20 (a) Initial situation with a window size seven.
(b) After seven frames sent and received, but not acknowledged.
(c) Initial situation with a window size of four.
(d) After four frames sent and received, but not acknowledged.
The Relationship between the Size
of Sliding Window and Protocol
If the size of sending window is one,and the size of receiving
window is one, the protocol is stop-and-wait protocol (one bit
sliding window protocol).
If the size of sending window is larger then one,and the size of
receiving window is one, the protocol is go back N.
If the size of sending window is larger then one,and the size of
receiving window is larger then one, the protocol is selective
repeat.
3.6 Example Data Link Protocols
• HDLC – High-Level Data Link Control
• The Data Link Layer in the Internet
3.6.1 High-Level Data Link Control
In this section we will examine a group of closely related protocols
that are a bit old but are still heavily used. They are all derived
from the data link protocol first used in the IBM mainframe world:
SDLC (Synchronous Data Link Control) protocol. After
developing SDLC, IBM submitted it to ANSI and ISO for
acceptance as U.S. and international standards, respectively. ANSI
modified it to become ADCCP (Advanced Data Communication
Control Procedure), and ISO modified it to become HDLC (Highlevel Data Link Control). CCITT then adopted and modified
HDLC for its LAP (Link Access Procedure) as part of the X.25
network interface standard but later modified it again to LAPB, to
make it more compatible with a later version of HDLC. These
protocols are based on the same principles. All are bit oriented, and
all use bit stuffing for data transparency. They differ only in minor.
Frame Format
The Control field is used for sequence numbers, acknowledgements, and
other purposes, as discussed below.
The Data field may contain any information. It may be arbitrarily long,
although the efficiency of the checksum falls off with increasing
frame length due to the greater probability of multiple burst errors.
The Checksum field is a cyclic redundancy code using the technique we
examined in Sec. 3-2.2.
The frame is delimited with another flag sequence (01111110). On idle
point-to-point lines, flag sequences are transmitted continuously.
The minimum frame contains three fields and totals 32 bits,
excluding the flags on either end.
Fig.3-24 Frame format for bit-oriented protocols.
Three Kinds of Frames
Fig.3-25 Control field of
(a) An information frame.
(b) A supervisory frame.
(c) An unnumbered frame.
Three Kinds of Frames
The P/F bit stands for Poll/Final. When used as P, the computer is
inviting the terminal to send data. All the frames sent by the
terminal, except the final one, have the P/F bit set to P. The final
one is set to F.
Type field of Supervisory frames ：
Type 0 is an acknowledgement frame , called RECEIVE READY. It
calls for retransmission of all outstanding frames starting at
Next .
Type 1 is a negative acknowledgement frame called REJECT.
Type 2 tells the sender to stop sending, called RECEIVE NOT
READY.
Type 3 is the SELECTIVE REJECT. It calls for retransmission of
only the frame specified.
High-Level Data Link Control (4) Three
Kinds of Frames
Unnumbered frame type :
DISC allows a machine to announce that it is going down.
SNRM (Set Normal Response Mode) is an unbalanced (i.e.,
asymmetric) mode in which one end of the line is the master
and the other the slave.
SABM (Set Asynchronous Balanced Mode) resets the line and
declares both parties to be equals.
FRMR (FRaMe Reject) indicates that a frame with a correct
checksum but impossible semantics arrived.
UA (Unnumbered Acknowledgement) is provided for acknowledged
of commands.
SAMB,P
A
B
UA
I,0,0
I,1,0
I,2,0
I,3,0
I,0,2
I,1,3
I,4,2
I,5,2
I,6,2
REJ,4
I,4,2
I,5,2
I,6,2
RNR,7
RR,7
I,7,2
I,0,2
DISC,P
UA,F
X
3.6.2 The Data Link Layer in the Internet
The Internet consists of individual machines (hosts and routers) and the
communication infrastructure that connects them. Within a single
building, LANs are widely used for interconnection, but most of the
wide area infrastructure is built up from point-to-point leased lines.
In Chap. 4, we will look at LANs; here we will examine the data
link protocols used on point-to-point lines in the Internet.
In practice, point-to-point communication is primarily used in two
situations. First, thousands of organizations have one or more LANs,
each with some number of hosts (personal computers, user
workstations, servers, and so on) along with a router (or a bridge,
which is functionally similar). Often, the routers are interconnected
by a backbone LAN. Typically, all connections to the outside world
go through one or two routers that have point-to-point leased lines
to distant routers. The second situation in which point-to-point lines
play a major role in the Internet is the millions of individuals who
have home connections to the Internet using modems and dial-up
telephone lines.
The Data Link Layer in the Internet
Fig.3-26 A home personal computer acting as an internet host.
PPP – Point to Point Protocol
PPP handles error detection, supports multiple protocols, allows IP
addresses to be negotiated at connection time, permits
authentication, and has many other features. PPP provides three
features:
A framing method that unambiguously delineates the end of one
frame and the start of the next one.
A link control protocol for bringing lines up, testing them,
negotiating options, and bringing them down again gracefully
when they are no longer needed. This protocol is called LCP
(Link Control Protocol).
A way to negotiate network-layer options in a way that is
independent of the network layer protocol to be used. The
method chosen is to have a different NCP (Network Control
Protocol) for each network layer supported.
PPP – Point to Point Protocol
The PPP frame format was chosen to closely resemble the HDLC
frame format, since there was no reason to reinvent the wheel. The
major difference between PPP and HDLC is that PPP is character
oriented rather than bit oriented. In particular, PPP uses byte stuffing
on dial-up modem lines, so all frames are an integral number of bytes.
It is not possible to send a frame consisting of 30.25 bytes, as it is
with HDLC. The PPP frame format is shown in Fig. 3-27.
Fig.3-27 The PPP full frame format for unnumbered mode operation.
PPP – Point to Point Protocol
All PPP frames begin with the standard HDLC flag byte (01111110), which is byte
stuffed if it occurs within the payload field. Next comes the Address field, which
is always set to the binary value 11111111 to indicate that all stations are to accept
the frame. Using this value avoids the issue of having to assign data link
addresses.
The Address field is followed by the Control field, the default value of which is
00000011. This value indicates an unnumbered frame. In other words, PPP does
not provide reliable transmission using sequence numbers and acknowledgements
as the default. In noisy environments, such as wireless networks, reliable
transmission using numbered mode can be used.
Since the Address and Control fields are always constant in the default configuration,
LCP provides the necessary mechanism for the two parties to negotiate an option
to just omit them altogether and save 2 bytes per frame.
The fourth PPP field is the Protocol field. Its job is to tell what kind of packet is in the
Payload field. Codes are defined for LCP, NCP, IP, IPX, AppleTalk, and other
protocols. The default size of the Protocol field is 2 bytes, but it can be negotiated
down to 1 byte using LCP.
The Payload field is variable length, up to some negotiated maximum. If the length is
not negotiated using LCP during line setup, a default length of 1500 bytes is used.
After the Payload field comes the Checksum field, which is normally 2 bytes, but a 4byte checksum can be negotiated.