TLEN 5330 DATA COMMUNICATIONS 1

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TLEN 5330 DATA COMMUNICATIONS 1

• Lecture Session 10
• (Stallings Chapter 7)
• Michael H. Borsuk

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• DATA LINK CONTROL
• FLOW CONTROL CONCEPTS
• Amount of data to be sent is often more than the
  maximum size of the data fields of the frames of
  our communications protocol. Data needs to be
  divided into frames.
• The Physical Layer definition of the OSI Model
  deals with bit timing and detection of bits using
  asynchronous and synchronous methods but not with
  blocks of data.
• The Data Link Layer is concerned with (among
  other things) the definition and reception of
  frames.
• Frames are numbered within a sequence. Three
  bit sequences (8 frames) are common but there can
  be fewer or more for example, long propagation
  times for satellite transmission involving long
  waits for frame acknowledgment messages.
• Receivers buffers (temporary memory) can be
  overloaded if data is received too fast for use
  by receiver or its attached processors.
• Frames can be received too quickly, out of
  sequence, or with uncorrectable errors. The
  transmitter must know about this and be able to
  resend bad frames.

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• FLOW CONTROL SCHEMES
• Stop and Wait Flow Control send frame and wait
  for acknowledgment from receiver before sending
  next frame.
• Implies that not too much time is taken
  waiting for each acknowledgment.
• Sliding Window Flow Control send frames on the
  fly and not worry about receiving acks until
  entire sequence is sent.
• Requires that there can only be a limited
  number of frames in a sequence so that too much
  data isnt sent when there are problems in
  reception and it is required to resend the entire
  sequence.
• Propagation delays suggest schemes where not all
  frames are acknowledged individually (e.g. ack 2
  frames at a time) and/or the receiver
  acknowledges a sliding window of frames requiring
  that the transmitter buffers window until ack is
  received.
• The window size seems to breathe as frames may
  not be sent and acks may not be received at a
  constant rate.
• Full Duplex transmission allows acks to be sent
  along with data going the other way
  simultaneously.
• Flow control is method of ensuring the receiver
  buffer isnt overloaded.

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• DIFFERENT ACKNOWLEDGMENT MESSAGES REQUIRED
• Error control deals with damaged or missing
  frames but idea is the same for both flow and
  error control
• Acknowledge correct receipt of individual frames
  or group of frames
• Send various messages to the originating end when
  the receiver thinks something is wrong (e.g. ask
  to resend one or more individual frames or entire
  sequence if there are uncorrectable errors)
• There is the possibility that the receiver gets
  two frames with the same sequence number before
  all frames in a sequence are received
• What if an ack message is not received by the
  transmitter?
• What if frame is missing within sequence? Does
  this differ from two frames with the same
  sequence number? What are possible actions?

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• MORE DETAILS RE SLIDING WINDOW CONTROL
• Acknowledge a correct receipt (RRx for Receive
  Ready for frame x)
• Send a rejection (REJx which also indicates which
  frame is missing or deemed to have uncorrectable
  errors.)
• The Go-back-N ARQ (for Automatic Repeat Request)
  protocol simply asks the transmitter to "go back"
  in the sequence enough to straighten the
  situation out when frames or acknowledgments are
  damaged or lost
• The Selective-Reject ARQ method requests that
  only specific frames are to be resent.
• Lots of variations different procedures due to
  propagation time, machine specifics, etc. There
  can be different time outs, how many frames are
  acknowledged, other details. Simplest situation
  is to acknowledge all frames, but this adds to
  overhead.

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• 1. Go-back-N ARQ
• As timer (in this example) allows 3 frames to be
  sent ahead of acknowledgments to account for
  propagation delay.
• B acknowledges every other frame if all is ok.
  Might have been set for every frame.
• RR from A to B (with P bit in HDLC 1) means
  here that A wants to know where it should
  restart. B replies that it wants to start over.
• 2. Selective reject ARQ
• B can ask for specific retransmissions.

FLOW CONTROL 3 FRAME TIMER EXAMPLES
Oh, you didnt get 4
OK, now I have 4, sent 6
OK, continue
Here comes 0.
Im confused
What did you get?
Start sequence over
I got 0-2, send 3
N 3
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• High-Level Data Link Control, HDLC
• This is the essence (and essential) complete
  Layer 2 Data Link Layer protocol--devised long
  before the OSI model was conceived of!
• SDLC (a variation of HDLC) was invented by IBM to
  replace the older Bisynchronous (BSC or BiSync)
  Protocol.
• The SDLC protocol became the layer 2 protocol for
  IBM's SNA (System Network Architecture) used for
  big main frames for many years.

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• WHY STUDY HDLC?
• HDCL is real rather than theoretical or a model
• HDLC is also the basis for synchronous PPP (point
  to point protocol) used by many servers to
  connect to modems over phone lines so that
  dial-up users can assess the Internet.
• Link Access Procedure Balanced (LAPB)
  implementation--where balanced means peer to
  peer--and LAPB is the basis for X.25 which is the
  most important (and widely used) standard for
  implementation of packet switched Wide Area
  Networks.
• X.25 also often called, "frame relay", is the
  glue holding a lot of the Internet together
  (along with ATM). So don't dismiss HDLC as being
  archaic. Note that we will study Frame Relay
  later in the course.
• Multiport version of HDLC and is easily
  understood as well.
• FYI, LAPD ("D" not "B") is the protocol behind
  ISDN which is a telephony based service, and LAPM
  is for modem to modem communications. These are
  similar but not the same as HDLC.

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• HDLC SPECIFICS
• HDLC uses the sliding window error control
  method.
• There are three categories of frames in HDLC.
• Unnumbered (Command/Response) frames are used to
  set up and terminate the link
• Information frames transport the data (and of
  course encapsulate the higher layers of protocol
  like TCP/IP on the Internet)
• Supervisory frames perform flow control and error
  recovery functions.

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(No Transcript)
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• HDLC FIELDS
• Flag Field - Every frame begins and ends with a
  "flag", the bit pattern 01111110 (six 1s in a
  row). The same flag can end one frame and begin
  the next.
• Because data can have five bits in a row and be
  confused with the flag, bit stuffing is used.
• The transmitter adds an extra 0 bit after each
  occurrence of five 1s in a row that occurs
  naturally in the data. The receiver removes every
  0 it finds after 5 1s.
• Whenever the receiver sees five 1s in a row it
  removes the next bit if it's a 0 or considers it
  a flag if it's a 1 as long as the next bit is a
  0. This last task is necessary because the
  transmitter uses the pattern 01111111 (seven 1s)
  as an "abort" signal.

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• HDLC FIELDS - continued
• Address Field - In command frames, this is the
  destination station. For response frames, this
  field identifies the station sending the
  response.
• Control field - Contains commands and responses.
• Diagram shows control field specifics. Dont
  worry about all the details but try to understand
  the nature of the functions.

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HDLC Control field
Secondary are multipoint slave stations.
These are 5 bits that allow for 32 additional
commands.
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• HDLC FIELDS continued
• Information Field - Any sequence of bits in
  theory but usually multiples of 8 bits, such as
  but not limited to an ASCII characters, of
  course.
• Frame Check Sequence field - The CRC for the
  frame.

NOMINAL END SESSION 10
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AUGUST 2003 NEW YORK CITY BLACKOUT A SMALL
VERSION OF THIS HAPPENED DURING THE DISCUSSION OF
THE FOLLOWING SLIDE ON 10 FEBRUARY 2005
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• How HDLC works (most general case for
  multipoint)
• The normal sequence of messages in HDLC consists
  of one or more frames containing I-fields ("I"
  for Information) from the source to the
  destination. This is the data actually being
  exchanged.
• The reception of these frames is acknowledged by
  the destination by its sending an ACK frame back.
  ACKs may be for each frame in a sequence or for
  more than one.
• The value of N(R) indicates that the station
  transmitting the N(R) has received correctly all
  I-frames numbered up to N(R) - 1. I-frames and
  S-frames are numbered from 0 to 7 (3 bits).
• The response frames can acknowledge several
  received frames at a time (up to 7 maximum for a
  3 bit sequence). These can appended to I-frames
  coming from the destination to the source
  simultaneously.
• Some HDLC systems require individual frames to be
  resent rather than entire sequences or just a few
  to catch up.

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• HDLC OPERATION - continued
• For control purposes in HDLC, one end is
  designated as the "primary" station and takes the
  responsibility for managing data flow and link
  error-recovery procedures.
• Primary stations send command frames.
• Other stations (in a multi-station situation or
  "the other station" in a point to point
  situation) communicate using response frames.
  This communication uses the Select bit in the
  control field of an I-frame, or a primary can
  allow a secondary to send by sending a Poll bit.
• A scheme is built in where secondaries can
  respond only to specific requests (NRM for Normal
  Response Mode) or ARM (Asynchronous Response
  Mode) where they can initiate a transmission
  without permission or a poll from the primary. In
  NRM, the secondary explicitly indicates the last
  frame to be sent by setting the final bit in the
  control field. In IP (Internet Protocol), the
  concept of connectionless datagrams appears.

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• Notes
• Basically, HDLC is a real system employing the
  concepts of earlier in this chapter and for that
  matter the earlier ones. There are many
  variations of the details of HDLC operation in a
  specific system.
• To ensure bit synchronization, the bit encoding
  for HDLC is usually NRZi. This encoding method
  results in a transition whenever the next bit is
  a zero but the signal remains at the same level
  when the next bit is a one.
• Since Bit stuffing" (to ensure unique flags)
  ensures that no data has more than five 1s in a
  row, there is always at least one transition
  every 5 bit times at a minimum in HDLC.
• That means that the situation of no bit
  transitions occurring for a long period is
  avoided, resulting in not needing a separate
  clock line or Manchester encoding. Cool huh?

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Dont hit the panic button just yet. It is more
important in Data Communications 1 to understand
the concepts of sliding window flow control
rather than the specific variations (or most
details) of HDLC.

• Definitely remember the following
• Acknowledgments and rejections are sent often
  appended to data messages flowing in the opposite
  direction to control the transmission on the
  fly.
• There are a number of frames in a sequence. 3
  bits is frequently used.
• Frames, acknowledgments, and rejection messages
  may get damaged or lost. Each problem requires an
  appropriate response to recover, but specifics
  differ.
• Propagation time is taken into account in various
  ways.
• Various methods are used in different specific
  implementations HDLC.
• HDLC will come up frequently as it is part of
  many existing protocols/systems.

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A word about
IBM Typewriter circa 1930s
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• A personal comment on IBM
• I mention International Business Machines
  frequently in Data Communications 1. A matter of
  fact, I label some innovations as very IBM from
  time to time. Why?
• IBM rose from a typewriter and sorting machine
  company to the premier high technology company in
  one generation. As the major supplier of main
  frame computers to industry and government, IBM
  was the market leader. All the followers were
  very far behind. It was said that, No one ever
  got fired by buying from IBM. Of course, IBM
  invented the modern PC as well, but that business
  never sat well with IBM management. IBM recently
  sold its PC business to a Chinese company. IBM
  never seemed to like the open system concept.
• In its heyday, IBM engineered its equipment to be
  EXTEMELY hardy (at a price). Its sales force was
  unique and the envy of the world. While many made
  fun of the salesmens silted style (and white
  shirt and narrow tie dress), the products lasted
  forever. Other companies spun off from IBM and
  competed but only when IBM let them. Examples are
  Digital Equipment Corporation, Amdahl, EDS, and
  more recently StorageTek here in Colorado. Most
  of these companies are now gone, but they were
  very innovative and often sold better equipment
  at a much less expensive prices.
• While the IBM of today is quite different from
  the IBM Im talking about, it will always be
  remembered for excellently engineered and
  reliable but expensive solutions. IBMs
  Token-Ring LAN almost never failed compared to
  modern Ethernets, but it was complicated.
• I think of Dick Hammings contributions as kind
  of the opposite of IBM. His real time computing
  methods, the Hamming Codes, and Hamming Distance
  schemes are very elegant but decidedly quick and
  dirty, fast but low accuracy. Couldnt be less
  like IBM. In a way, he represented the beginning
  of the end of IBMthe triumph of cleverness over
  ironclad solutions. In modern systems we can have
  both good engineering and cleverness.

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The IBM Thomas J. Watson Research Center
(Yorktown New York)
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POINT TO POINT PROTOCOL Reference
http//www.cisco.com/univercd/cc/td/doc/cisintwk/i
to_doc/ppp.htm

• Nice review of Link Control and introduction to
  Multiplexing
• PPP provides a method for transmitting datagrams
  (packets) over serial point-to-point links.
• PPP contains three main components.
• A method for encapsulating datagrams over serial
  links. PPP uses the High-Level Data Link Control
  (HDLC) protocol as a basis for encapsulating
  datagrams over point-to-point links.
• A Link Control Protocol (LCP) to establish,
  configure, and test the data link connection.
• PPP is designed to allow the simultaneous use of
  multiple network layer protocols (e.g. TCP/IP,
  IPX/SPX, NETBUI, etc.)

Link Control Protocol The PPP Link Control
Protocol (LCP) is the most important protocol in
the PPP suite. It is responsible for configuring,
maintaining and terminating the overall PPP link.
The two devices using PPP employ a set of LCP
frames to conduct LCP operations. (From The
TCP/IP Guide)
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• PPP General Operation
• To establish communications over a point-to-point
  link, the originating PPP first sends LCP frames
  to configure and (optionally) test the data link.
  
• After the link has been established and optional
  facilities have been negotiated as needed by the
  LCP, the originating PPP sends NCP frames to
  choose and configure one or more network layer
  protocols. (PPP works with other than just
  TCP/IP.)
• When each of the chosen network layer protocols
  has been configured, packets from each network
  layer protocol can be sent over the link.
• The link will remain configured for
  communications until explicit LCP or NCP frames
  close the link, or until some external event
  occurs (for example, an inactivity timer expires
  or a user intervenes).

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• PPP Physical Layer Requirements
• PPP is capable of operating across any DTE/DCE
  interface. Examples include EIA/TIA-232-C
  (formerly RS-232-C), EIA/TIA-422 (formerly
  RS-422), EIA/TIA-423 (formerly RS-423), and
  International Telecommunication Union
  Telecommunication Standardization Sector (ITU-T)
  (formerly CCITT) V.35, and of course V.90, V.91
  etc. modems on analog telephone lines.
• The only absolute requirement imposed by PPP is
  the provision of a duplex circuit, either
  dedicated or switched, that can operate in either
  an asynchronous or synchronous bit-serial mode,
  transparent to PPP link layer frames.
• PPP does not impose any restrictions regarding
  transmission rate other than those imposed by the
  particular DTE/DCE interface in use.

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• PPP Link Layer
• PPP USES HDLC
• PPP uses the principles, terminology, and frame
  structure of the International Organization for
  Standardization (ISO) HDLC procedures (ISO
  3309-1979), as modified by ISO 33091984/PDAD1
  "Addendum 1 Start/Stop Transmission."
• ISO 3309-1979 specifies the HDLC frame structure
  for use in synchronous environments.
• ISO 33091984/PDAD1 specifies proposed
  modifications to ISO 3309-1979 to allow its use
  in asynchronous environments.
• The PPP control procedures use the definitions
  and control field encodings standardized in ISO
  4335-1979 and ISO 4335-1979/Addendum 1-1979.

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The PPP Frame
Flag01111110 Address11111111, this is the
broadcast address. PPP does not assign
individual station addresses. Routers deal with
multiple users via Number Address Control (NAT)
and unix port numbers. More on that later in
course. Controlusually 00000011 which means
modem is sending unsequenced frames. Theres no
need for sequencing usually for modem to ISP
connection since it is a single user and stop
start flow control is used on link. (Outlook or
Outlook Express asks you for this when you set
connection parameters.) ProtocolTwo bytes that
identify the protocol encapsulated in the
information field. DataZero or more bytes that
contain the datagram. FCS2 bytes (16 bits). Some
implementations use 4 bytes (32 bits).
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• The Link Control Protocol of PPP
• There are three main stages of link life in
  PPP, and LCP plays a key role in each one
• Link Configuration The process of setting up and
  negotiating the parameters of a link.
• Link Maintenance The process of managing an
  opened link.
• Link Termination The process of closing an
  existing link when it is no longer needed (or
  when the underlying physical layer connection
  closes).
• This and the next two illustrations are from The
  TCP/IP Guide

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THE LIFE STAGES OF PPP
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THE LINK CONTROL PROTOCOL OF PPP TIME SEQUENCE
CHART
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QUIZ 2 IS DUE BEFORE SESSION 13