Computer Networks

1
Computer Networks
2
Generic Types of Computer Networks

• Depending on the number of nodes and their
  proximity, three types of computer networks can
  be identified
• Local area network (LAN) It connects hundreds of
  computers, and the distance is up to a few
  kilometers.
• Metropolitan area network (MAN) It connects
  thousands of computers in a metropolitan area
  within a distance of hundreds of kilometers.
• Wide area network (WAN) It connects tens of
  thousands of computers distributed throughout a
  country or the world at a typical distance of
  thousands of kilometers.

3
Network Architecture (Layering)

• To reduce their design complexity, most networks
  are organized as a series of layers, i.e.,
  network protocols are designed in terms of
  layered architecture (layering).
• The major advantage of layering is that it
  clearly delineates the responsibilities of
  various protocols, by dividing responsibilities
  hierarchically among layers, with each layer
  offering services needed by the layer above.
• The key to protocol families is that
  communication occurs logically at the same layer
  of the protocol in both sender and receiver, but
  it is implemented via services of the lower level
  (peer-to-peer).

4
Layering Contd.

• The danger in layering is the considerable
  latency added to message delivery. However,
  protocol families are used to define a standard,
  not to force how the standard is implemented.
• The most popular network architectures are OSI
  architecture and TCP/IP (transmission control
  protocol/internet protocol) architecture.
• Application
• Presentation Application
• Session Transport
• Transport Network
• Network DataLink
• Data Link Physical
• Physical
• OSI Layers TCP/IP Layers

5
(No Transcript)
6
OSI reference model

• 7 Application
  Application 7
• 6 Presentation
  Presentation 6
• 5 Session
  Session 5
• 4 Transport
  Transport 4
• 3 Network 3 3 3
  Network 3
• 2 Data Link 2 2 2 Data
  Link 2
• 1 Physical 1 1 1
  Physical 1
• Host A
  Host B
• The common purpose of layers 4-7 is to provide
  interoperability all the system elements can
  exchange data regardless of the vendor of the
  equipment (open system).
• Connectivity is provided in the layers 1-3 of the
  model, which provides a working connection
  between the sender and receiver, i.e., the
  ability to move data anywhere in the network,
  regardless of the transmission technology or
  medium.

7
OSI Application layer

• This layer's responsibility is to interface the
  user application with the rest of the layers in
  the model. The Application layer is also
  responsible for providing an API (Application
  Programming Interface) to the user applications
  so the programmers who write code for the user
  interface don't have to worry about the
  implementation details of the interface. This
  means that the Application layer takes the
  responsibility of the networking details away
  from the user application so the user application
  does not have to know anything about the
  underlying implementation of the network. Some
  examples of user applications are file transfer
  services, printing services, e-mail services,
  network management consoles, client-server
  processes, and so on.

8
OSI Presentation layer.

• This layer's responsibility is to provide
  encoding standards for the network. The
  Presentation layer is also responsible for
  negotiating between the Application layer and the
  rest of the protocol stack. It provides a
  standard encoding streamer for the Application
  layer so that communication between the
  Application layer and the rest of the protocol
  stack is standardized across different operating
  environments. In other words, the Presentation
  layer provides translation and conversion
  functions to successfully transfer data to the
  underlying protocol stack. As an example, if the
  Application layer of a PC sends information in
  ASCII format, the Presentation layer is
  responsible for formatting the information in the
  standard network type. This standard network
  type, which is generic for the underlying
  protocol stack across different operating
  environments, would be transferred without
  further conversion. At the receiving end, it is
  again the Presentation layer's responsibility to
  convert the generic network format to a format
  that the receiving application can understand.
  Data encryption and decryption can also take
  place at the Presentation layer.

9
OSI Session layer.

• This layer's responsibility is to provide a
  communication channel between hosts. It provides
  a definition for managing the individual network
  channels, also called sessions, between two
  hosts. The Session layer is responsible for
  establishing a session between the hosts, as well
  as maintaining and ending the session. Some
  examples of the Session layer protocol are RPC
  (Remote Procedure Call), AppleTalk, and NFS
  (Network File System).

10
OSI Transport layer.

• This layer's responsibility is to control the
  transmission of data on the network. In other
  words, it provides flow control mechanisms to
  ensure data integrity between the nodes. The flow
  control mechanism acknowledges the receipt of
  every segment from the sending host and the
  proper sequencing of the segments. If the sender
  does not receive an acknowledgement from the
  recipient, the flow control mechanism at the
  sender's end is responsible for resending the
  segment. On the whole, the Transport layer's
  responsibility is to segment the data received
  from the Session layer and forward it to the
  Network layer. In addition, the Transport layer
  receives segmented data from the Network layer to
  reassemble the segments to forward them to the
  Session layer. The Transport layer is also
  responsible for establishing a logical connection
  between the destination node and the source node.
  Some examples of the Transport layer are TCP and
  UDP (User Datagram Protocol).

11
OSI Network layer.

• This layer's responsibility is to ensure the
  addressing of the hosts. It also ensures the
  routing of information between hosts across
  networks. In other words, the Network layer
  handles all of the transmission and traffic
  management among hosts. It also provides address
  resolution for the segments forwarded by the Data
  Link layer.

12
OSI Data Link layer.

• This layer's responsibility is to define how data
  is accessed from a physical medium. It provides a
  mechanism to format the information presented
  from a physical medium so the information can be
  passed to the Network layer. The information
  presented from a physical medium can be in the
  form of bits. The Data Link layer collates this
  information and formats it into frames. A frame
  is a unit of information that contains the
  destination address, the source address, an error
  checksum, and the data itself. The Data Link
  layer is also responsible for converting the
  information obtained from the Network layer into
  bits to forward it to the Physical layer. In
  addition, the Data Link layer is responsible for
  ensuring that the messages traversing the network
  reach the appropriate physical devices. This is
  possible because the Data Link layer manages the
  unique identity of the physical device on the
  network. It uses the concept of hardware
  addressing (MAC address) to identify a physical
  device. Some examples of the Data Link layer
  protocol are ARP (Address Resolution Protocol)
  and RARP (Reverse Address Resolution Protocol).

13
OSI Physical layer.

• This layer's responsibility is to manage the
  hardware details of sending and receiving binary
  data over a physical channel. The physical
  channel is typically made up of wires such as
  twisted-pair and fiber optic cables. It can also
  be made up of wireless media such as infrared or
  radio waves. In general, the Physical layer
  provides a specification for interfacing with a
  physical channel based on the electrical and
  mechanical functions of the medium. The
  connectors at the Physical layer have different
  topologies defined for different network designs.
  Topologies are the structures in which you set up
  your network. Some examples are the star
  topology, the ring topology, and the bus
  topology. One example of the Physical layer is
  the Ethernet standard, which is the network
  protocol that defines how different devices on
  the network communicate with each other over the
  Physical layer.

14
TCP/IP reference model

• 5 layers
• Application layer. This layer's responsibility is
  to provide a common interface for any user
  application to communicate with the underlying
  layers. In other words, the Application layer is
  responsible for providing an interface between
  the user application and the network.

• Transport layer. This layer's responsibility is
  to control the flow of data between two
  communicating hosts. The Transport layer is
  responsible for breaking down data into packets
  and sending and receiving them from the Network
  layer.
• Network layer. This layer's responsibility is to
  route packets across the network. It is also
  responsible for some message control and group
  management.
• Link layer. This layer's responsibility is to
  handle the hardware-related details of the
  system. In other words, the Link layer is
  responsible for interfacing the operating system
  to the network interface card within the computer.

15
TCP/IP enabled communica
16
Internetwork communication
17
Homework

• Compare OSI and TCP/IP model

18
LAN Components

• 3 general characteristics
• A diameter of not more than a few kilometers.
• A total data rate of at least several Mbps.
• Completely ownership by a single organization.
• The medium in LAN is usually a twisted pairs of
  copper wires, or coaxial cable, or fiber optics.
  It may also be wireless transmission.
• Hardware in LAN includes network interface cards
  (NICs), servers, communication devices.
• NIC connects a machine to LAN.
• Server provides service to other machines.
• Communication devices include repeaters, bridges,
  routers, and gateways etc..

19
LAN components

• Repeaters copy individual bits between cable
  segments.
• Bridges connect LANs together.
• Routers or gateways connect LANs to WANs or WANs
  to WANs and resolve incompatible addressing.

LAN server
Repeater
Host
Other LAN
Router
Bridge
To WAN router
20
LAN Communication Concepts

• In LAN, network control is distributed among the
  devices on the network, it resides in the NIC
  firmware in each machine.
• LAN communication may be connectionless or
  connection-oriented.
• Connectionless messages (datagrams) are sent with
  the expectation that they will be received
  correctly. There is no acknowledgment of correct
  receipt. A higher layer must ask for
  retransmission if the message is received
  incorrectly.
• Connection-oriented communications include the
  acknowledgment of message as correct before they
  are passed on to the recipient.

21
Comm. Concepts contd.

• LANs have five major communications
  characteristics
• Medium the means by which data is sent.
• Transmission technique In baseband technique,
  the LAN signal is carried directly on the medium
  in broadband system, the LAN signal is modulated
  on to an analog carrier signal, which allows
  several LANs to share the same medium.
• Network topology the layout of the cabling.
• Access control method contention (Ethernet) and
  token passing (token ring) for shared medium.
• Data Rate the raw ability to transfer
  information in megabits per second.

22
Comm. Concepts contd.

• In LANs, the machines can be connected by shared
  medium or switched (point-to-point) medium.
• Shared medium Multiple computers share a single
  interconnection medium (Ethernet).
• Switched medium It allows communication directly
  from source to destination, without intermediate
  nodes to interfere with these signals.

23
WAN

• Wide area networks (WANs) carry message at a
  lower speed between computers that are separated
  by large distance.
• Many WANs and LANs can be combined to produce a
  single internetwork a communication system that
  interconnects large collections of geographically
  dispersed computers.
• The computers interconnected by a WAN are called
  host computers. The communication medium is a set
  of communication circuits linking a set of
  dedicated computers called packet switches or
  packet switching exchanges (PSEs).
• The OSI layer architecture, TCP/IP layer
  architecture, or other layer architectures can be
  used in building WANs.

24
WAN

• In WANs (packet networks), a message is divided
  into packets before transmission and the packets
  are reassembled at the receiving computer
  (transport layer). A packet consists of a header
  and a data field. The header contains a transport
  address composed of the network address of a host
  and a port number.
• The PSEs operate the network by forwarding
  packets from one PSE to another along a route
  from sender to the recipient. PSEs are
  responsible for defining the route (network
  layer).

25
WAN

• Every packet of data is stored temporarily by
  each PSE along its route before it is forwarded
  to another PSE (store-and-forward communication).
  The routing operations introduce a delay at each
  point in the route, and the total transmission
  time for a message depends on the route it
  follows.
• Two types of data transport service can be
  provided connection-oriented -- a virtual
  connection is set up between a sending and a
  receiving process and is used for the
  transmission of a stream of data connectionless
  individual messages (datagrams) are transmitted
  to specified destinations.

26
Intranet
12
15
87
19
10
DATA
87
T2
10
A
SW2
LAN
LAN
SW1
LAN
LAN
SW3
D
27
Internet
A
D
S1
S3
A
S2
F2
LAN
F1
LAN
S1
WAN
F1
F3
F2
F1
LAN
F2
LAN
S3
D
28
Introduction to DLL

• Receives service from physical layer and provides
  service to the network layer.
• Receives service from network layer and provides
  service to physical layer.
• Responsible for carrying data from one hop to the
  next hop.

12
15
87
19
10
DATA
87
T2
10
29
Duties
bridge
15
87
12
19
Duties of DLL
packetinzing
Error control
Access control
Addressing
Flow control
Frame or cell
MAC or VC
Prevent conflict or collision
CRC
DLL is for point to point, or node to node on a
common link LAN and WAN operate in DLL
30
IEEE standards

• 802 project
• LLC (logical Link Control)
• MAC (Media Access Control)
• 802.3CSMA/CD
• 802.4 TOKEN BUS
• 802.5 TOKEN RING
• 802.6 DQDB
• 802.11 WIRELESS LAN

MAC
DLL
LLC
PHYSICAL LAYER
PHYSICAL LAYER
INTERNET
IEEE
31
Design issues

• How to perform the duties stated in the last
  slide?
• DL can be designed to offer various services. 3
  main responsibilities
• Unacknowledged Connectionless service.
• Acknowledged connectionless service.
• Connection oriented service.

32
Connection oriented service

• A connection is established before the data is
  sent. Each frame sent is numbered, and DL
  guarantees that the frame is indeed received.
  Each frame is received only once and in right
  order.
• Transfers have 3 distinct phases.
• Connection establishes by initializing variables
  and counters needed to keep track of frames
  received.
• One or more frames are transmitted
• Connection released by freeing variables, buffers
  and other resources to maintain the connection.

33
Framing

• Packetizing. Hand over the frame to physical
  layer. Or create frames from raw data received
  from physical layer.
• Breaks up bit streams into discrete frames and
  compute checksums for each frame.
• Frame gap in addition to this
• Character count rarely used now
• Starting and ending character with character
  stuffing
• Start, end flags with bit stuffing
• Physical layer coding violation

34

• Problem with character count
• count character may be corrupted.
• Character delimiter
• ASCII character sequence used as frame delimiter
• When delimiter appears in the data consecutive
  delimiter character is used as a escape sequence.
• Suitable for 8-bit character and ASCII code
  transfer.

35

• Start, end flags with bit stuffing
• Start bit pattern 01111110
• If there are consecutive 5 1s on the data, DLL
  automatically stuff a 0 bit after the consecutive
  5 1s in the outgoing bit pattern. Receiver remove
  this 0 bit after the consecutive 5 1s.
• Use different encoding for data and frame
  separator.
• Manchester encoding represent 1 by H-L and 0 by
  L-H. H-H and L-L are not used for data. One can
  use H-H-L-L as frame delimiter.

36
Error control

• Need feedback for sending frames.
• Waiting time for feedback timer.
• Retransmit if frame or the acknow is lost.
• Potential danger to retransmitting.
• Use sequence number to each outgoing frame.
• Whole issue is of managing timers and sequence
  numbers so as to ensure that each frame is
  ultimately passes to the NL at the destination
  exactly once.

37
Flow control

• How does a recv handle the situation when a
  sender sends frames faster than it can receive.
• Sender need feedback from recv to control its
  frame rate.

38
Access control

• Link management

39
Elementary data link protocolsFlow control
protocols
40

• Physical, data link and Network layers are
  independent. Uses message to communi.
• Connection oriented, reliable channel.
• Infinite supply of data.
• No processing delay.
• Simplex mode.
• DL waits for a packet form NL. When and if DL
  recv a pack from NL, it encapsulates it into a
  frame adding some control bits (header), and then
  handed over to physical layer. Transmitting HW
  appends checksum bits and then transmits frame on
  the cable.
• DL in rcvr waits for a frame from physical layer.
  DL may wait in an infinite loop or for an
  interrupt from physical layer. Recv HW recv a
  frame and computes the checksum. If checksum is
  ok frame is recvd undamaged and passed to DL. DL
  checks if the destination matches with its own
  id. If everything is ok, DL drops the frame
  header and passed the packet to NL.

41
Definitions

• const LastBit . determines the pkt size
• doomsday false repeat forever
• MaxSeq . Highest Seq 2n -1
• type bit 0..1
• SequenceNr 0 .. MaxSeq
• packet packed array 0 .. LastBit of
  bit
• FrameKind (data, ack, nak)
• Frame packed record
• kind FrameKind
• seq SequenceNr
• ack SequenceNr
• info packet
• end

42
Definitions contd.

• procedure wait (var event EvType)
• begin wait for an event to happen return its
  type in event end.
• procedure FromNL(var p packet)
• begin Fetch info from NL for transmission end
• procedure ToNL(p packet)
• begin delivers info from inbound frame to NL
  end
• procedure FromPhysL(var r frame)
• begin get an frame from PhysL and copy it to r
  end
• procedure ToPhysL(s frame)
• begin pass the frame s to PhysL for
  transmission end
• procedure StartTimer(k SequencNr)
• begin start the clock and enable TimeOut event
  end

43
Definitions contd.

• procedure StopTimer(kSequenceNr)
• begin Stop the clock and disable TimeOut event
  end
• procedure StartAckTimer
• begin Start aux timer for sending separate acks
  end
• procedure StopAcktimer
• begin Stop aux timer and disable NLIdle event
  end
• procedure EnableNL
• begin allows NL to cause a NLReady event end
• procedure DisableNL
• begin Forbids NL from causing a NLReady event
  end
• procedure inc(var k SequenceNr)
• begin increment k circularly end

44
Unrestricted simplex protocol

• Assumptions Data trans in one direction only.
  Channel error free. No processing delay. Infinite
  buffer space.
• Sender DL protocol
• type EvType (FrameArival)
• procedure sender1
• var s frame
• buffer packet
• begin
• repeat
• FromNL (buffer)
• s.info buffer
• ToPhysL(s)
• until doomsday
• end

45

• Receiver end DL
• procedure receiver1
• Var r frame
• event EvType
• begin
• repeat
• wait(event)
• FromPhysL(r)
• ToNL(r.info)
• until doomsday
• end

46
Simplex wait-and-Stop protocol

• The main problem we have to deal with is how to
  prevent sender from flooding the recevr with data
  faster than it can handle.
• Assumption recev has limited buffer, needs time
  to process frames. Needs mechanism to control the
  rate of frame-flow from sender.
• If the recvr requires a time t to execute
  FromPhysLToNL, the sender must transmit at an
  average rate less than one frame per t time.
  Moreover, recvr HW my not have automatic buffer
  and queuing.
• Recvr sends a dummy frame as the feedback gt Ack.
• Sender wait for Ackn before it sending the next
  frame.
• A half-duplex physical channel is sufficient here.

47

• type Evtype (FrameArrival)
• procedure sender2
• var sframe
• buffer packet
• event EvType
• begin
• repeat
• FromNL(buffer)
• s.info buffer
• ToPhysL(s)
• wait(event)
• until doomsday
• end

procedure receiver2 var s,r frame event
EvType begin repeat wait(event) FromPhysL(r
) ToNL(r.info) ToPhysL(s) until
doomsday end
48
Stop and Wait protocol for Noise channel

• Sending device keeps a copy of the last frame
  transmitted until it recvs an ack for that frame.
• Both data frames and ack frames are numbered
  alternatively 0 and 1. A data 0 frame is ack by
  an ACK 1 frame, indicating that it has recved
  data frame 0 and expecting data frame 1.
• If the recvr detects an error in the recevd
  frame, it simply discard the frame and send no
  ack. If the recvr recives a frame out of order,
  it knows that a frame is lost. It discards the
  out-of order recvd frame.
• Sender has control variable S, that holds the
  number iof the recently sent frame (0 or 1). The
  rcvr has a control variable, R, that holds the
  number of the next frame expected (0 or 1).
• The sender starts a timer when it sends a frame.
  If an ack is not received within timeout period,
  the sender assumes that the frame is lost and
  resends it.
• Rcvr sends only positive ack for frames received
  safely. Ack number always defines the next frame
  expected.

49
Operation Normal
Data 0
S0
R0
Ack 1
S1
Data 1
R1
Ack 0
time
50
Lost frame

• A receiver remains silent and keeps it value of
  R.
• After the time out interval at the sender ends,
  sender sends another copy of the frame.

51
Lost Acknowledgement

• If the sender receives a damaged Ack, it discards
  it.
• When the timeout for Ack is over sender
  retransmits the frame.
• However, receiver discards the duplicate frame
  and sends the Ack again.

52
Delayed Ack

• Ack is received by the sender after the timeout
  for ack 0. The sender already retransmitted a
  copy of the frame 0.
• Since receiver expects the frame 1, it simply
  discards the duplicate frame 0 and sends ack 1.

53
Piggybacking

• Bidirectional transmission.
• Piggybacking combines data frame with
  acknowledgment

54
Sliding window protocols

• To improve efficiency multiple frames
  (outstanding frame) are sent
• Go back N
• Selective repeat
• Frames are numbered sequentially from 0 to 2N-1.
  If N is 3, frames are 0,1,2,3,4,5,6,7,0,1,2,3,4,5
  ,6,7,0,1,2,3,..
• Sliding window concept is used to hold the
  outstanding frame.

Senders sliding window
55
Receivers 1 bit sliding window

• Always 1 bit window.
• Hold outstanding frame the frame to be received
  next.

56
Control variables

• S seq.no of recently sent frame.
• SF seq. no of the first frame in the window.
• SL Last frame in the window.
• R seq no of the frame it expects to receive.

57
Acknowledgement

• If a frame is damaged or is received out of
  order, the recv is silent and will discard all
  subsequent frames.
• The silence of the recv. cause the timer of the
  unacknowledged frame in the sender to expire.
  Then sender go back to send all frames, beginning
  with the unacknowledged one.

58
Go back N normal operation
59
Lost or damaged frame
60
Sender window size

• Window size must be less than 2m.
• Otherwise recv can recv erroneous packet.
• Piggybacking is possible to implement
  bidirectional transmission.
• Q Explain the problem that can result in if the
  window size is larger than or equal to 2m.

61
Selective repeat

• In noisy channel Go back N protocol is
  inefficient. Retransmission is high.
• Selective repeat protocol does no sent N frames
  only the damaged frame is retransmitted.
• Sender and receiver window size must be less than
  or equal to 2m/2.
• Use Nack to report damaged frame.

3 0 1 2 3 0 1
3 0 1 2 3 0 1
Frame acknowledged
Frames waiting to be sent
Frames received acknolgd
Frames that cannot be accepted
RF
RL
SF
S
SL
Sender window
Receiver window
62
Selective repeat operation

• Frame 0 and 1 are received bcoz they are in the
  range at the receiver.
• When frame 3 is received, it is also accepted,
  however, receiver sends nack for 2.
• Sender then sends only frame 2.
• Try yourself what happes for lost and delayed
  acks and nacks??

63
Explain that the size of sender and receiver
window must be at most one-half of 2m.
64
HDLC configuration and Transfer modes
HDLC High level data link control. Half-duplex
and full-duplex. Provides 2 common modes of
transmission
NRM Normal Response mode
One primary many secondary. Secondary can only
responds. Primary can send commands. Used for
both p2p and multipoint.
ABM Asynchronous balanced mode
P2p. Each station can function as a primary and
secondary.
65
HDLC frame format
Flag
Flag

• Three types of frame
• I frames Information frames used to transport
  user data control info relating to data.
• S frames Supervisory frames used to transport
  only control information.
• U frames Unnumbered frames for system
  management.
• Bit oriented. Use bit stuffing.
• Address 1 or several bytes long. Ethernet uses
  more bytes for both sender and receiver
  addresses.
• Control
• 1 or 2 bytes long. Sequence no., ackn., etc. flow
  control.
• Data
• Information. Its length can vary from network to
  network but always fixed within each network.
• Checksum
• Error control
• On idle, flags sequences are transmitted
  continuously
• Minimum frame length three fields (32 bits)flag
  bits.

66
HDLC contd.

• Seq frame sequence no. 3 bit sliding window.
  Next piggybacked ackn. P/F poll/final, P is
  used when computer is inviting the terminal to
  send data. All the frames sent by the terminal
  set the bit to P, except the final one, which is
  set to F.

Information
Supervisiony
Unnumbered
Control fields for three kinds of frames
67

• In some of the protocols the P/F bit is used to
  force the other machine to send a supervisory
  frame immediately rather than waiting for the
  reverse traffic onto which to piggyback the
  window information.
• Type for different kinds of supervisory frames.
  Type 0 is RECEIVE READY, 1 is REJECT. Next field
  indicates frames to be retransmitted, Type 2
  receive not ready, Type 3 - selective reject.
• Unnumbered Frames used for connectionless
  services. Differs greatly on implementation.

68
PPP

• Point to Point Protocol
• Purposes
• router to router traffic
• home user to ISP
• provide three main features
• Framing. Frame format also handles error
  detection
• Link control protocols for bringing lines up,
  testing them, negotiating options, and bringing
  them down when necessary. This protocol is called
  link control protocol (LCP). Supports sychronous
  and asynchronous circuits and bit and byte
  oriented encoding.
• A way to support different network layer
  protocols. Network Control protocol (NCP).
• Moreover, PPP suports multiple protocols, allows
  IP address to be negotiated at connection time,
  permits authentication.