Local and Wide Area Networks and the Internet

1
Local and Wide Area Networksand the Internet
2
Overview of Networks

• Distributed applications run over networks that
  link computers together.
• In information Technology, A network is a series
  of points or nodes interconnected by
  communication paths.
• Networks can be characterized in terms of either
  the topology or spatial distance.
• A network can also be characterized by the type
  of data transmission technology it uses.

3
Networks by size

• LAN (Local Area Network)
• Unswitched (Does not use routers)
• Usually covers a building
• MAN (Metropolitan Area Network)
• Unswitched
• Covers a City
• WAN (Wide Area Network)
• Switched (Uses routers)
• May cover a county, state, country or the whole
  world

4
Reasons for Networks

• Resource Sharing
• High Reliability
• Save Money
• Improve Corporate Communication

5
Performance Issues for Distributed Networks

• Performance issues are concerned with the speed
  at which messages can be transferred over the
  network. There are two key concerns
• Latency is the delay after a send operation is
  executed and before data starts to arrive at the
  destination node.
• Data Transfer Rate is the speed that data can be
  transferred over the network, usually defined in
  bits-per-second (bps1).
• 1 In abbreviations, b is used for bits and B for
  bytes.

6
Network speed

• The time required to transmit a message is
• transmission timelatencylength/data transfer
  rate
• The total system bandwidth is a way of measuring
  the capacity or throughput of a network. It
  refers to the volume of traffic that can be
  transferred across the network in a period of
  time, usually 1 second. It is frequently
  expressed in millions or billions of bits per
  second, as Mbps or Gbps.

7
Latency on large networks

• Switching delays at routers and the time required
  to find and set up a communication path can cause
  latency delays on large networks such as the
  Internet that are several orders of magnitude
  larger than local area networks.
• Most of us are familiar with the Internet 404
  errors that occur when the time to access
  information is longer than the timeout allowed
  for searching. Some of this is from latency
  delays, although most of it is from busy hosts or
  missing nodes.

8
Scalability

• As a network grows in the number of nodes, it is
  a severe challenge to maintain performance.
  Bottlenecks and complexity often degrade
  throughput.
• Scalability refers to the ability of a network to
  grow in size without substantial loss of
  performance.

9
Reliability

• The original DARPANET, from which the Internet
  evolved, was designed as a military network to
  survive a nuclear attack. A key concern was
  reliability, in this case the ability to continue
  to transfer messages in the event of failures on
  the network. Error tolerant and error free
  communications are key reliability concerns.

10
Security

• Networks are also concerned with protection from
  unauthorized use, loss or compromise of data and
  with external threats to the ability to transfer
  messages or the integrity of messages. These
  topics are the subject of future lectures on
  distributed system security.

11
Mobility

• Personal Digital Assistants, Lap Top Computers
  and cellular phones are examples of mobile
  devices that may need access to a network at
  different locations. Networks must be designed to
  allow this to occur in an efficient, secure and
  productive manner.

12
Quality of Service

• Metrics have been established to measure the
  reliability and bandwidth of networks. These
  often focus on throughput or bandwidth adjusted
  for possible failures and expressed as a minimum
  acceptable quality of service and a desired level.

13
Multicasting

• Most networks are designed for point to point
  transfer of information between two nodes. There
  may also be a requirement for one-to-many
  communication, and some networks are designed to
  do this in an efficient manner.

14
Local Area Network Topologies

• Bus Topology
• Ring Topology
• Star Topology
• Tree Topology

15
Network TopologiesSource Tanenbaum, Computer
Networks, Figure 1.6
16
Network Performance Coulouris et al, Table 3-1
Type Example Range BW Mbps Latency ms
LAN Ethernet 1-2 km 10-1000 1-10
WAN IP routing worldwide 0.01-600 100-500
MAN ATM 2-50 km 1-150 10
Internet Internet worldwide 0.5-600 100-500
WPAN Bluetooth 10-30 m 0.5-2 5-20
WLAN WiFi 150-1500m 2-54 5-20
WMAN WiMax 5-50 km 1.5-20 5-20
WWAN GSM, 3G worldwide 0.01-2 100-500
17
Local Area Network Bandwidth
Network applications increase the demands of
using high-resolution graphics, video and other
rich media data types, pressure is growing at the
desktop, the server, the hub and the switch for
increased bandwidth. There are various categories
of bandwidth-intensive applications, for example
Scientific modeling, publication, and medical
imaging applications
Data warehousing and backup applications
Internet and intranet applications
Mission-critical business applications
18
Gigabit Ethernet Standards
2- IEEE 802.3ab
1- IEEE 802.3z
2- Copper Cabling Specifications
1- Fiber Cabling Specifications
19
Gigabit Ethernet limitations
Due to collisions in CD/MA and signal
attenuation, all versions of Ethernet, including
Gigabit Ethernet as well as Ethernet and Fast
Ethernet including have a limited working
distance, making them more suitable for local
area networks than wide area networks..
20
Gigabit Ethernet Data Transfer

• From Table 3-1 in the text, we see that Ethernet
  has a latency of 5 to 10 ms. Gigabit Ethernet
  has a bandwidth of 1000 Mbps. How long will it
  take to transfer a file of 10,000 bytes? Assume
  no parity, so 1 byte 8 bits. Latency 0.005 to
  0.01 seconds plus data transfer (80,000 /
  1,000,000,000 0.00008 seconds). Since the
  latency is several orders of magnitude slower
  than the transfer time, latency dominates.

21
WiFi Data Transfer

• Compare the Gigabit Ethernet example to WiFi at 2
  Mbps. How long will it take to transfer the same
  file of 10,000 bytes? Latency 0.005 to 0.02
  seconds plus data transfer (80,000 / 2,000,000
  0.04 seconds) for a total transfer time of 0.045
  to 0.06 seconds.
• Note that both examples ignore handshaking,
  connection overhead, error correction and other
  delays that we will discuss later in the course.

22
Classroom Exercise

• How long will it take to transfer 25 pictures
  from a camera to a photo printer using Bluetooth?
  Assume each picture is 1 MB and that an
  acknowledgement of 10 bytes has to go back to the
  camera to acknowledge each picture before the
  next can transfer. Bluetooth has a bandwidth of
  0.5 to 2 Mbps and a latency of 5-20 ms. Dont
  forget B byte and b bit

23
Metropolitan Area Networks

• Distributed Queue Data Bus
• IEEE 802.6 Broadcast medium with dual cables

24
WAN Technologies

• A Wide Area Network (WAN) is a data
  communications network that covers a relatively
  broad geographic area and often uses transmission
  facilities provided by common carriers, such as
  telephone companies.
• A WAN is an interconnection of LANs.
• A WAN functions at the lower three layers of the
  OSI model.

25
Wide Area Networks

• Subnets (usually point-to-point, store-and
  forward, packet switching using routers)
• Transmission Lines (circuits, channels, trunks)
• Switching Elements (packet switching nodes,
  intermediate systems, data switching exchanges)
• Routers are used to connect LANs to WANs

26
Some WAN Backbones Orfali et al, Table 4-3
(abridged)

• T1 (DS1) 1.54 Mbit/s North America
• E1 2.04 Mbit/s CCITT
• E3 34.36 Mbit/s CCITT
• T3 (DS3) 44.73 Mbit/s North America
• OC1 51.84 Mbit/s Sonet fiber
• OC3 155.52 Mbit/s Sonet fiber
• OC96 4.976 Gbit/s Sonet fiber
• OC192 10 Gbit/s Sonet fiber
• OC768 40 Gbit/s Sonet fiber

27
WAN Capacity

• David Willis compares sharing files over WANs to
  herding hippos through a garden hose. (Orfali et
  all, page 59)
• I had a Terabyte of Data in Missouri and a T3
  connection to my backup system in Georgia. It
  takes over 62 hours to send a TB over T3 with a
  perfect connection, 100 efficiency, no other
  traffic and no transmission overhead. A week is
  more likely. Sending tapes by Fed-Ex was faster.
  That is what we did.

28
Wireless Networks

• Mobile
• Stationary
• Cellular (CDPD)

29
Network PrinciplesPackets

• In order to send many messages across a network,
  individual messages can be broken into smaller
  chunks, called packets. Packets usually have a
  maximum size, to allow nodes in the network to
  reserve enough memory buffer space to ensure that
  the message can be received, to allow sharing of
  the network, and to improve reliability and fault
  detection.

30
Network PrinciplesContention

• A basic problem in almost every network is
  resource contention. One of the most basic
  resources is connections between nodes. Unless
  you have a fully connected network (slide 15d),
  you are likely to have a time when two or more
  messages want to use the same connection at the
  same time. A fully connected network is not
  practicalcan you imagine spending your first few
  months at NJIT connecting 12,000 individual wires
  between your computer and every other computer on
  campus?

31
Connection Sharing

• There are two basic approaches to sharing
  connections switching and multiplexing.
• Switching, like the public switched telephone
  network, (PSTN) sets up a circuita reserved path
  through the network for the duration of the
  message.
• Multiplexing allows multiple messages to use the
  same connection. There are two basic formstime
  division multiplexing and frequency division
  multiplexing.

32
Frequency Division Multiplexing

• Frequency Division Multiplexing (FDM) works like
  radio. Each radio station uses a different
  carrier frequency and imposes a signal on that
  frequency. You tune your radio to the frequency
  of the station you want to hear. The PSTN used to
  depend heavily on FDM for analog circuits,
  sending 24 calls over a T1 line at different
  frequencies. Frequencies of light have different
  colors. Todays laser optic circuits can have
  many different colors of laser light traveling on
  the same circuit at the same time with FDM.

33
Time Division Multiplexing

• The alternative to FDM is Time Division
  Multiplexing, or TDM. During the Second World
  War, the French Resistance got coded messages
  over BBC radio by TDM. For example, at 735 pm
  The next song is dedicated to Clara, might mean
  one thing while This song request is from Harry
  might be a different request. But the particular
  resistance unit listens for its message at
  specific times. There are several different forms
  of TDM, including CDSM, Tokens and Frame Relay.

34
CDSM

• In Collision Detection Shared Multiplexing, or
  CDSM, a node wishing to send a message over a
  connection first listens for traffic, and if it
  does not hear any, tries to send its message.
  As it tries it continues to listen so that it can
  detect another node that also tries at the same
  time, creating a collision that garbles the
  message. If it detects a collision, it stops
  sending and waits for a period of time before
  trying again. Each node waits a random amount of
  time so that the same two nodes dont continue to
  collide indefinitely.

35
Tokens

• Token ring networks, (slide 15b) pass a token
  around the network from station to station. The
  token can either have a message attached or not.
  A node that wishes to send a message must wait
  until it receives a token without a message, then
  it attaches the message to the token and passes
  it on. It works something like trying to find an
  unoccupied taxi in New York City during rush hour.

36
Packet Switching

• Frame relay and ATM are among several
  technologies that send messages across a
  connection with very precise timing. Messages
  are broken into packets, sending a packet from
  one message, then another packet from the same or
  a different message. Messages can be reassembled
  from packets at their destination.

37
Addresses

• Nodes on a network need to have an identifier, so
  that messages can be sent to the proper node.
  These identifiers are called addresses. A
  telephone number is an address. So is a Uniform
  Resource Locator (URL). The Internet uses IPV4
  and IPV6 addresses as primary identifiers. IP
  addresses are covered in the Sockets lecture.

38
Packet Delivery

• There are two ways that packets can be delivered
  to their destination
• Datagram packet delivery
• Virtual circuit (or stream) packet delivery
• Both are explained in the socket lecture.

39
Digital TDM

• Several packets can be sent across a connection
  in the same time period by sending a byte from
  one message, followed by a byte from the next.
• In the PSTN, a DS1 line is the digital equivalent
  of an analog T1 line. It sends 24 messages at a
  time, alternating bytes, and converts them back
  into individual messages at the destination by
  reassembly.

40
Network PrinciplesProtocols

• In order to accomplish communications, networks
  need standard rules to define what is to be done
  and how to do it. These rules are formally
  specified in documents called protocols. A
  protocol must be specific enough that two nodes,
  technologies, systems or other parties can
  communicate without any difficulty.

41
Parts of a Protocol

• Protocols have two parts
• A specification of the sequence of the messages
  that must be exchanged.
• A specification of the format of the data in each
  part of the message.
• Protocols are implemented with a pair of software
  modules on the sending and receiving ends.

42
Protocols and Interfaces Source Tanenbaum,
Computer Networks, Figure 1.9
43
The OSI Protocol Model

• In 1992, the International Standards Organization
  (ISO) defined a protocol suite defining a seven
  layer reference model for open systems. By
  specifying agreed upon layer boundaries, it is
  possible to divide network tasks into common
  segments such that collections of cooperating
  behaviors can accomplish the tasks performed by
  each segment. These seven layers are shown in the
  diagram on the next slide.

44
(No Transcript)
45
The Seven OSI Layers

• Application Layer defines the communication needs
  of specific applications such as FTP or HTTP.
• Presentation Layer includes encryption and such
  tasks as placement of fields in a display.
• Session Layer includes reliability and adaptation
  such as failure detection and automatic recovery.
• Transport layer defines connection oriented and
  connectionless protocols at the message level.

46
Open Systems Interconnection Model

• The seven layers
• in the OSI model
• from the
• Abdus Salam Center
• Network tutorial

47
OSI Low Level Layers

• Network Layer transfers packets between nodes
  using a protocol specific to the particular
  network. This may include setting up connections
  between routers
• Data Link Layer manages the transfer of packets
  between nodes connected by a physical link.
• Physical Layer specifies the circuits and
  hardware that carry electrical, light or
  electromagnetic signals between nodes.

48
How OSI Works Source Tanenbaum, Computer
Networks, Figure 1.19
49
Headers and trailers

• Each level is packaged as data to other levels
  with a header attached.

Headers
Trailer
50
Physical Layer

• The physical layer just sends bits that might be
  encoded as different voltage layers for a
  specified instant of time on an electric wire or
  as pulses of light on a fiber optic line. There
  are many possible ways to distinguish a 1 or a 0
  on a communications medium.

51
Data Link Layer

• The Data Link layer groups bits into frames or
  other units and adds additional bits of
  information to group the bits, indicate the
  beginning and end of a character, assign sequence
  numbers for ordering, and provide for error
  detection and correction with parity and
  checksums. Sequence numbers, parity bits and
  checksums are all examples of overhead.

52
Network Layer

• The Network Layer adds information that allows
  the receiver of a message to identify traffic
  that belongs to it and allows intermediate
  devices to route information to the proper
  destination. The most common form uses Internet
  Protocol, which uses IP addresses and ports to
  identify clients and servers. Each message
  contains the addresses and port numbers of both
  the client and the server as overhead.

53
Transport Layer

• The Transport Layer can add information for
  synchronization, breaking messages into chunks,
  acknowledgement of receipt, timeouts and
  retransmission of data not acknowledged. The
  most common transport protocols are TCP and UDP.

54
Session Layer

• The Session Layer is an enhancement of the
  Transport Layer and can add information for
  dialog control, synchronization, error recovery,
  and similar functions. The session and lower
  levels are all concerned with getting a bit
  stream across a connection reliably.

55
Presentation Layer

• The Presentation Layer is the lowest layer that
  is concerned with the meaning of the bits
  transmitted. It identifies collections of bits
  with identifiers so that they can be assigned
  meaning. Data can be collected into fields and
  records and assigned labels.

56
Application Layer

• While the Application Layer was originally
  designed to contain a collection of standardized
  network applications like electronic mail and
  file transfer, it has become a general purpose
  container for applications and protocols that do
  not fit into the lower layers. It lacks a clear
  separation between applications, application
  specific protocols, and general purpose protocols
  such as File Transfer Protocol.

57
Packet Assembly

• The Network layer is responsible for preparing
  packets to move across the network. One
  requirement is to break up messages into packets
  that can be no larger than the Maximum Transfer
  Unit (MTU), including both the header and the
  data field. For example, the MTU for Ethernet is
  1500 bytes. The IP protocol MTU is 64 KB,
  although most systems are set for 8 KB to allow
  for smaller I/O buffers. If IP packets are sent
  over Ethernet, they must be fragmented to the
  Ethernet MTU size.

58
Transmission Control Protocol Layers

• The early specifications of network layers were
  defined before the OSI model and only include
  four layers, as shown on the next slide.
• This was done for the Defense Advanced Research
  Projects Agency (for DARPANET). When anti-war
  sentiment was common on college campuses during
  the Vietnam war, it was renamed the Advanced
  Research Projects Agency (and ARPANET).
• When portions of ARPANET were opened to public
  use, those portions became the Internet.

59
Comparing TCP/IP to OSI
60
Initial TCP/IP Networks and Protocols
61
TCP Finite State Machine
62
Classroom Exercise

• Use the state chart on the previous page. What is
  the minimum time to transfer one packet using TCP
  with a latency of 10 ms? Assume small messages
  and fast transmission like Gigabit Ethernet, so
  that data transfer time is negligible compared to
  connection latency.
• Note that actual timing is affected by other
  network traffic and timeout settings. There are
  multiple timeout settings in TCP.

63
Bibliography

• George Coularis, Jean Dollimore and Tim Kindberg,
  Distributed Systems, Concepts and Design, Addison
  Wesley, Fourth Edition, 2005
• Andrew Tannenbaum and Maarten Steen, Distributed
  Systems, Principles and Paradigms, Prentice Hall,
  2002
• DARPA RFC 793, September 1981, figure 6, page 23
• Orfali, R., Harkey, D., Edwards, J, The Essential
  Client Server Survival Guide, Second Edition,
  Wiley, 1996