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					                    Introduction to This Class
•   Instructor: Stephan Bohacek
•   bohacek@udel.edu, 302-831-4274
•   http://www.eecis.udel.edu/~bohacek
•   Textbook: Kurose and Ross: Computer Networks (4th or 5th edition?)
•   Web page has
     –   syllabus
     –   class notes
     –   videos lectures of some topics (in progress)
     –   homework assignments
     –   project assignments
     –   announcements
• Issues
     – Programming languages: C++ or java?
     – Computers:
          • >=2GB ram?
          • Windows or Linux?
           Introduction to Data Networking
    Kurose and Ross: Computer Networking: A Top Down Approach

• Chapter 1: Overview and              • Chapter 4: Network layer
  general principles
   – Protocol stack                       – IP and IPv6
   – Sharing                              – NAT
       • Statistic multiplexing
       • Packet switching                 – Routing
       • Circuit switching                     • Intra-network routing
   – Performance of packet                     • Inter-network routing
     switching networks
• Chapter 2: Application               • Chapter 5: Datalink and
  layer                                  MAC layer
   – TCP and UDP,
     multiplexing and ports               –   Multiple access
   – Applications                         –   ARP
       • http, ftp, email, DNS, P2P,      –   Ethernet
         DHT
• Chapter 3: Transport layer              –   Switches and hubs
   – Tools for reliable transport         –   Link layer routing
   – TCP
                          The Internet
• What is the longest time that you have gone without using “the
  Internet?”
• What is the Internet?
• How do you use the Internet?
• Do you only watch TV over the Internet, or do you have cable?
• Skype?
• Facebook?
• IM?
• Twitter?
• Smart phone?
• Do you only have a data plan on your phone?
                       Networking Basics
•   Core components of the Internet – the protocol stack
•   Multiplexing, circuit switching, and packet switching
•   Loss and delays
•   The structure of the Internet
•   This lecture covers much of chapter 1 in the textbook.
                       Networking Basics
•   Core components of the Internet – the protocol stack
•   Multiplexing, circuit switching, and packet switching
•   Loss and delays
•   The structure of the Internet
•   This lecture covers much of chapter 1 in the textbook.
                            Core components
•   End-hosts
•   Applications
     –   ?




•   Packets
     –   TCP
     –   UDP
•   Routers and gateways
    and groups of routers
    (ISPs)
•   Links
     –   ?




•   Protocols
                            Core components
•   End-hosts
•   Applications
     –   Web
     –   Email
     –   File transfer
     –   File sharing
•   Packets
     –   TCP
     –   UDP
•   Routers and gateways
    and groups of routers
    (ISPs)
•   Links
     –   Fiber
     –   Coaxial
     –   Twisted pair
     –   Wireless
•   Protocols
          Application Layer – where the applications live
•   End-hosts             •   Email:
•   Applications               –   Rules/protocols for how an end-host gets mail from the mail server
     –   Web              •   Web:
     –   Email                 –   Rules/protocols for how the end-hosts gets a web page from the web servers
     –   File transfer    •   Question:
     –   File sharing          –   How is a networking application different from a non-networking application
•   Packets                        (e.g., MS Word). That is, why, when talking about networking application, do
     –   TCP                       we focus on protocols, but do not focus on protocols when discussing non-
                                   networked applications such as MS-Word?
     –   UDP
                               –   Answer: The networking applications must communicate, and rules are required
•   Routers and gateways           to define the communication.
    and groups of routers •   Roles that end-hosts play:
    (ISPs)
                               –   Client, server, and peer
•   Links                      –   The client asks the server for a service.
     –   Fiber                         •   E.g., The client asks the server to send a mail for it.
     –   Coaxial                       •   The client asks the server for a web page
     –   Twisted pair                  •   The client asks the server to translate a web address to an IP address.
     –   Wireless              –   Peer: A host can act as both a client and a server. But usually in one transaction,
                                   the host takes only one role
•   Protocols
                                  Layers 2-4
•   End-hosts
•   Applications                       Which are the end-host?
     –   Web
     –   Email
     –   File transfer
     –   File sharing
•   Packets
     –   TCP
                             client
    Routers and gateways 
     –   UDP
•
    and groups of routers
                                                                server

•
    (ISPs)
    Links                 
     –
     –
         Fiber
         Coaxial                            Routers
     –
     –
         Twisted pair
         Wireless
                          
                          
•   Protocols
                                          Layers 2-4
•   End-hosts
                              Goal: move messages from server to the client     Why is this a good approach?
                                                                                1.Small problems are easier to
•   Applications              Approach: break the problem into little pieces.     understand/solve.

     –   Web                  Each piece is a layer in the “protocol stack”     2.Different solutions can be
                                                                                  mixed and matched
     –   Email
     –   File transfer
     –   File sharing
•   Packets
     –   TCP
                                    client
    Routers and gateways
     –   UDP
•
    and groups of routers
                                                                                      server

•
    (ISPs)
    Links                 
     –
     –
         Fiber
         Coaxial          
     –
     –
         Twisted pair
         Wireless
                          
                          
•   Protocols
                                               Layers 2-4
•   End-hosts
•   Applications
     –   Web
     –   Email                        client
     –   File transfer                                                                               server
     –   File sharing
•   Packets
                          
                              Top down approach of breaking problems into small pieces
     –   TCP                  4. Transport layer
                                  • Reliability: The server must make sure that the client gets the data
    Routers and gateways
     –   UDP
                                    Congestion control (or lack there of)
•
                          
                                  • Congestion Control: The server should send data as fast as possible, but not too fast
    and groups of routers         • TCP provides these features (services), while UDP does not

•
    (ISPs)
    Links                    3. Network layer (could be called the routing layer, but it isn’t)
                                  • The packets must find their way through the network.

                          
                                  • Each packet has the IP address of the destination
     –   Fiber
                                  • By examining the IP address, routers decide where to send the packet next
     –   Coaxial

                          
                              2. Link Layer or MAC layer (link layer and MAC layer)
     –
                          
         Twisted pair             • Links connect the routers/gateways and end-hosts
     –   Wireless                 • This layer provides logical and control for communicating across links.
•   Protocols                     • Services that this layer might provide include
                                     • congestion control, media access, error detection/correction
                                                Layers 2-4
•   End-hosts                 Top down approach of breaking problems into small pieces
                              …..
•   Applications
                              2.    Link Layer or MAC layer (link layer and MAC layer)
     –   Web                      •     Links connect the routers/gateways and end-hosts
     –   Email                    •     This layer provides logical and control for communicating across links.
     –   File transfer            •     Services that this layer might provide include congestion control, media access,
     –   File sharing                   error detection/correction
•   Packets
     –   TCP
                          
    Routers and gateways
     –   UDP
•
    and groups of routers
                          
•
    (ISPs)
    Links                 
     –
     –
         Fiber
         Coaxial          
     –
     –
         Twisted pair
         Wireless
                          
                          
                               •   Media access. The “air” is a shared medium. If two nodes transmit at the same time,
                                   there will be a collision. Thus, a scheme must be developed to determine which
•   Protocols                      node transmits when.
                               •   Error detection/correction. If interference does occur, then errors might occur. If an
                                   error is detected, then
                                   1. the error could be corrected with forward error correction, or
                                   2. the receiving link could request a retransmission
                                                   Layers 2-4
•   End-hosts
•   Applications
     –   Web
     –   Email                         client
     –   File transfer                                                                                              server
     –   File sharing
•   Packets
                          
                              Top down approach of breaking problems into small pieces
     –   TCP                  4.     Transport layer
                                   1.     Reliability: The server must make sure that the client gets the data

    Routers and gateways
     –   UDP                         Congestion control (or lack there of)
•
                          
                                   2.     Congestion Control: The server should send data as fast as possible, but not too fast
                                   3.     TCP provides these features (services), while UDP does not
    and groups of routers     3.     Network layer (could be called the routing layer, but it isn’t)

•
    (ISPs)
    Links                         1.
                                   2.
                                          The packets must find their way through the network.
                                          Each packet has the IP address of the destination


                          
                                   3.     By examining the IP address, routers decide where to send the packet next
     –   Fiber                2.     Link Layer or MAC layer
     –   Coaxial                   1.     Links connect the routers/gateways and end-hosts

                          
                                   2.     This layer provides logical and control for communicating across links.
     –
                          
         Twisted pair              3.     Services that this layer might provide include
     –   Wireless                      1.     congestion control, media access, error detection/correction
                              1.     Physical layer
•   Protocols                      1.     Logical bits are encoded as physical quantities, e.g., as voltage levels, as shifts in phase, …
                                   2.     This course does not cover the physical layer
                                        Protocols
•   End-hosts               protocols define format, order of msgs sent and received
•   Applications                 among network entities, and actions taken on msg
     –   Web
                                                transmission, receipt
     –   Email
     –   File transfer
     –   File sharing
•   Packets
     –   TCP                       Hi
                                                              TCP connection
     –   UDP
                                                              request
•   Routers and gateways
                                   Hi
    and groups of routers                                     TCP connection
    (ISPs)
                                Got the                       response
•   Links                        time?
                                                              Get http://www.awl.com/kurose-ross
     –   Fiber
     –   Coaxial                 2:00
     –   Twisted pair
                                                                     <file>
     –   Wireless
•   Protocols                                      time
                        Internet protocol stack


•   application: supporting network applications
     – FTP, SMTP, HTTP
•   transport: process-process data transfer          application
     – TCP, UDP
•   network: routing of datagrams from source to      transport
    destination
     – IP, routing protocols
•   link: data transfer between neighboring network    network
    elements
     – PPP, Ethernet                                     link
•   physical: bits “on the wire”

                                                       physical
                     ISO/OSI reference model


•   presentation: allow applications to interpret meaning of
    data, e.g., encryption, compression, machine-specific
    conventions                                                application
•   session: synchronization, checkpointing, recovery of
                                                               presentation
    data exchange
•   Internet stack “missing” these layers!                       session
     – these services, if needed, must be implemented in
         application                                            transport
     – needed?
                                                                 network
                                                                   link
                                                                 physical
                  Layers 1-5 (7)

• Why is L3 Communications called L3?

• What does the L7 filter web page discuss? Why is
  it called L7
                                                                                                                      switch
                                                                                                                      router
                                            Link
                                            PHY
                                                                                    pkt    Network              pkt
                              pkt   Link                                                                                 Network
                                                   Link   pkt                       pkt   Link Link    pkt      pkt     Link Link     pkt
                              pkt   PHY            PHY    pkt                       pkt                pkt
                                                                                          PHY PHY               pkt     PHY PHY       pkt
                                                                       Link   pkt
                                                                       PHY    pkt
Application   It was a dark and…
                                                                Link   pkt
 Transport    It was a dark and…
                                                                PHY    pkt
 Network            pkt
   Link             pkt
   PHY              pkt                                                                         Link
                                                                                          pkt
                                                                                                             Link     pkt
                                                                                          pkt   PHY
                                                                                                             PHY      pkt
                                           Link
                                           PHY
                                                                                                                        Application    It was a dark and…

                                                                                                                         Transport     It was a dark and…
                                                                                                                         Network             pkt
                                                                                                                           Link              pkt
                                                                                                                           PHY               pkt
                 Today – networking basics
•   Core components of the Internet – the protocol stack
•   Multiplexing, circuit switching, and packet switching
•   Loss and delays
•   The structure of the Internet
•   This lecture covers much of chapter 1 in the textbook.
     Circuit switching versus Packet switching
• Packet switching brought the networking revolution
• Circuit switching
• Virtual circuit networking
    – A half-way point between packet switched and circuit switched
      networking
                                   Circuit switching
•   Circuit switching
     – Old style phone system
     – Each connection gets its own wire or
       bandwidth
     – Note: calls must be set-up.
     – E.g.,
          • Me: operator, get my the president.
          • Operator: one moment please.
          • Then she plugs a cable into a socket so
            now I have a physical wired between me
            and the president.
     – Instead of each connection getting a whole
       wire, connections can share a wire via
       multiplexing

     – The first automatic circuit switching was
       developed by Almon Strowger – an
       undertaker. There were two undertakers in
       a small town and the switch board
       operator was the wife of the other
       undertaker. So Strowger invented an
       automatic circuit switch to rid both
       husband and wife of employment.
              Frequency division multiplexing

         On each hop, the connection gets its own bandwidth

phone          end office    toll office     end office              phone




        300 3400      100300 103400   200300 203400       300 3400




             Frequency division multiplexing is used in
             • TV & radio
             • Cell phones (not so much today)
                           Time division multiplexing
                                        1 byte for each
                                     channel every 1/8000
phone       bytes                           seconds
                                               Or
                                     24×7×8000+overhead         1 byte for each channel
                                         = 1.544Mbps
         4321                            (DS1 or T1)
                                                                 every 1/8000 seconds
                                                                           Or
                                                               28×24×7×8000+overhead
         4321                                                    = 44.736Mbps (DS-3)


                                 3    2      1

         4321
        1 byte every 1/8000
               seconds
        Or 7×8000=56Kbps
         (1Kbps=1000bps)                                     There are standard bit-rates that support
         (7 bits of data & 1                                 multiplexing different numbers of calls
           bit of control)                                   Multiplex 28 DS1
                                                             = 28*24*64kbps + overhead = 44.736Mbps DS-3

                                                            Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1
                                                            STS is electrical and oc is optical
                                                            OC3 = 155.52Mbps (150.336 payload)
                                                            OC12 = 633.08 Mbps (601.344 payload)
                                                            OC48 = 2.488Gbps (2.405Gbps)
                                                            OC192 = 9.953Gbps (9.6Gbps payload)
                                       Time division multiplexing
                                                            1 byte for each
                                                         channel every 1/8000
    phone              bytes                                    seconds
                                                                   Or
                                                         24×7×8000+overhead         1 byte for each channel
                                                             = 1.544Mbps
                     4321                                    (DS1 or T1)
                                                                                     every 1/8000 seconds
                                                                                               Or
                                                                                   28×24×7×8000+overhead
                     4321                                                            = 44.736Mbps (DS-3)


                                                    3      2      1

                     4321
                   1 byte every 1/8000
                         seconds
                   Or 7×8000=56Kbps
                    (1kbps=1000bps)                                              There are standard bit-rates that support
                   (7 bits of data 1 bit                                         multiplexing different numbers of calls
                        of control)                                              Multiplex 28 DS1
                                                                                 = 28*24*64kbps + overhead = 44.736Mbps DS-3
•    Note all the control overhead: if the bit is 1, then payload is
     control.                                                                   Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1
•    Lots of control is needed to setup a circuit. How is it possible to        STS is electrical and oc is optical
     get channels at each hop?                                                  OC3 = 155.52Mbps (150.336 payload)
•    Also, if there is not data, then nothing is sent. This wastes data.        OC12 = 633.08 Mbps (601.344 payload)
•    But the circuit is yours, guaranteed!                                      OC48 = 2.488Gbps (2.405Gbps)
                                                                                OC192 = 9.953Gbps (9.6Gbps payload)
                          Packet switching - Statistical multiplexing



•    Data is in packets, not streams.
•    Must be digital
•    Each packet has an address
•    A switch/router reads the whole packet, then reads the address and forwards the packet –
     store and forward

Packet format
specification specifies
where the address is


                   data 1




     client
                                                                               Server: address = 1
                   Packet switching - Statistical multiplexing


•   Data is in packets, not streams.
•   Must be digital
•   Each packet has an address
•   A switch/router reads the whole packet, then reads the address and forwards the packet –
    store and forward                              If destination
                                                  is 1, then next
                             If destination
                                                     hop is C
                            is 1, then next
                                hop is B               B     If destination
                                                            is 1, then next
                                  A                  data 1      hop is
               data 1             data 1                             C
                                                                    data 1

    client                                            D
                                                                              Server: address = 1
                              F                              E
                     Packet switching - Statistical multiplexing


•   Data is in packets, not streams.
•   Must be digital
•   Each packet has an address
•   A switch/router reads the whole packet, then reads the address and forwards the packet –
    store and forward
•   No reservations are needed. First come first serve.
•   Major benefit:
     –   If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else
         can use it.
•   Major drawback:
     –   What happens if two packets arrive at a switch and both need to go to the same output interface.
         Picture. One packet is either dropped, or is placed in a buffer. Either way, something bad has
         happened, the packet is gone or is delayed. This would never happen on a circuit switched
         network.  queuing delay and packet loss 
                   Packet switching - Statistical multiplexing


•   Data is in packets, not streams.
•   Must be digital
•   Each packet has an address
•   A switch/router reads the whole packet, then reads the address and forwards the packet –
    store and forward




               data 1             data 1



     client   other 1
                                                                              Server: address = 1



    other client
                   Packet switching - Statistical multiplexing


•   Data is in packets, not streams.
•   Must be digital
•   Each packet has an address
•   A switch/router reads the whole packet, then reads the address and forwards the packet –
    store and forward



                                                    data
                                                   other 1

                                  other 1
               data 1             data 1
                                                                data
                                                               other 1


     client   other 1
                                                                              Server: address = 1



    other client
                     Packet switching - Statistical multiplexing


•   Data is in packets, not streams.
•   Must be digital
•   Each packet has an address
•   A switch/router reads the whole packet, then reads the address and forwards the packet –
    store and forward
•   No reservations are needed. First come first serve.
•   Major benefit:
     –   If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else
         can use it.
•   Major drawback:
     –   What happens if two packets arrive at a switch and both need to go to the same output interface.
         One packet is either dropped, or is placed in a buffer. Either way, something bad has happened,
         the packet is gone or is delayed. This would never happen on a circuit switched network. 
         queuing delay and packet loss 
                                 Packet vs. Circuit Switching
If usage is random (e.g., web surfing) statistical multiplexing is better.
Suppose that
1. We have a 5Mbps link
2. Each user needs 50kbps
3. And each user is active 20% of the time. (Note that this condition does not matter for circuit switching. Why?)
How many users can be accommodated under circuit switching and how many can be accommodated under packet
switching?

                                   Circuit switching case
The total number of users that can be accommodated with circuit switching is 5×106/50×103 = 100 users
                                        Packet Switching Case
      Now suppose there are 200 users, what is the probability that there are 150 or more active users?
      In this case, there would be a problem, since the network cannot support more than 100 active users.


Simpler questions: What is the probability of 150 particular users being active and 50 other being inactive?
                                             0.2150 1  0.2 
                                                             200 150




How many different ways can I select these 150 active users?

                    n      n!
                    
                    k  k!n  k !    Is the number of ways that you can select k out of n
                    
                   200      Is the number of ways that you can select 150 people out of 200
                   150

            The probability of any 150 users being active and the rest in active is

                                  200
                                  150
                                        0. 2 150  0. 2 150 This is the Binomial distribution
                                                 1      200
                                       Packet Switching Case
       What is the probability of more than 100 users being active?


      The probability of 101 users being active plus, 102 users being active, plus …., plus 200 users being active,
      which is


          k
           200
             101
                      200
                       k
                            0. 2 k  0. 2 k 10  This is the binomial complimentary cumulative distribution
                                   1      200     14




                   We conclude that if there are 200 users, then in “pretty much always” things will work fine


   Suppose that there are 300 users:       k
                                            300
                                              101
                                                      300
                                                       k
                                                               0. 2 k  0. 2  k 10 
                                                                      1       300     8            Still pretty good


                                                                                               Might be acceptable
Suppose that there are 400 users:      400
                                        k101
                                                400
                                                 k
                                                      0. 2 k    0. 2 
                                                               1         400k
                                                                                 0. 004       performance (if there is some
                                                                                               other mechanism to recover!)


       Therefore: circuit switching could support 100 users, while
       packet switching can support 400 users. A factor of 4 more!!!
                   Packet Switching vs. Circuit Switching
    A couple of things:

                                         What does this probability really mean?

                                                 300
                                                        300  k
                                                 
                                                            0.2 1  0.2300k  108
                                                             
                                                k 101  k 



 This means that
 • when you walk into the switching center, the probability of finding overload is 10 -8.
 • Or, if you random access the link, the probability of finding it in overload.
 • Once you find it in overload, or not, the probability that is will be in overload in the next second is more complicated
    and requires queuing theory. This analysis might reveal worst performance.


In this example, we assumed 20% user utilization (they were active 20% of the time)
If it the user utilization is smaller, then the difference between packet switching and circuit switching is even greater. But if
it is larger, then there is less of a difference.
What is your user utilization?
•For web surfing
•For cell phone usage
•VoIP call
•For music streaming
•P2P
        Packet Switching vs. Circuit Switching
• If loss and delay are permissible and usage is random, then packet
  switching is better than circuit switching.
• If usage is very regular (e.g. TV!), circuit switching is best.
• If losses and delay are not permissible, then circuit switching is best
  (e.g., remote controlled surgery).
• With packet switching, congestion control is required. Also, there is
  more overhead for each packet.
• For circuit switching, once the circuit is setup, it can be very efficient.
  But circuits must be set-up.
• So, for short file transfer, packet switching is good but for long file
  transfers, circuit switching might be better.
    Packet Switching vs Statistical Multiplexing
•   There is a subtle difference between packet switching and statistical
    multiplexing.
•   Statistical multiplexing means to use the resource as needed.
     • This leads to the performance improvements mentioned but also the
         complications (delay and loss).
•   Statistical multiplexing requires packet switching to put data into chunks
•   Circuit switching can work with data packets/chunks, but there is no need for
    an address
•   The phone network uses circuit switching, but the circuits are statistically
    multiplexed between users.
•   In packet switching, links are statistically multiplexed.
                     Packet Switching: Statistical Multiplexing




             100 Mb/s
   A         Ethernet        statistical multiplexing          C

                                   1.5 Mb/s
       B
                queue of packets
                waiting for output
                       link


                                  D                           E

Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand 
   statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
                                       Time division multiplexing
                                                            1 byte for each
                                                         channel every 1/8000
    phone              bytes                                    seconds
                                                                   Or
                                                         24×7×8000+overhead         1 byte for each channel
                                                             = 1.544Mbps
                     4321                                    (DS1 or T1)
                                                                                     every 1/8000 seconds
                                                                                               Or
                                                                                   28×24×7×8000+overhead
                     4321                                                            = 44.736Mbps (DS-3)


                                                    3      2      1

                     4321
                   1 byte every 1/8000
                         seconds
                   Or 7×8000=56Kbps
                    (1kbps=1000bps)                                              There are standard bit-rates that support
                   (7 bits of data 1 bit                                         multiplexing different numbers of calls
                        of control)                                              Multiplex 28 DS1
                                                                                 = 28*24*64kbps + overhead = 44.736Mbps DS-3
•    Note all the control overhead: if the bit is 1, then payload is
     control.                                                                   Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1
•    Lots of control is needed to setup a circuit. How is it possible to        STS is electrical and oc is optical
     get channels at each hop?                                                  OC3 = 155.52Mbps (150.336 payload)
•    Also, if there is not data, then nothing is sent. This wastes data.        OC12 = 633.08 Mbps (601.344 payload)
•    But the circuit is yours, guaranteed!                                      OC48 = 2.488Gbps (2.405Gbps)
                                                                                OC192 = 9.953Gbps (9.6Gbps payload)
             Packet-switching: store-and-forward


                 L
                     R        R         R

                                  Example:
• takes L/R seconds to            • L = 7.5 Mbits
  transmit (push out) packet      • R = 1.5 Mbps
  of L bits on to link at R bps   • transmission delay = 15 sec
• store and forward: entire
  packet must arrive at router
  before it can be transmitted
  on next link
• delay = 3L/R (assuming        more on delay shortly …
  zero propagation delay)
                 Today – networking basics
•   Core components of the Internet – the protocol stack
•   Multiplexing, circuit switching, and packet switching
•   Loss and delays
•   The structure of the Internet
•   This lecture covers much of chapter 1 in the textbook.
 Losses and delay in packet switched networks
• Losses
   – Transmission losses
        • In fiber links, bit-error is 10-12 or better (i.e., less).
              – What is the probability of packet error when there are 1500 bytes in a packet?
                  » 1 - (1 - 10-12)1500×8 = 1.2×10-8
        • In wireless links, the bit-error rate can be very high
   – Congestion losses.
        • If too many packets arrive at the same time, then the buffers will fill up and
          packets are lost.
        • Increasing the link speeds or reducing the number of users can reduce the
          probability of loss.
        • Increasing the size of the buffer reduces losses, but also increases delay.
• Delay                                                                 packet being transmitted (delay)

   –   Queuing delay
   –   Transmission delay             A

   –   Propagation delay
                                          B
   –   Processing delay                                                packets queueing (delay)

                                                       free (available) buffers: arriving packets
                                                       dropped (loss) if no free buffers
                                     Queuing delay
                                                    packet being transmitted (delay)

         A

             B
                                               packets queueing (delay)
                                free (available) buffers: arriving packets
                                dropped (loss) if no free buffers


    •   Queuing delay occurs for the same reason as congestion losses.
    •   The more the network is utilized, the high the queueing delay (and losses)
    •   Utilization =  := actual use / maximum possible use
Suppose that
• the link bit-rate is Z,
• there are X users
• Each users uses data rate Y with probability p, and use no bandwidth with probability 1-p.

                                            = X×(Y×p)/Z
                      Queuing delay
    Is it possible to have a network run at full utilization?

          No! The average delay would be infinite!




From queuing theory
Delay = /(1- )




                                                                
                    Delay in packet switched networks
•   Delay
     –   Queuing delay        How long does it take to transmit a packet?
     –   Transmission delay   How long does it take to get all the bits from node on to the wire/air/fiber?
     –   Propagation delay
     –   Processing delay
                                            Suppose
                                            •Link bit rate is 10 Mbps
                                            •Packet size is 1400 bytes
                                            How long to transmit the packet?


                                       1400 *8 bits / packet
                                                               = .0011 sec = 1.1 ms
                                        10*10^6 bits / sec
                        Delay in packet switched networks
•   Delay
     –   Queuing delay                     How long does it take for a bit to travel along a wire/fiber/through the air?
     –   Transmission delay
     –   Propagation delay
     –   Processing delay

     –   Suppose
            •   Speed of light in a vacuum 3108 m/s while in a fiber it is 2108 m/s
            •   How long does it take to transmit a bit from NY to LA = 3962km
                    –   20ms propagation delay
            •   How about from NY to Jakarta, Indonesia = 16,179km
                    –   80ms
            •   How about to a Geostationary satellite?
                    –   35,786,000 m above the equator
                    –   35,786,000 m /3e8 = 120ms (each way), 240ms up and back
                    –   On the edge of coverage, the delay can be 280ms
                    –   Note: for voice, the maximum delay is 250ms one-way
                    –   For the Iraq war, two satellite hops were often used, resulting in a one-way delay over500ms
            •   Medium orbit satellites (e.g., GPS)
                    –   60ms (each way)
            •   Low-earth orbit satellites (low-earth? What about middle-earth?)
                    –   Iridium at 10ms (each way_
                             » Note, Iridium paid 5 billion for the network and sold for 25million (1/2%->on sale 99.5% off, everything must go)
                    – Teledesic. 10ms
            –   Solar powered aircraft – 0.125ms (each way)
transmitter   receiver
          Start of bit 0


transmitter                receiver
          0
                Transmitter is transmitting bit 0



transmitter 0                                       receiver
    Transmitter has finished transmitting bit 0, and is starting to transmit bit 1



        Start of bit 1

transmitter
          1                                                          receiver
               0
                        Transmitter is transmitting bit 2




transmitter 2   1                                           receiver
                    0
… and so on …
 The transmitter is transmitting bit 8
 The receiver is starting to receive bit 0




transmitter   8     7     6     5     4      3   2   1   0   receiver
 The transmitter is transmitting bit 9.
 The receiver has completed receiving bit 0 and is now starting to receive bit 1




transmitter   9    8     7     6    5     4     3     2     1     0receiver
… and so on …
                          Question: what is the transmission delay?
                          Answer: The time to transmit a bit = 1/bit-rate
         If bit rate = 10Mbps, then the time to transmit a bit is 1/10×106=10-7sec=100nsec

                  Question: What is this?

        transmitter   9     8     7     6     5     4     3     2     1     0receiver

               Answer: This is the length of a bit on the wire (in in the air)
      Question: How is the length of a bit related to the bit rate and transmission delay?
 Question: How far does the beginning of the bit travel while rest of the bit is being transmitted?

 Question: How long does it take to transmit the bit?

Answer: If the link is wireless, then the signal is moving at 3108m/sec.
In this case, the start of the bit has traveled 3108m/sec  1/bit-rate
e.g., bit-rate is 10Mbps, the beginning of the bit has traveled: 3108m/sec / (10010-9sec) = 30m
e.g., bit-rate is 10Gbps, the beginning of the bit has traveled: 3108m/sec / (10010-12sec) = 0.03m
     If the propagation delay is fixed, but the bit-rate is increased.
     Or, equivalently
     If the propagation delay is fixed, but the transmission delay is decreased.




transmitter 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
          16                                                      receiver



  Higher bit rate => low transmission delay (less time to transmit a bit)
  => More bits fit on the wire (or the air)
      If the propagation delay is fixed, but the bit-rate is deceased.
      Or, equivalently
      If the propagation delay is fixed, but the transmission delay is increased.




transmitter
        1                  2                       1               receiver



   Lower bit rate => higher transmission delay (more time to transmit a bit)
   => Fewer bits fit on the wire (or the air)
      If the propagation delay is fixed, but the bit-rate is deceased even more.
      Or, equivalently
      If the propagation delay is fixed, but the transmission delay is increased even more.




transmitter                            1                             receiver



   The receiver receives the bits as the transmitter is still transmitting them.
   The wire (or air) doesn’t hold any complete bits
                          Question: what is the propagation delay?
       Answer: The time for the start (or end) of a bit to move from the transmitter to receiver

                   The beginning of bit 0


                                                                                receiver
        6transmitter 253 222
84734734734714714014014014560140111140140111 3 00 2
 5869582582582582587876565656014016033 5140140000
 9586958253478258736273333333 000
  695869 9 9 936936253827 7 7 222222
         666936936
6734734734782514714014014014565654545454323232323232321212121010101010 0 0 0

                                           00:00:00
                                           00:00:30
                                           00:00:29
                                           00:00:28
                                           00:00:27
                                           00:00:26
                                           00:00:25
                                           00:00:24
                                           00:00:23
                                           00:00:22
                                           00:00:21
                                           00:00:20
                                           00:00:19
                                           00:00:18
                                           00:00:17
                                           00:00:16
                                           00:00:15
                                           00:00:14
                                           00:00:13
                                           00:00:12
                                           00:00:11
                                           00:00:10
                                           00:00:09
                                           00:00:08
                                           00:00:07
                                           00:00:06
                                           00:00:05
                                           00:00:04
                                           00:00:03
                                           00:00:02
                                           00:00:01




                                                   Of course,
                 the time for the beginning of a bit to travel from the transmitter to receiver
                                                is the same as
                    the time for the end of a bit to travel from the transmitter to receiver
                Question: How long to get a bit from transmitter to receiver?
                Or: Once a host starts transmitting, how long until the bit is received?




       transmitter
                0                                                           receiver


Three things
1. Transmit the bit
2. Bit must propagate to receiver
3. Receive bit
Duration = 2TtransmissionDelay + TpropagationDelay ???
No
              Question: How long to get a bit from transmitter to receiver?
              Or: Once a host starts transmitting, how long until the bit is received?




      transmitter
               0                                                          receiver
                     0


Three things
1. Transmit the bit
2. The end of the bit must propagate to receiver – at which point, the bit has been received
3. Receive bit (already done)
Duration = TtransmissionDelay + TpropagationDelay
              Question: How long to get a packet from transmitter to receiver?
              Or: Once a host starts transmitting, how long until the packet is received?




       transmitter
                0                                                          receiver
                        0


Three things
1. Transmit the packet
2. The end of the packet must propagate to receiver – at which point, the packet has been received
3. Receive packet (already done)
Duration = packetSize  TtransmissionDelayPerBit + TpropagationDelay




                          Important: This delay is incurred at each hop.
                     a N hop path will have a transmission delay at each hop
       Mild correction:         Maybe a bit that has
                                value 1 looks like this

     voltage


transmitter                                               receiver

                                                  x
                          length
                           of bit
                        Delay in packet switched networks

•   Delay                        Routers take a bit of time to process packets.
     –   Queuing delay
     –   Transmission delay
                                 • moving packets inside the router
     –   Propagation delay       • Finding which is the next hop
     –   Processing delay
                                 • Applying security or QoS
                                 How to measure delay?
  •   Ping: > ping 216.109.124.73
  •   Ping gives help
  •   (linux) Ping –I 10 216.109.124.73 > file.txt
  •   Then read it in excel and plot delay

  •   Traceroute (linux), tracert (windows)
  •   Traceroute 216.109.124.73 gives the routers and an estimate of the delay to each router.

  •   Question? Does it take larger packets longer to transmit than shorter packets?
  •   Of course, it does.
  •   Can we test it with ping?
  •   Not really. But try it with ping and wireshark


1.Open wireshark                                     8.    Open file in excel
2.Select correct interface                           9.    Delete things we don’t want
3.Start recording                                    10.   Save file
4.There are too many packets                         11.   Open in matlab
5.Filter out everything that is not icmp             12.   Plot
6.Run ping for a bit                                 13.   Repeat for large packets
7.In wireshark export only what is displayed
   Estimating the distribution of queuing delay with
              Wireshark and other tools
                  you                                                   google

       RTT = 2Tpropagation + Ttransmisison_1 + Ttransmisison_2 +…+ Q1 + Q2 + …

1. ping -n 250 google.com                          8.    Open data in something like excel
    – type “ping” for help                         9.    Remove everything but the times
2. Get/open wireshark                              10.   Resave as times.txt
3. Find the correct interface                      11.   Load times.txt in matlab
4. Start collecting data                           12.   u = diff(times);
    – Notice packet are being captured             13.   plot(u)
5. Start ping                                      14.   u = u(u<.2);
6. When ping finishes, stop capture                15.   plot(u)
7. Export data                                     16.   What is min(u)?
    – Select displayed packets option              17.   q = u – min(u);
        (lower left)
                                                   18.   hist(q,20)
    – Deselect Packet Details (lower
        right)
                 Today – networking basics
•   Core components of the Internet – the protocol stack
•   Multiplexing, circuit switching, and packet switching
•   Loss and delays
•   The structure of the Internet
•   This lecture covers much of chapter 1 in the textbook.
                      Internet structure: network of networks



•   roughly hierarchical
•   at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless),
    national/international coverage
     – treat each other as equals




    Tier-1
    providers
                                    Tier 1 ISP
    interconnect
    (peer)
    privately
                        Tier 1 ISP                Tier 1 ISP
     Tier-1 ISP: e.g., Sprint



POP: point-of-presence

    to/from backbone

                peering
…                …
                 .
              …
…

       …



   to/from customers
                        Internet structure: network of networks


 •   “Tier-2” ISPs: smaller (often regional) ISPs
      – Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs




                                                                             Tier-2 ISPs
Tier-2 ISP pays              Tier-2 ISP                                      also peer
                                                         Tier-2 ISP          privately with
tier-1 ISP for
connectivity to                         Tier 1 ISP                           each other.
rest of Internet
 tier-2 ISP is
customer of
tier-1 provider           Tier 1 ISP                   Tier 1 ISP        Tier-2 ISP

                      Tier-2 ISP                  Tier-2 ISP
                        Internet structure: network of networks




 •   “Tier-3” ISPs and local ISPs
      – last hop (“access”) network (closest to end systems)


                    local
                     ISP     Tier 3                            local
                                                 local            local
                              ISP                               ISP
                                                  ISP              ISP
Local and tier-              Tier-2 ISP                  Tier-2 ISP
3 ISPs are
customers of                           Tier 1 ISP
higher tier
ISPs
connecting
them to rest
                            Tier 1 ISP                   Tier 1 ISP       Tier-2 ISP
of Internet
                                                                                local
                      Tier-2 ISP                 Tier-2 ISP
                                                                                 ISP
                local         local                local
                 ISP           ISP                  ISP
                    Internet structure: network of networks


•   a packet passes through many networks!



                 local
                  ISP     Tier 3                        local
                                             local            local
                           ISP                           ISP
                                              ISP              ISP
                          Tier-2 ISP                 Tier-2 ISP

                                   Tier 1 ISP


                         Tier 1 ISP                  Tier 1 ISP       Tier-2 ISP
                                                                            local
                   Tier-2 ISP                Tier-2 ISP
                                                                             ISP
             local         local               local
              ISP           ISP                 ISP
                          ISPs and the structure of the Internet

•   http://som.csudh.edu/fac/lpress/netapps/hout/oneWilshire/index.htm
MEET ME ROOM
Said to be the most interconnected space in the world and the most expensive real
estate in North America, the “Meet Me Room” (a telco industry term) is the heart of
One Wilshire. Here the primary fiber optic cables are routed, split, and shared.
Because of the presence of so many telcos in this room and the ability to freely
interconnect between them, rackspace here becomes extremely valuable. For
comparison, the average price for office space in downtown Los Angeles is $1.75 per
square foot per month. At the Meet Me Room, $250 per square foot would be a
bargain.
          CABLE RISERS
Some 1,800 known conduits contain the fiber optic cables that
flow through the building’s stairwells and vertical utility
corridors, called “risers.” Cable connects the commercial telco
tenants on floors 5 through 29 to the 4th floor Meet Me Room,
and to a new, “wireless” Meet Me Room constructed on the
30th floor.
      SURFACE CABLE MAP
Whenever a permit is pulled by a city contractor for any
underground repairs outside One Wilshire, the various telco
companies with cable in the area come out and paint the cable
routes on the asphalt, creating a visible graphic of the complexity
of what lies just under the surface.
        HVAC

Computers generate a lot of heat, and maintaining a stable, cool temperature and
a low humidity is essential in telco hotels, so tenants sometimes demand to install
their own cooling systems to safeguard their equipment. At One Wilshire, these
units are installed primarily on the third floor roof. A new closed loop cooling
system has been installed on the 30th floor roof.
CABLE MINING

As tenants’ needs change, cables can go unused. Cable mining is performed to
thin out the obsolete cables and future congestion is alleviated through the
installation of dedicated new ducts.
 ELECTRICITY

Power is supplied by DWP, but in the event of a blackout, the building’s
five generators will kick in. It takes the generators three seconds to start
up and stabilize. During this brief period, the entire building runs on
batteries. There are 11,000 gallons of diesel stored on site, enough to run
the generators for 24 hours before being refueled.
     MICROWAVE
On the roof, microwave antennas link up One Wilshire to transmission towers located around
the city. Though fiber’s higher capacity has given it dominance over microwave at One Wilshire,
microwave’s relatively low cost over long distances continues to make it economical for some
applications. The roof’s clear line of sight to the south, west, and to other high-rises, along
with the ability to interface with the fiber inside, continues to make One Wilshire an attractive
location for microwave-based transmission.
  READING A ROOF

Much can be learned about a building’s function by examining its roof. The
existence of telco hotels in the region around One Wilshire is indicated by the
presence of new and extensive cooling units on the roofs of adjacent buildings,
many of which were nearly vacant until the telco companies moved in.
POINT OF ENTRY
The main fiber optic cables connecting One Wilshire to the world enter the building
from under the street through closets in the walls of the building’s parking garage.
Given the importance of the building to the global communications network, access
to the parking garage is controlled, and the building is said to be monitored
continuously by federal security officials.

				
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