Internet Overview: roadmap

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Internet Overview roadmap

• 1.1 What is the Internet?
• 1.2 Network edge
• end systems, access networks, links
• 1.3 Network core
• circuit switching, packet switching
• 1.4 Delay, loss and throughput in Internet
• 1.5 Protocol layers, service models
• 1.6 Networks under attack security

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The Network Core

• Internet mesh of interconnected routers
• How is data transferred through net?
• circuit switching dedicated circuit per call
  telephone net
• packet-switching data sent thru net in discrete
  chunks

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Network Core Circuit Switching

• End-end resources reserved for call
• dedicated bandwidth resources no sharing
• circuit-like (guaranteed) performance
• call setup required

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Network Core Circuit Switching

• Total network resources (e.g., bandwidth) divided
  into pieces
• pieces allocated to each call
• resource piece idle if not used by owning call
  (no sharing)
• dividing link bandwidth into piecesHOW?
• frequency division multiplexing (FDM)
• Users use different frequency channels
• time division multiplexing (TDM)
• Users use different time slots

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Circuit Switching FDM and TDM
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Numerical example 1

• You need to send a file of size 640,000 bits to
  your friend. You are using a circuit-switched
  network with TDM. Suppose, the circuit-switch
  network link has a bit rate of 1.536 Mbps (1Mb
  106 bits) and uses TDM with 24 slots. How long
  does it take you to send the file to your friend?
• Lets work it out!

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Packet Switching
100 Mb/s Ethernet
C
A
1.5 Mb/s
B
queue of packets waiting for output link
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Network Core Packet Switching

• each end-end data stream divided into packets
• user A, B packets share network resources
• each packet uses full link bandwidth
• resources used as needed

• resource contention
• aggregate resource demand can exceed amount
  available
• congestion packets queue, wait for link use
• store and forward packets move one hop at a time
• Node receives complete packet before forwarding

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Packet switching versus circuit switching

• Packet switching allows users to use the network
  dynamically!
• resource sharing
• simpler, no call setup
• With excessive users
• Excessive congestion
• packet delay and loss

How do delay and loss occur in Internet/network?
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How do delay and loss occur?

• packets queue in router buffers
• store and forward packets move one hop at a time
• Router receives complete packet before forwarding
• packets queue, wait for turnDELAY

A
B
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Four sources of packet delay

• 1. nodal processing
• check bit errors
• determine output link

• 2. queueing
• time waiting at output link for transmission
• depends on congestion level of router

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Delay in packet-switched networks

• 4. Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• 3. Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

Note s and R are very different quantities!
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Total delay

• dproc processing delay
• typically a few microsecs or less
• dqueue queuing delay
• depends on congestion
• dtrans transmission delay
• L/R, significant for low-speed links
• dprop propagation delay
• a few microsecs to hundreds of msecs

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Numerical example 2
L
B
A
R
R
R

• Example A wants to send a packet to B. The
  packet size is, L 7.5 Mb (1 Mb 106 bits). The
  link speed is, R 1.5 Mbps. How long does it
  take to send the packet from A to B? Assume zero
  propagation delay.
• Lets work it out!

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Packet loss

• queue (aka buffer) preceding link in buffer has
  finite capacity
• packet arriving to full queue dropped (aka lost)
• lost packet may be retransmitted by previous
  node, by source end system, or not at all

buffer (waiting area)
packet being transmitted
A
B
packet arriving to full buffer is lost
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Throughput

• throughput rate at which information bits
  transferred between sender/receiver

Rs
Rs
Rs
R
Rc
Rc
Rc
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Numerical example 3 Throughput

• Example A has requested for a packet (size
  640,000 bits) from server B. The packet will come
  through an intermediate router C. It takes 0.1
  second for the packet from B to C and 0.4 seconds
  from C to A. (Note 1Mb106 bits). Assume zero
  propagation delay.
• What is the throughput from B to C?
• What is the throughput from C to A?
• What is the average throughout from B to A?
• Lets work it out!

B
Rs
Rs
Rs
C
Rc
Rc
Rc
A