Transcript 4th Edition: Chapter 1 - John Jay College of Criminal Justice

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
Lecture 2
1-1
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”
Lecture 2
1-2
Network Core: Circuit Switching
 End-end resources
reserved for “call”



dedicated bandwidth
resources: no sharing
circuit-like (guaranteed)
performance
call setup required
Lecture 2
1-3
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 “pieces”…HOW?
 frequency division multiplexing (FDM)
• Users use different frequency channels

time division multiplexing (TDM)
• Users use different time slots
Lecture 2
1-4
Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
Lecture 2
time
1-5
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?
Let’s work it out!
Lecture 2
1-6
Packet Switching
100 Mb/s
Ethernet
A
B
C
1.5 Mb/s
queue of packets
waiting for output
link
D
E
Lecture 2
1-7
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
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
Lecture 2
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
1-8
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?
Lecture 2
1-9
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 turn…DELAY
A
B
Lecture 2
1-10
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
transmission
A
propagation
B
nodal
processing
queueing
Lecture 2
1-11
Delay in packet-switched networks
3. Transmission delay:
 R=link bandwidth (bps)
 L=packet length (bits)
 time to send bits into
link = L/R
transmission
A
4. Propagation delay:
 d = length of physical link
 s = propagation speed in
medium (~2x108 m/sec)
 propagation delay = d/s
Note: s and R are very
different quantities!
propagation
B
nodal
processing
queueing
Lecture 2
1-12
Total delay
dtotal  dproc  dqueue  dtrans  dprop
 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
Lecture 2
1-13
Numerical example 2
L
A
R
R
R
B
 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.
Let’s work it out!
Lecture 2
1-14
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)
A
B
packet being transmitted
packet arriving to
full buffer is lost
Lecture 2
1-15
Throughput
 throughput: rate at which information bits
transferred between sender/receiver
Rs
Rs
Rs
R
Rc
Rc
Rc
Lecture 2
1-16
Numerical example 3: Throughput
B
Rs
Rs
Rs
C
Rc
Rc
Rc
A
 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: 1Mb=106 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?
Let’s work it out!
Lecture 2
1-17