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


Hardware view of Internet

Components of Internet

Structural view


Client-server model
Peer-to-peer model
Lecture 2
1-2


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
1-3


End-end resources reserved for “call”


 dedicated bandwidth resources: no sharing circuit-like (guaranteed) performance call setup required
Lecture 2
1-4


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
1-5

FDM
Example:
4 users frequency time
TDM frequency
Lecture 2 time
1-6


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
= 10 6 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
1-7

A
100 Mb/s
Ethernet
B
1.5 Mb/s queue of packets waiting for output link
D E
C
Lecture 2
1-8

Circuit switching
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation 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
Lecture 2 resource contention:
 aggregate resource demand can exceed amount available
 store and forward: packets move one hop at a time

Node receives complete packet before forwarding
 congestion: packets queue, wait for link use
1-9


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
1-10

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
1-11


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
1-12

3. Transmission delay:

R=link bandwidth (bps)


L=packet length (bits) time to send bits into link = L/R
4. Propagation delay:


 d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) propagation delay = d/s
A transmission
Note: s and R are very different quantities!
propagation
B nodal processing queueing
1-13

d total
 d proc
 d queue
 d trans
 d prop



 d proc

= processing delay typically a few microsecs or less d queue

= queuing delay depends on congestion d trans

= transmission delay
= L/R, significant for low-speed links d prop

= propagation delay a few microsecs to hundreds of msecs
Lecture 2
1-14

L
A
R
B

Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 6 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
1-15

L
A
R R
B

Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 6 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
1-16

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 = 10 6 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.

What if there are three packets from A?
Let’s work it out!
Lecture 2
1-17



 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
A buffer
(waiting area) packet being transmitted
B packet arriving to full buffer is lost
Lecture 2
1-18


Throughput: rate at which information bits transferred between sender/receiver
R s
R s
R s
R
R c
R c
R c
Lecture 2
1-19

R s
R c
R s
R c
C
B
R s
A
R c

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=10 6 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
1-20