Lecture 2 Introduction 1-1
Chapter 1: 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 packet-switched networks
1.5 Protocol layers, service models1.6 Networks under attack: security
Lecture 2 Introduction 1-2
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 Introduction 1-3
Network Core: Circuit Switching
End-end resources reserved for “call”
link bandwidth, switch capacity
dedicated resources: no sharing
circuit-like (guaranteed) performance
call setup required
Lecture 2 Introduction 1-4
Network Core: Circuit Switching
network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls 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 Introduction 1-5
Circuit Switching: FDM and TDM
FDM
frequency
time
TDM
frequency
time
4 users
Example:
Lecture 2 Introduction 1-6
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 Introduction 1-7
Packet Switching
A
B
C100 Mb/sEthernet
1.5 Mb/s
D E
queue of packetswaiting for output
link
Lecture 2 Introduction 1-8
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
Bandwidth division into “pieces”
Dedicated allocationResource reservation
Lecture 2 Introduction 1-9
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
What are delay and loss in Internet/network?
Lecture 2 Introduction 1-10
Take home messages
Think, what would be the problem if excessive number of users are trying to access a circuit switch network?
Advantages and disadvantages between circuit-switch and packet-switch networks…
Lecture 2 Introduction 1-11
How do loss and delay 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
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets dropped (loss) if no free buffers
Lecture 2 Introduction 1-12
Four sources of packet delay
1. nodal processing: check bit errors determine output link
A
B
propagation
transmission
nodalprocessing queueing
2. queueing time waiting at output
link for transmission depends on
congestion level of router
Lecture 2 Introduction 1-13
Delay in packet-switched networks3. 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 (~2x108 m/sec) propagation delay = d/s
A
B
propagation
transmission
nodalprocessing queueing
Note: s and R are very different quantities!
Lecture 2 Introduction 1-14
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
proptransqueueproctotal ddddd
Lecture 2 Introduction 1-15
Numerical example 2
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!
R R R
L
A B
Lecture 2 Introduction 1-16
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 allA
B
packet being transmitted
packet arriving tofull buffer is lost
buffer (waiting area)
Lecture 2 Introduction 1-17
Throughput throughput: rate at which information
bits transferred between sender/receiver
Rs
Rs
Rs
Rc
Rc
Rc
R
Lecture 2 Introduction 1-18
Numerical example 3: Throughput
Rs
Rs
Rs
Rc
Rc
Rc
A
B 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!
C