26
CS410 Networks and Networking Lecture 02 1: Introduction 1
CS410 Networks and Networking
Lecture 02
1: Introduction 1
Quiz Define Network?
What is a link?
What is a distributed application? Give one or two examples.
What is the difference between a host and an end system?
Web Server is a type of end system. T F
What is a protocol?
What are two major models of network applications?
What are some of the physical media that Ethernet can run over?
1: Quiz 2
Introduction 1-3
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, network structure
1.4 Delay, loss and throughput in packet-switched networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Computer Network
End System
Access Network
Links
Introduction 1-4
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Introduction 1-5
The Network Core
mesh of interconnected routers
the fundamental question: how is data transferred through net?
circuit switching: dedicated circuit per call: telephone net
packet-switching: data sent thru net in discrete “chunks”
Introduction 1-6
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
Introduction 1-7
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”
frequency division
time division
Introduction 1-8
Circuit Switching: FDM and TDM
FDM
frequency
time
TDM
frequency
time
4 users
Example:
Introduction 1-9
Numerical example
How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links (there are several links in series from
A to B) are 1.536 Mbps
Each link uses TDM with 24 slots/sec
500 msec to establish end-to-end circuit
Each circuit has a transmission rate =1.536/24 = 64 kbps
Time to transmit the file = 640000/64000 = 10 sec
Total Time = Connection Time + Transmission Time
= 10 + 0.5
= 10.5 sec
Let’s work it out!
Introduction 1-10
Network Core: Packet Switching
each end-end data stream divided into packets
user A, B packets share network resources
each packet travels at the full link transmission rate
resources used as needed
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
Introduction 1-11
Packet Switching
frequency
time
4 users
Example:
the packets from different sources flowing on a link do not have to follow any fixed, pre-defined pattern
Introduction 1-12
Packet-switching: store-and-forward
takes L/R seconds to transmit (push out) packet of L bits on to link at R bps
store and forward: entire packet must arrive at router before it can be transmitted on next link
delay = 3L/R (assuming zero propagation delay)
Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay = 15
sec
R R R
L
more on delay shortly …
Packet Switching: Queuing & Forwarding Table
How does the router determine the link onto which it should forward the packet?
Each router has a forwarding table that maps destination addresses to outbound links
Introduction 1-13
A B C
1010
010
0011 A
B
C
Introduction 1-14
Network Core: Packet Switching
Resource contention: aggregate resource demand can exceed amount available
congestion: packets queue, wait for link use
Store time: store and forward; packets move one hop at a time Node receives complete packet before forwarding
Introduction 1-15
Packet Switching: Statistical Multiplexing
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.
A
B
C 100 Mb/s Ethernet
1.5 Mb/s
D E
statistical multiplexing
queue of packets waiting for output
link
Introduction 1-16
Packet switching versus circuit switching
1 Mb/s link
each user: 100 kb/s when “active”
active 10% of time
circuit-switching: 10 users
packet switching: with 35 users,
probability > 10 active at same time is less than .0004
Packet switching allows more users to use network!
N users
1 Mbps link
Q: how did we get value 0.0004?
Introduction 1-17
Packet switching versus circuit switching
great for bursty data
resource sharing
simpler, no call setup
excessive congestion: packet delay and loss
protocols needed for reliable data transfer, congestion control
Cannot make any end-to-end guarantees for bandwidth
Q: How to provide circuit-like behavior?
bandwidth guarantees needed for audio/video apps
still an unsolved problem (chapter 7)
packet switching
Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
Introduction 1-18
Internet structure: network of networks
a packet passes through many networks!
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local ISP
local ISP
local ISP
local ISP
local ISP Tier 3
ISP
local ISP
local ISP
local ISP
Introduction 1-19
Internet structure: network of networks
end systems connect into the internet via local (access) ISP or Tier 3 ISP
Internet structure is
roughly hierarchical
Local and Tier 3 ISPs are customers of higher tier ISPs connecting them to the rest of the Internet
Tier 1 ISP
Tier-2 ISP
Tier 3 ISP
local ISP
local ISP
Introduction 1-20
Internet structure: network of networks
at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage
treat each other as equals (all of them are interconnected)
same as any network with routers and links connected to other networks
high speed links 622 Mbps – 10 Gbps
High efficiency routers to forward packets at high speed
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-1 providers interconnect (peer) privately
Introduction 1-21
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 routes traffic through the Tier 1 ISPs to reach a large portion of the global internet
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider
Tier-2 ISPs also peer privately with each other.
Introduction 1-22
Internet structure: network of networks
“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local ISP
local ISP
local ISP
local ISP
local ISP Tier 3
ISP
local ISP
local ISP
local ISP
Introduction 1-23
Internet structure: network of networks Points of Presence (POPs): Groups of routers, owned by Tier 1 or Tier 2 ISPs, located at different geographical locations where other ISPs or customers can connect and access the internet
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP Tier-2 ISP
Tier-2 ISP
local ISP
local ISP
local ISP
local ISP
local ISP Tier 3
ISP
local ISP
local ISP
local ISP
POPs
Introduction 1-24
Tier-1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
…
. …
…
…
POP: point-of-presence
1: Introduction 25
Network Core: Definitions Link: Transmits data based on
transmission rate (bits/second)
Circuit: Each link is created by a certain number of circuits
Packet: Segmented data with header bytes (created by sending end system)
Packet Switch: Forwards a packet on a communication link Routers (core) and Link-
Layer Switches (access network)
Bandwidth: width of the frequency spectrum
Homework Posted
1: Introduction 26
Demo – WireShark
