Introduction to Computer Networking
Ho Dac Phuong, MSC
hdphuong@vnu.edu.vn
7680811
Part I: Introduction
Chapter goal:
get context, overview, “feel” of networking
more depth, detail later in course
approach:
descriptive
use Internet as example

Overview:
what’s the Internet
what’s a protocol?
network edge
network core
access net, physical media
performance: loss, delay
protocol layers, service models
backbones, NAPs, ISPs
history
ATM network
What’s the Internet: “nuts and bolts” view
millions of connected computing devices: hosts, end-systems
pc’s workstations, servers
PDA’s phones, toasters
running network apps
communication links
fiber, copper, radio, satellite
routers: forward packets (chunks) of data thru network

What’s the Internet: “nuts and bolts” view
protocols: control sending, receiving of msgs
e.g., TCP, IP, HTTP, FTP, PPP
Internet: “network of networks”
loosely hierarchical
public Internet versus private intranet
Internet standards
RFC: Request for comments
IETF: Internet Engineering Task Force
local ISP
company
network
regional ISP
router
workstation
server
mobile
What’s the Internet: a service view
communication infrastructure enables distributed applications:
WWW, email, games, e-commerce, database., voting,
more?
communication services provided:
connectionless
connection-oriented
cyberspace [Gibson]:
“a consensual hallucination experienced daily by billions of operators, in every nation, ...."
What’s a protocol?
human protocols:
“what’s the time?”
“I have a question”
introductions

… specific msgs sent
… specific actions taken when msgs received, or other events
network protocols:
machines rather than humans
all communication activity in Internet governed by protocols
protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
What’s a protocol?
a human protocol and a computer network protocol:

Q: Other human protocol?
Hi
Hi
TCP connection
req.
A closer look at network structure:
network edge: applications and hosts
network core:
routers
network of networks
access networks, physical media: communication links
The network edge:
end systems (hosts):
run application programs
e.g., WWW, email
at “edge of network”
client/server model
client host requests, receives service from server
e.g., WWW client (browser)/ server; email client/server
peer-peer model:
host interaction symmetric
e.g.: teleconferencing
Network edge: connection-oriented service
Goal: data transfer between end sys.
handshaking: setup (prepare for) data transfer ahead of time
Hello, hello back human protocol
set up “state” in two communicating hosts
TCP - Transmission Control Protocol
Internet’s connection-oriented service

TCP service [RFC 793]
reliable, in-order byte-stream data transfer
loss: acknowledgements and retransmissions
flow control:
sender won’t overwhelm receiver
congestion control:
senders “slow down sending rate” when network congested
Network edge: connectionless service
Goal: data transfer between end systems
same as before!
UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service
unreliable data transfer
no flow control
no congestion control
App’s using TCP:
HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email)

App’s using UDP:
streaming media, teleconferencing, Internet telephony
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”
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

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
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
transmit over link
wait turn at next link
Network Core: Packet Switching
Packet-switching versus circuit switching: human restaurant analogy
other human analogies?

A
B
C
10 Mbs
Ethernet
1.5 Mbs
45 Mbs
statistical multiplexing
queue of packets
waiting for output
link
Network Core: Packet Switching
Packet-switching:
store and forward behavior
Packet switching versus circuit switching
1 Mbit link
each user:
100Kbps when “active”
active 10% of time

circuit-switching:
10 users
packet switching:
with 35 users, probability > 10 active less that .004

Packet switching allows more users to use network!
N users
1 Mbps link
Packet switching versus circuit switching
Great for bursty data
resource sharing
no call setup
Excessive congestion: packet delay and loss
protocols needed for reliable data transfer, congestion control
Q: How to provide circuit-like behavior?
bandwidth guarantees needed for audio/video apps
still an unsolved problem (chapter 6)
Is packet switching a “slam dunk winner?”
Packet-switched networks: routing
Goal: move packets among routers from source to destination
we’ll study several path selection algorithms (chapter 4)
datagram network:
destination address determines next hop
routes may change during session
analogy: driving, asking directions
virtual circuit network:
each packet carries tag (virtual circuit ID), tag determines next hop
fixed path determined at call setup time, remains fixed thru call
routers maintain per-call state
Access networks and physical media
Q: How to connection end systems to edge router?
residential access nets
institutional access networks (school, company)
mobile access networks
Keep in mind:
bandwidth (bits per second) of access network?
shared or dedicated?
Residential access: point to point access
Dialup via modem
up to 56Kbps direct access to router (conceptually)
ISDN: intergrated services digital network: 128Kbps all-digital connect to router
ADSL: asymmetric digital subscriber line
up to 1 Mbps home-to-router
up to 8 Mbps router-to-home
ADSL deployment: UPDATE THIS
Residential access: cable modems
HFC: hybrid fiber coax
asymmetric: up to 10Mbps upstream, 1 Mbps downstream
network of cable and fiber attaches homes to ISP router
shared access to router among home
issues: congestion, dimensioning
deployment: available via cable companies, e.g., MediaOne
Institutional access: local area networks
company/univ local area network (LAN) connects end system to edge router
Ethernet:
shared or dedicated cable connects end system and router
10 Mbs, 100Mbps, Gigabit Ethernet
deployment: institutions, home LANs soon
LANs: chapter 5
Wireless access networks
shared wireless access network connects end system to router
wireless LANs:
radio spectrum replaces wire
e.g., Lucent Wavelan 10 Mbps
wider-area wireless access
CDPD: wireless access to ISP router via cellular network
Physical Media
physical link: transmitted data bit propagates across link
guided media:
signals propagate in solid media: copper, fiber
unguided media:
signals propagate freelye.g., radio
Twisted Pair (TP)
two insulated copper wires
Category 3: traditional phone wires, 10 Mbps ethernet
Category 5 TP: 100Mbps ethernet
Physical Media: coax, fiber
Coaxial cable:
wire (signal carrier) within a wire (shield)
baseband: single channel on cable
broadband: multiple channel on cable
bidirectional
common use in 10Mbs Ethernet
Fiber optic cable:
glass fiber carrying light pulses
high-speed operation:
100Mbps Ethernet
high-speed point-to-point transmission (e.g., 5 Gps)
low error rate
Physical media: radio
signal carried in electromagnetic spectrum
no physical “wire”
bidirectional
propagation environment effects:
reflection
obstruction by objects
interference
Radio link types:
microwave
e.g. up to 45 Mbps channels
LAN (e.g., waveLAN)
2Mbps, 11Mbps
wide-area (e.g., cellular)
e.g. CDPD, 10’s Kbps
satellite
up to 50Mbps channel (or multiple smaller channels)
270 Msec end-end delay
geosynchronous versus LEOS


Delay in packet-switched networks
packets experience delay on end-to-end path
four sources of delay at each hop
nodal processing:
check bit errors
determine output link
queueing
time waiting at output link for transmission
depends on congestion level of router
Delay in packet-switched networks
Transmission delay:
R=link bandwidth (bps)
L=packet length (bits)
time to send bits into link = L/R
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 quantitites!
Queueing delay (revisited)
R=link bandwidth (bps)
L=packet length (bits)
a=average packet arrival rate
traffic intensity = La/R
La/R ~ 0: average queueing delay small
La/R -> 1: delays become large
La/R > 1: more “work” arriving than can be serviced, average delay infinite!