Introduction Introduction Srinidhi Varadarajan What is a network? - - PowerPoint PPT Presentation
Introduction Introduction Srinidhi Varadarajan What is a network? What is a network? Carrier of information between connected entities What does a network consist of? End hosts connected to the network Routers/switches that move
Carrier of information between connected entities What does a network consist of?
– End hosts connected to the network – Routers/switches that move data through the network – Physical links that carry information
– Applications that communicate with each other to provide services
A set of connected networks is called an
The Internet is a specific example of an
– The Internet is characterized by the use of a common network protocol (IP).
An internetwork need NOT be connected to the
Network edge:
– millions of connected computing devices running
network apps – pc’s workstations, servers – PDA’s phones, toasters
Network core:
– routers: forward packets (chunks) of data thru network
Media:
– communication links: fiber,
copper, radio, satellite local I SP company net work regional I SP rout er workst at ion server mobile
protocols: control
– e.g., TCP, IP, HTTP, FTP, PPP
Internet: “network of
– loosely hierarchical – public Internet versus private intranet
Internet standards
– RFC: Request for comments – IETF: Internet Engineering Task Force local I SP company net work regional I SP rout er workst at ion server mobile
communication infrastructure enables distributed
– 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 the time?” “I have a question” introductions
machines rather than
all communication
network edge:
network core:
– routers – network of networks
access networks,
– run application programs – e.g., WWW, email – at “edge of network”
– client host requests, receives service from server – e.g., WWW client (browser)/ server; email client/server
– host interaction symmetric – e.g.: teleconferencing
handshaking: setup
– set up “state” in two communicating hosts
TCP - Transmission
– Internet’s connection-
reliable, in-order byte-
– loss: acknowledgements and retransmissions
flow control:
– sender won’t overwhelm receiver
congestion control:
– senders “slow down sending rate” when network congested
– same as before!
UDP - User Datagram
HTTP (WWW), FTP (file
streaming media,
mesh of interconnected
the fundamental
link bandwidth,
dedicated resources:
circuit-like
call setup required
pieces allocated to
resource piece idle if
dividing link
user A, B packets share
each packet uses full link
resources used as
aggregate resource
congestion: packets
store and forward:
Bandwidth division into “pieces” Dedicated allocation Resource reservation
A B C
10 Mbs Et hernet 1.5 Mbs 45 Mbs
D E
st at ist ical mult iplexing
queue of packet s wait ing f or out put link
Segmented Packet Switching Message 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!
Great for bursty data
Excessive congestion: packet delay and loss
Q: How to provide circuit-like behavior?
Goal: move packets among routers from source to
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
residential access nets institutional access
mobile access
bandwidth (bits per
shared or dedicated?
Dialup via modem
ISDN: integrated services
ADSL: asymmetric digital
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
company/univ local area
Ethernet:
deployment: institutions,
shared wireless access
wireless LANs:
– radio spectrum replaces wire – e.g., Lucent Wavelan 10 Mbps
wider-area wireless
– CDPD: wireless access to ISP router via cellular network
physical link:
guided media:
– signals propagate in solid media: copper, fiber
unguided media:
– signals propagate freelye.g., radio
two insulated copper
– Category 3: traditional phone wires, 10 Mbps ethernet – Category 5 TP: 100Mbps ethernet
wire (signal carrier) within a
wire (shield)
– baseband: single channel
– broadband: multiple channels on cable
bidirectional common use in 10Mbs
Ethernet
glass fiber carrying light
pulses
high-speed operation:
– 100Mbps Ethernet – high-speed point-to-point transmission (e.g., 5 Gbps)
low error rate
signal carried in
electromagnetic spectrum
no physical “wire” bidirectional propagation environment
effects:
– reflection – obstruction by objects – interference
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
packets experience
four sources of delay
nodal processing:
– check bit errors – determine output link
queueing
– time waiting at output link for transmission – depends on congestion level of router
propagat ion t ransmission nodal processing queueing
R=link bandwidth
L=packet length (bits) time to send bits into
Propagation delay:
d = length of physical link s = propagation speed in
medium (~2x108 m/sec)
propagation delay = d/s
propagat ion t ransmission nodal processing queueing
R=link bandwidth (bps) L=packet length (bits) a=average packet arrival
rate
La/R ~ 0: average queuing delay small La/R <= 1: delays become large La/R > 1: more “work” arriving than can be
many “pieces”:
t icket (pur chase) baggage (check) gat es (load) runway t akeof f airplane rout ing t icket (complain) baggage (claim) gat es (unload) runway landing airplane rout ing airplane rout ing
Layers: each layer implements a service – via its own internal-layer actions – relying on services provided by layer below t icket (pur chase) baggage (check) gat es (load) runway t akeof f airplane rout ing t icket (complain) baggage (claim) gat es (unload) runway landing airplane rout ing airplane rout ing
Count er -t o-count er deliver y of per son+bags baggage-claim-t o-baggage-claim deliver y people t r ansf er : loading gat e t o ar r ival gat e runway-t o-r unway deliver y of plane air plane r out ing f r om sour ce t o dest inat ion
t icket (pur chase) baggage (check) gat es (load) runway t akeof f airplane rout ing t icket (complain) baggage (claim) gat es (unload) runway landing airplane rout ing
airplane rout ing
airplane rout ing airplane rout ing
explicit structure allows identification,
modularization eases maintenance, updating of
layering considered harmful?
application: supporting network
– ftp, smtp, http
transport: process-process data
– tcp, udp
network: routing of datagrams from
– ip, routing protocols
link: data transfer between
– ppp, ethernet
physical: bits “on the wire”
applicat ion t ransport net work link physical applicat ion t ransport net work link physical applicat ion t ransport net work link physical applicat ion t ransport net work link physical net work link physical
distributed “entities”
entities
applicat ion t ransport net work link physical applicat ion t ransport net work link physical applicat ion t ransport net work link physical applicat ion t ransport net work link physical net work link physical dat a dat a
take data from
add
send datagram
wait for peer to
analogy: post
dat a
t r anspor t t r anspor t
ack
applicat ion t ransport net work link physical applicat ion t ransport net work link physical applicat ion t ransport net work link physical applicat ion t ransport net work link physical net work link physical dat a dat a
M M M M Ht Ht Hn Ht Hn Hl M M M M Ht Ht Hn Ht Hn Hl message segment dat agr am f r ame