CSE401N
Computer Networks
Lecture-2
Network Structure[KR-1.2+1.3+1.4]
S. M. Hasibul Haque
Dept. of CSE
BUET
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A closer look at network structure:
 network edge:
applications and
hosts
 network core:
 routers

network of
networks
 access networks,
physical media:
communication links
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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.: Gnutella, KaZaA
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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 connectionoriented service
Why not connected?
TCP service [RFC 793]

reliable, in-order bytestream data transfer


flow control:


loss: acknowledgements
and retransmissions
sender won’t overwhelm
receiver
congestion control:

senders “slow down sending
rate” when network
congested
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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
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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”
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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
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Network Core: Circuit Switching
network resources
(e.g., bandwidth)
divided into “pieces”
 pieces allocated to calls
 resource piece
idle if
 dividing link bandwidth
into “pieces”
 frequency division
 time division
not used by owning call
(no sharing)
 dividing link bandwidth
into “pieces”
 frequency division
 time division
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Network Core: Circuit Switching
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Circuit Switching: TDMA and TDMA
Example:
FDMA
4 users
frequency
time
TDMA
frequency
time
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Circuit Switching: Resources
(Frequency and Time)
 Divide link bandwidth—
the resource--into
“pieces”
 frequency division
multiplexing (FDM)
 time division
multiplexing (TDM)
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Circuit Switching: The Process
 Three phases
1.
2.
3.
circuit establishment
data transfer
circuit termination
 If circuit not available: “busy signal”
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Timing Diagram of Circuit Switching
Host 1
Node 1
Node 2
processing delay at Node 1
circuit
establishment
Host 2
propagation delay
from Host 1
to Node 1
propagation delay
from Host 2
To Host 1
data
transmission
DATA
circuit
termination
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Delay Calculation in Circuit-Switched Networks
 Propagation delay: delay for the first bit to go
from source to destination
 Transmission delay: time to pump data onto link at
reserved rate
Propagation delay:
 d = length of physical link
 s = propagation speed in
medium (~2x105 km/sec)
 propagation delay = d/s
Transmission delay:
 R = bandwidth (bps)
 L = packet length
(bits)
 time to send a packet
into link = L/R CSE401N
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An Example
 Propagation delay

suppose the distance between host 1 and host 2 is 4000 km, then
one-way propagation delay is:
4000 km
200, 000 km/ s
 20ms
 Transmission delay

suppose we reserve one slot of a T1 line, which
• has a bandwidth of 1.536 Mbps
• is divided into 24 slots, and thus
• each reserved slot has a bandwidth of 64 Kbps

then the transmission delay of a file with 6.4 Kbits is
6.4 kbits
64 kbps
 100ms
 Suppose the setup message is very small, and the total setup
processing delay is 200 ms
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An Example (cont.)
 Then the delay to transfer a 6.4 Kbits file from host
1 to host 2 (from the beginning until host receives
last bit of the file) is:
20  200  20  20  100  360 ms
20 + 200
20
20
DATA
100
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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,
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
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
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Packet Switching
Each end-to-end data flow divided into packets
 Packets have the following structure:
Header
Data
Trailer
• Header and Trailer carry control information (e.g., destination
address, check sum)
• (where is the control information for circuit switching?)
 At each node the entire packet is received, stored
briefly, and then forwarded to the next node
(Store-and-Forward Networks)
 Each packet is passed through the network from
node to node along some path (Routing)
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Packet Switching
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Inside a Packet Switching Router
 A node in a packet switching network
incoming links
node
outgoing links
Memory
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Packet Switching: Resources
 Each packet waits for its turn at the output link
 On its turn, a packet uses full link bandwidth
 Resources used
as needed
 Aggregate resource demand can exceed amount
available
 Congestion: packets queue, wait for link use
Bandwidth division into “pieces”
Resource reservation
Dedicated allocation
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A Taxonomy of Packet-Switched
Networks According to Routing

Goal: move packets among routers from source to
destination

we’ll study several routing algorithms later in the course
 Two types of packet switching
 datagram network
• each packet of a flow is switched independently

virtual circuit network:
• all packets from one flow are sent along a pre-established path
(= virtual circuit)
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Datagram Packet Switching
 Example: IP networks
 Each packet is independently switched
 each packet header contains complete
destination address
 receiving
a packet, a router looks at the
packet’s destination address and searches its
current routing table to determines the next
hop
 routes may change during session
 routers do not keep any state about a flow
An example of datagram-style routing in daily life?
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Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 7
Host E
Node 4
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Timing Diagram of Datagram Switching
Host 1
transmission
time of Packet 1
at Host 1
Node 1
Packet 1
Host 2
Node 2
propagation
delay from
Host 1 to
Node 1
Packet 2
Packet 1
Packet 3
processing
and
queueing
delay of
Packet 1
at Node 2
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
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Virtual-Circuit Packet Switching
 Example: Asynchornous Transfer Mode (ATM) networks
 Hybrid of circuit switching and datagram switching



each packet carries a short
tag (virtual-circuit (VC) #),
tag determines next hop
fixed path determined at
Virtual Circuit setup time,
remains fixed thru flow
routers maintain per-flow
state
Incoming
Interface
Incoming
VC#
Outgoing
Interface
Outgoing
VC#
1
12
2
22
1
16
3
1
2
12
3
22
…
 What advantages do virtual circuit have over datagram?


Guarantees in-sequence delivery of packets
However: Packets from different virtual circuits may be
interleaved
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Virtual-Circuit Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Node 6
Node 7
Host E
Node 4
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Virtual-Circuit Packet Switching
 Three phases
1.
2.
3.
VC establishment
Data transfer
VC disconnect
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Timing Diagram of Virtual-Circuit Switching
Host 1
Node 1
Host 2
Node 2
propagation delay
between Host 1
and Node 1
VC
establishment
Packet 1
Packet 2
Packet 1
data
transfer
Packet 3
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
VC
termination
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Discussion: Datagram Switching vs. Virtual
Circuit Switching
 What are the benefits of datagram
switching?
 What are the benefits of virtual circuit
switching?
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Delay at a Router in Packet Switching
 A packet experiences delay at each hop
 Four types of delay at each hop
 nodal processing delay: check errors & routing
 queueing: time waiting for its turn at output link
 transmission delay: time to pump packet onto a link at link speed
 propagation delay: router to router propagation
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Delay in Datagram Networks
Host 1
transmission
time of Packet 1
at Host 1
Node 1
Packet 1
Packet 2
Packet 3
Node 2
propagation
delay between
Host 1 and
Node 2
Packet 1
Host 2
nodal
processing
and
queueing
delay of
Packet 1 at
Node 2
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
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Network Core: Packet Switching
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
45 Mbs
E
Packet-switching versus circuit switching: human
restaurant analogy
 other human analogies?
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Packet switching versus circuit switching
Packet switching allows more users to use network!
 1 Mbit link
 each user:
 100Kbps when “active”
 active 10% of time
 circuit-switching:
 10 users
N users
1 Mbps link
 packet switching:
 with 35 users,
probability > 10 active
less than .0004
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Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
 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)

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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?
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Residential access: point to point access
 Dialup via modem
 up
to 56Kbps direct access to
router (conceptually)
 ISDN: integrated services
digital network: 128Kbps alldigital connect to router
 ADSL: asymmetric digital
subscriber line
 up to 1 Mbps home-to-router
 up to 8 Mbps router-to-home
 ADSL deployment: happening
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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

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Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
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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 happening now
 LANs: chapter 5
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Wireless access networks
 shared
wireless access
network connects end
system to router
 wireless LANs:


radio spectrum replaces
wire
e.g., Lucent Wavelan 11
Mbps
router
base
station
 wider-area wireless
access

CDPD: wireless access to
ISP router via cellular
network
mobile
hosts
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Home networks
Typical home network components:
 ADSL or cable modem
 router/firewall
 Ethernet
 wireless access
point
to/from
cable
headend
cable
modem
router/
firewall
Ethernet
(switched)
wireless
laptops
wireless
access
point
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Physical Media
 physical link:
transmitted data bit
propagates across link
 guided media:

signals propagate in
solid media: copper,
fiber
 unguided media:
 signals propagate
freely, e.g., radio
Twisted Pair (TP)
 two insulated copper
wires


Category 3: traditional
phone wires, 10 Mbps
Ethernet
Category 5 TP:
100Mbps Ethernet
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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
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
Ethernet
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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
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Thank YOU
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