2. Recommended Texts
Texts:
1. A. L.eon-Garcia and I. Widjaja: Communication Networks
2. A. S. Tanenbaum: Computer Networks
3. D. Bertsekas and R. Gallagar: Data Networks
References:
1. Kurose and Ross: Computer Networking
2. W. Stallings: Data and Computer Communication; Prentice-Hall, 1997.
2
3. Introduction
• An overview of networks, the Internet, services and
protocols
• Circuit Switching, Packet Switching
• Store-&-Forward Operation in Packet Switching,
Pipelining
• Layered Network Architecture, Summary overview of
the TCP/IP Network Layers
3
4. 4
• Network Devices
– Routers, servers, switches,
multiplexers, hubs, modems, …
• Communication links
– fiber, copper, radio, satellite, ..
• Protocols control sending,
receiving of messages
– e.g., TCP, IP, HTTP, FTP, PPP
Elements of the Internet
Local ISP
Campus
LAN
Regional ISP
router
workstation
server
mobile
The Internet is a loosely hierarchical,
network of networks
5. 5
Circuit Switching
Direct connection through
interconnected wires
Circuit
Switched
Communication
Network
Packet
Switched
Packet switching preferred in
computer communications
Packet Switching
Data divided into packets and
sent through a network – no
direct connection between the
end nodes
6. 6
Communication Through a Public Circuit
Switched Network
Modem
Modem
Typical way of setting up a Circuit-Switched connection between
computers using a Public Switched Network such as the PSTN
(Public Switched Telephone Network)
7. Circuit Switching
• Dedicated communication path between the end
nodes set up through circuit switches of the
network
• Three phases
– Circuit establishment set up circuit
– Data transfer use circuit to send/receive data
– Circuit disconnect disconnect the circuit
Must have sufficient switching capacity and channel
capacity to establish connection
8. 8
Circuit Switching
• Inefficient
– Channel capacity dedicated for duration of connection
– If no data is being transmitted, capacity is wasted
• Set up (connection) takes time
• Once connected, transfer is transparent
– Data is not examined by network, no delay
– Data rate is fixed for circuit-switching network
– Both ends must operate at the same rate
9. 9
Communication Through a Packet Switching Network
• Network consists of
interconnected routers or
switches
• Data transmitted in small packets
– Data is organized into packets
with size typically up to 1000
octets
– Each packet contains a portion
of user data (Data) plus some
control information (Header)
• Packets are received, stored
briefly (buffered) and passed on
to the next node
– Store and forward causes
packet delay
Note that the Header is actually
“overhead” as what we really want to
send is just the Data.
However, in packet switching the header
is required to make sure that the packet
reaches the destination
packets
Destination
packets
packets
packets
Routers or
Switches
Source
10. 10
Using Packet Switching
Intermediate nodes(routers) store the
packet, decide the route, and forward
the packet: processing delay
packet header contains routing
information
11. 11
Packet Switching Implementations
• Station breaks long message into packets
• Packets sent one at a time to the network
• Packets handled in two approaches
– Datagram Service
• For each packet, each node makes its own decision as to
how to forward it so that it eventually reaches its
destination
– Virtual circuit
• A path for the packet is pre-determined for the sender-
receiver pair and the packet always follows this path
12. 12
Datagram Packet Switching
• Each packet contains a full destination address
• Packets can take any practical route
• Each packet treated independently
• Packets may arrive out of order (receiver must re-order the
packets)
• Packets may go missing (end-nodes handle recovery of
missing packets)
3
2
1
1 3 2
Store-&-Forward Operation required at
each node for each packet
13. 13
Virtual Circuit Packet Switching
• Preplanned route established before any packets sent
– Call setup establishes the route for data connection
– All packets in a connection follow the established route
• Each packet contains a virtual circuit identifier instead of
destination address
• No routing decisions required for each packet
• Virtual circuit is not a dedicated path (may be shared)
3
2
1
3 2 1
Store-&-Forward Operation required at
each node for each packet
14. 14
Virtual Circuit vs Datagram Switching
• Virtual circuit
– Network can provide sequencing and error control
– Packets are forwarded more quickly
• No routing decisions to make
– Less reliable
• Loss of a node loses all circuits through that node
• Datagram
– No call setup phase
• More efficient for short message or fewer packets
– More flexible
• Routing can be used to avoid congested parts of the
network
Intermediate nodes will need to do Store-&-
Forward for both kinds of packet switching
15. 15
Advantages of Packet Switching
• Line efficiency
– Single node to node link can be shared by many packets
over time
– Packets queued and transmitted as fast as possible
• Data rate conversion
– Each station connects to the local node at its own speed
– Nodes buffer data if required to equalize rates
• Packets are accepted even when network is busy
– Delivery may slow down
• Priorities can be used
• Robust against link and node failure
16. 16
Store-&-Forward Operation in Packet
Switching
A BSwitch
R bits/second R bits/second
• A transmits a message of F
bits to B in one packet
• Header of h bits attached to
make a packet of F +h bits
• Link Speed R bits per second
(bps)
• No link congestion so no
queueing delay
Example
Time to transfer
message
from A to the Switch
seconds
F h
R
Time to transfer
message
from the Switch to B
seconds
F h
R
Total Time to Transfer
message from A to B
2 seconds
F h
R
Note that having more
intermediate switches
will cause more delays!
Can we do better than this?
17. 17
Store-&-Forward Operation in Packet
Switching
A BSwitch
R bits/second R bits/second
•
Example (continued)
Time to transfer a
packet
from A to the Switch
2 seconds
F h
R
Time to transfer a
packet
from the Switch to B
Total Time to Transfer
message from A to B
23 seconds
F h
R
2 seconds
F h
R
Pipelining
(illustrated in the next slide)
•A transmits message of F
bits as two segments of F/2
bits each
• Header of h bits attached to
each segment to form packet
of F/2 +h bits
• After receiving packet 1, the
Switch starts forwarding it to
B while it receives packet 2.
After packet 2 is received (and
packet 1 is transmitted), the
Switch starts forwarding
packet 2 to B.
Message may be divided into
more than two segments
18. 18
“Pipelining”
Pipelining packets (of a
multi-packet message)
may improve overall
delay performance when
packets go through
store-&-forward nodes
in between
A B
Store-&-Forward
Switch/Node
time
onepackettwopackets
19. 19
Some Simple Thumb-rules for Pipelining in
Store-&-Packet Transmissions
• Generally better to divide a long message into smaller segments
even though the header bits have to be added to each segment
to form each packet (i.e. more overhead bits)
• This is because Pipelining can then be used to reduce the overall
message transfer delay
• This would be even more effective when there are multiple
switches between the source and the destination nodes
• Avoid making the segments too small as then the added
overhead of h bits per packet will begin to have a detrimental
effect (i.e. higher delays)
20. 20
What is a protocol?
• Communications between computers requires very specific
unambiguous rules
• A protocol is a set of rules that governs how two or more
communicating parties are to interact
• The purpose of a protocol is to provide some type of
communications services, e.g.
• Internet Protocol (IP) – packet transfer
• Transmission Control Protocol (TCP) – transfer reliability
• Hyper Text Transfer Protocol (HTTP) – web service
• Simple Mail Transfer Protocol (SMTP) – email service
21. 21
Example of a Protocol
(Calling Directory Assistance)
“Do you have an address?”
Caller
“What name?”
Give Name
Caller
replies
System replies
with number
System
replies
System
replies
System
replies
Operator
replies
“Thank you, please hold”
Give Address
Caller
replies
Give City Name
Dials Directory Assistance
“What city”?
Caller
replies
Operator
replies
Caller
waits
“Thank you, please hold”
Caller
waits
Caller
dials
22. 22
Protocol “Layers”
Networks are complex!
• many “pieces”:
– hosts
– routers
– links of various media
– applications
– protocols
– hardware, software
Question:
Is there any hope of organizing
the structure of network?
This is where “layering”
helps!
Each layer does a small bit of the
work required and the overall work is
divided between the layers
Basic Philosophy: Layer n at one node talks to Layer n at another node
using the Layer n Protocol
23. 23
Internet protocol stack
• Application: supporting network
applications
– FTP, SMTP, HTTP
• Transport: host-host data transfer
– TCP, UDP
• Network: routing of datagrams from
source to destination
– IP, routing protocols
• Link: data transfer between neighboring
network elements
– PPP, Ethernet
• Physical: bits “on the wire”
Application
Transport
Network
Link
Physical
25. 25
Protocol layering and data
Each layer takes data from above
• adds header information to create new data unit
• passes new data unit to layer below
application
transport
network
link
physical
application
transport
network
link
physical
source destination
M
M
M
M
Ht
Ht
Hn
Ht
Hn
Hl
M
M
M
M
Ht
Ht
Hn
Ht
Hn
Hl
message
segment
datagram
frame
26. Internet Protocol (IP)
• This is the protocol most commonly used for the
Network Layer
• Routers interconnect different networks
• Host computers prepare IP packets and transmit them
over their attached network (complexity at edge)
• Routers forward IP packets across networks
• Best-effort IP transfer service, no retransmission
Net 1 Net 2
Router
27. 27
Addressing & Routing
• Hierarchical address: Net ID + Host ID (e.g. 128.100.11.1)
• IP packets routed according to Net ID
• Routers compute routing tables using distributed algorithm
G
G
G
G
G
G
Net 1
Net 5
Net 3
Net 4Net 2
H
H H
H
28. Transport Protocols
• Host computers run two transport protocols on top of IP to
enable process-to-process communications
• User Datagram Protocol (UDP) enables best-effort transfer of
individual blocks of information
• Transmission Control Protocol (TCP) enables reliable transfer of
a stream of bytes
Internet
Transport Protocol
29. 29
Internet Applications
• Internet applications run on either TCP or UDP
• TCP: HTTP (web); SMTP (e-mail); FTP (file transfer);
telnet (remote terminal)
• UDP: DNS, RTP (voice & multimedia)
• TCP & UDP incorporated into computer operating
systems
• Any application designed to operate over TCP or UDP
will run over the Internet!!!
