1. Communication Networks NETW 501
Lecture 3
Layered Architecture of Communication Networks: TCP/IP Model
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Important concepts to learn about
What is a Packet?
Packet Transmission time
Packet Propagation time
Circuit Switching
Packet Switching
Virtual Circuit Packet Switching

3. What is a Packet? A packet is a small block of data bits.
… … … … …
A packet is a small block of data bits.
Packets are sent individually
All the bits in a given packet are treated as single entity

4. Why Packets?
Packets allow for a more convenient transmission process by providing the following:
Coordination
Helps transmitter and receiver determine which section of data has been received correctly
Low overhead due to Retransmission
When bit error occurs, retransmission of the erroneous packet only is needed

5. Headers and Trailers
For a layer N+1, layer N does not understand the message content
So layer N adds bits (Headers and Trailers) that represents its language (i.e., protocol)
The header and trailer for each layer may represent:
Messages to help the receiver side understand what to do
Parameters to help the receiver side perform its service correctly

6. Headers = Overhead We want to send L bits We actually send
TD+HD+HN+HT+L bits
Header and/or trailer added by layer N is independent of the packet length from layer N+1
L bits
Application Data TH
Application Data
HT bits
Transport Data NH
HN bits
Network Data DH DT
HD bits
TD bits
Data Link Data
TD+ HD+HN+HT+L bits
TH: Transport Header
NH: Network Header
DH: Data link Header
DT: data link Trailer

7. Throughput Efficiency
For an Error Free Channel
Throughput Efficiency reflects the percentage of
information bits to
the total number of bits required for the information packet to arrive safely at the destination

8. Packet Transmission Time
It is the time required to put all packet bits over the physical channel
It is dependent on the modem transmission rate
Example
If Transmission Rate is 1 Mbps and Packet Size is 8000 bits then
Transmission Time is:
Tt = 8000/1*106=8 ms

9. Packet Propagation Time
It is the time it takes the electrical signals to travel from one machine to the other
It is dependent on the speed of light and the distance between the two machines
Example
Assume the speed of light over cable is 2.3*108 m/s and the distance between two machines is 1000 Km, then
Propagation Time is:
Tp = 1000*103/2.3*108=4.345ms

10. Example Physical Channel Machine A Machine B Tp=4.345 ms Tt=8 ms Time

11. Circuit Switching
The concept of circuit-switching comes from old telephone technology
For each connection, physical switches are set in the telephone network to create a physical “circuit”
An end-to-end path MUST be set up before any data can be sent

12. Phases of Circuit Switching
Circuit Establishment
A circuit is established between two end users with the help of intermediate switches
Information Transfer
Information is transmitted using the circuit established in the set-up phase
Circuit Disconnect
Circuit is terminated
Dedicated resources are released

13. 1. Circuit Establishment
Circuit Switching - Example
1. Circuit Establishment
2. Information Transfer
3. Circuit Disconnect
Switch C
Switch A
Switch D
Physical copper
connection set up
when call is made
Switch B
Switching offices

14. Circuit Establishment
Time Diagram (Circuit Switching)
Switch A
Switch B
Switch C
Switch D Tp
Circuit Establishment
5Tp Tt
Data Transfer
Circuit Disconnect
Time
Time
Time
Time
Time
Time

15. Circuit Switching: Pros and Cons
Switches are set up at the beginning of the connection (call) and maintained throughout the connection
Network resources are reserved and dedicated from sender to receiver
Propagation time is the only source of delay during data transfer
Not a very efficient strategy!
A connection “holds” a physical line even during “silence” periods (when there is nothing to transmit)

16. Packet Switching Networks
No physical connection is established in advance between the sender and receiver
Each packet is sent independently
Each packet is stored in a router, processed and then forwarded to the next hop
STORE AND FORWARD Operation

17. Packet Switching Networks - Example
Router E
Router C
Router A
Router D
Router Congested
Router B
Router F

18. Time Diagram (Packet Switching)
Router A
Router B
Router C
Router D Tp Tt 1 1
TPr 2 2
TPr  Processing Time 1 1
TPr 2 2 1
Data Transfer 1
TPr 2 2 1 1 2
TPr 2 1 1 2 2
Time
Time
Time
Time
Time
Time

19. Circuit Switching vs. Packet Switching
Must set up a connection (initial delay).
Resources are dedicated
Therefore they may be used inefficiently!
Performance is predictable as resources are reserved.
Packet switching
Very small set-up delay.
Efficient shared use of resources.
Possible congestion and consequent packet dropping
Performance is unpredictable and is a function of current traffic conditions.

20. Virtual Circuit Packet Switching
Compromise between circuit switching and packet switching
- Combines advantages of both
Preplanned route established before any packets sent
Connection Establishment Phase is required
Each packet contains a Virtual Circuit Identifier instead of destination address
No routing decisions required for each packet
Connection release is required after data transfer phase is completed

21. 1. Connection Establishment
Virtual Circuit Packet Switching - Example
1. Connection Establishment
2. Information Transfer
3. Circuit Disconnect
Router E
Router C
Router A
Router D
Router B
Router F

22. Connection Establishment
Time Diagram (Virtual Circuit Packet Switching)
Router A
Router B
Router C
Router D
Connection Establishment 1 1
TPr 2 2 1 1 2
TPr 2 1
Data Transfer 1
TPr 2 2 1 1 2
TPr 2 1 1 2 2
Connection Release
Time
Time
Time
Time
Time
Time

23. Virtual Circuit vs. Packet Switching
Virtual Circuits
No packet reordering occurs because all packets follow the same reserved path
Packets are forwarded more quickly
Less processing time because no routing decisions are necessary
Requires connection set-up and release times
If an intermediate router is down a new connection establishment must be performed
Packet Switching
No Set/Release Phases
More flexible
Can recover from down time of intermediate routers by selecting alternate routes without the need for any new connection establishment

24. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International