1. (Ownership, Service Paradigm, Measures of Performance)
Chapter 15
Network Properties
(Ownership, Service Paradigm, Measures of Performance)

2. Comer, 4e, Ch 15 and Ch 16 Comer 5e, Ch 26

3. Network Ownership And Service Type
Private
Owned by individual or corporation
Restricted to owner’s use
Typically used by large corporations
Public
Owned by a common carrier
Individuals or corporations can subscribe
“Public” refers to availability, not data

4. Advantages and Disadvantages
Private
Complete control
Installation and operation costs
Public
No need for staff to install/operate network
Dependency on carrier
Subscription fee

5. Public Network Connections
One connection per subscriber
Typical for small corporation or individual
Communicate with another subscriber
Multiple connections per subscriber
Typical for large, multi-site corporation
Communicate among multiple sites as well as with another subscriber

6. Virtual Private Network
A service
Provided over public network
Interconnects sites of single corporation
Acts like private network
No packets sent to other subscribers
No packets received from other subscribers
Data encrypted

7. Virtual Private Network

8. Frame Relay Pricing Permanent Virtual Circuits (PVCs)
Leased access line must be fast enough to handle all of the PVCs it is multiplexing
Example: if it multiplexes kbps PVCs, the access line must be 840 kbps (T1 line needed)
PVC
Leased
Access Line
PVC

9. Network Service Paradigm
Fundamental characteristic of network
Understood by hardware
Visible to applications
Two basic types of networks
Connectionless
Connection-oriented

10. Connectionless ( CL ) Sender Network Forms packet to be sent
Places address of intended recipient in packet
Transfers packet to network for delivery
Network
Uses destination address to forward packet
Delivers

11. Characteristics of Connectionless Networks
Packet contains identification of destination
Each packet handled independently
No setup required before transmitting data
No cleanup required after sending data
Think of postcards

12. Connection-Oriented (CO)
Sender
Requests “connection” to receiver
Waits for network to form connection
Leaves connection in place while sending data
Terminates connection when no longer needed

13. Connection-Oriented (CO) (continued)
Network
Receives connection request
Forms path to specified destination and informs sender
Transfers data across connection
Removes connection when sender requests
Think of telephone calls

14. Terminology In conventional telephone system In CO data network
Circuit
In CO data network
Virtual Circuit
Virtual Channel

15. Comparison of CO and CL CO CL More intelligence in network
Can reserve bandwidth
Connection setup overhead
State in packet switches
Well-suited to real-time applications CL
Less overhead
Permits asynchronous use
Allows broadcast / multicast

16. Two Connection Types Permanent Virtual Circuit (PVC)
Entered manually
Survives reboot
Usually persists for months
Switched Virtual Circuit (SVC)
Requested dynamically
Initiated by application
Terminated when application exits

17. Examples of Service Paradigm Various Technologies Use

18. Connection Multiplexing
Typical computer has one physical connection to network
All logical connections multiplexed over physical interconnection
Data transferred must include connection identifier

19. Connection Identifier
Integer value
One per active VC
Not an address
Allows multiplexing
Computer supplies when sending data
Network supplies when delivering data

20. Example Connection Identifier (ATM)
24 bits long (The full address is 160 bits)
Divided into two parts
Virtual Path Identifier
Virtual Channel Identifier
Known as (VPI/VCI)
Different at each end of connection
Mapped by switches

21. Illustration of ATM VC
Switch maps VPI/VCIs
17 to 12
96 to 8

22. Two Primary Performance Measures
Delay
Throughput

23. Delay Time required for one bit to travel through the network
Three types (causes)
Propagation delay
Switching delay
Queuing Delay
Intuition: “length” of the pipe

24. Throughput Number of bits per second that can be transmitted Capacity
Intuition: “width” of the pipe

25. Components of Delay Fixed (nearly constant) Variable
Propagation delay
Switching delay
Variable
Queuing delay
Depends on throughput
If delay is changing rapidly, we refer to it as Jitter

26. Relationship Between Delay and Throughput
When network idle
Queuing delay is zero
As load on network increases
Queuing delay rises
Load defined as ration of throughput to capacity
Called utilization

27. Relationship Between Delay and Utilization
Define
D0 to be the propagation and switching delay
U to be the utilization (0  U  1)
D to be the total delay
Then
High utilization known as congestion

28. Practical Consequence
Any network that operates with a utilization approaching 100% of capacity is doomed

29. Delay-Throughput Product
Time to cross network
Measured in seconds
Throughput
Capacity
Measured in bits per second
Delay * Throughput
Measured in bits
Gives quantity of data “in transit”

30. Summary Network can be Virtual Private Network Public Private
Uses public network
Connects set of private sites
Addressing and routing guarantee isolation

31. Summary (continued) Networks are Connection types
Connectionless
Connection-oriented
Connection types
Permanent Virtual Circuit
Switched Virtual Circuit
Two performance measures
Delay
Throughput

32. Summary (continued) Delay and throughput interact
Queuing delay increases as utilization increases
Delay * Throughput
Measured in bits
Gives total data “in transit”

33. Protocols and Protocol Layering
Chapter 16
Protocols and
Protocol Layering

34. Protocol Agreement about communication Specifies
Format of messages (syntax)
Meaning of messages (semantics)
Rules for exchange
Procedure for handling problems

35. Need for Protocols Hardware is low level Many problems can occur
Bits corrupted or destroyed
Entire packet lost
Packet duplicated
Packets delivered out of order

36. Need for Protocols (continued)
Need mechanisms to distinguish among
Multiple computers on a network
Multiple applications on a computer
Multiple copies of a single application on a computer

37. Set of Protocols Work together
Each protocol solves part of communication problem
Known as
Protocol suite
Protocol family
Designed in layers

38. Plan for Protocol Design
Intended for protocol designers
Divides protocols into layers
Each layer devoted to one subproblem
Example: ISO 7-layer reference model

39. Illustration of the 7-Layer Model
All
People
Seems To
Need
Data
Processing
Defined early
Now somewhat dated
Does not include internet layer!

40. ISO Layers Layer 1: Physical Layer 2: Data Link (media access)
Underlying hardware (Example: RS-232)
Layer 2: Data Link (media access)
Hardware frame definitions
Layer 3: Network
Packet forwarding
Layer 4: Transport
Reliability

41. ISO Layers (continued)
Layer 5: Session
Login and passwords
Layer 6: Presentation
Data representation
Layer 7: Application
Services for common applications

42. TCP/IP protocol suite

43. Layers and Protocol Software
Protocol software follows layering model
One software module per layer
Modules cooperate
Incoming or outgoing data passes from one module to another
Entire set of modules known as stack

44. Illustration of Stacks

45. Layers and Packet Headers
Each layer
Prepends header to outgoing packet
Removes header from incoming packet

46. Example of encapsulation

47. Scientific Layering Principle
Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source

48. Illustration of Layering Principle

49. Protocol Techniques For bit corruption For out-of-order delivery
Parity
Checksum
CRC
For out-of-order delivery
Sequence numbers
Duplication

50. Protocol Techniques (continued)
For lost packets
Positive acknowledgement and retransmission
For replay (excessive delay)
Unique message ID
For data overrun
Flow control

51. Flow Control Needed because Related to buffering Two forms
Sending computer system faster than receiving computer
Sending application faster than receiving application
Related to buffering
Two forms
Stop-and-go
Sliding window

52. Stop-And-Go Flow Control
Sending Side
Transmits one packet
Waits for signal from receiver
Receiving side
Receives and consumes packets
Transmits signal to sender
Inefficient

53. Acknowledgements & Timeouts

54. Stop & wait sequence numbers
Sender
Receiver
Sender
Receiver
Sender
Receiver
Frame 0
Frame 0
Frame 0
imeout
imeout
ACK 0 T T
ACK 0
ACK 0
Frame 0
Frame 1
Frame 0
imeout
imeout T
ACK 0
ACK 1
ACK 0 T
Frame 0
(c)
(d)
ACK 0
(e)
Simple sequence numbers enable the client to discard duplicate copies of the same frame
Stop & wait allows one outstanding frame, requires two distinct sequence numbers
CS 640

55. Sliding Window Flow Control
Receiving side
Establishes multiple buffers and informs sender
Sending side
Transmits packets for all available buffers
Only waits if no signal arrives before transmission
Sends signals as packets arrive

56. Illustration of Sliding Window on Sending Side
Window tells how many packets can be sent
Window moves as acknowledgements arrive

57. Sliding Window Example
Sender
Receiver 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 A3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 3 4 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 6 A4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14

58. Performance Stop-and-go Sliding window Slow
Useful only in special cases
Sliding window
Fast
Needed in high-speed network

59. Comparison of Flow Control

60. Why Sliding Window? Simultaneously Speedup Tw = min(B, TG * W) where
Increase throughput
Control flow
Speedup
Tw = min(B, TG * W)
where
B is underlying hardware bandwidth
TW is sliding window throughput
TG is stop-and-go throughput
W is the window size

61. Congestion Fundamental problem in networks
Caused by traffic, not hardware failure
Analogous to congestion on a highway
Principle cause of delay

62. Illustration of Architecture That Can Experience Congestion
Multiple sources
Bottleneck

63. Dealing with Congestion
Congestion results in filled buffers in packet switches => packets will be discarded => retransmission => more packets…..
Solutions
Increasing buffer space
Reducing the amount of packets in the network
Packet switches inform senders, or
Senders use packet loss as an indication

64. Congestion and Loss
Modern network hardware works well; most packet loss results from congestion, not from hardware failure