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SEPT, 2005 CSI 4118 1
Part 2.1
Network Properties
(Ownership, Service Paradigm, Measures of Performance)
Robert Probert, SITE, University of Ottawa
SEPT, 2005 CSI 4118 2
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
SEPT, 2005 CSI 4118 3
Advantages and Disadvantages
Private Complete control Installation and operation costs
Public No need for staff to install/operate network Dependency on carrier Subscription fee
SEPT, 2005 CSI 4118 4
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
SEPT, 2005 CSI 4118 5
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
SEPT, 2005 CSI 4118 6
Network Service Paradigm
Fundamental characteristic of network Understood by hardware Visible to applications
Two basic types of networks Connectionless Connection-oriented
SEPT, 2005 CSI 4118 7
Connectionless ( CL )
Sender 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
SEPT, 2005 CSI 4118 8
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
SEPT, 2005 CSI 4118 9
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
SEPT, 2005 CSI 4118 10
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
SEPT, 2005 CSI 4118 11
Terminology
In conventional telephone system Circuit
In CO data network Virtual Circuit Virtual Channel
SEPT, 2005 CSI 4118 12
Comparison of CO and CL
CO 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
SEPT, 2005 CSI 4118 13
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
SEPT, 2005 CSI 4118 14
Examples of Service ParadigmVarious Technologies Use
SEPT, 2005 CSI 4118 15
Connection Multiplexing
Typical computer has one physical connection to network
All logical connections multiplexed over physical interconnection
Data transferred must include connection identifier
SEPT, 2005 CSI 4118 16
Connection Identifier
Integer value One per active VC Not an address Allows multiplexing
Computer supplies when sending data Network supplies when delivering data
SEPT, 2005 CSI 4118 17
Two PrimaryPerformance Measures
Delay Throughput
SEPT, 2005 CSI 4118 18
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
SEPT, 2005 CSI 4118 19
LatencyQueueTransmitnPropagatioLatency
a vaccum in /103.0
cablea in /102.3
fibera in /102.0
light of Speed8
8
8
sm
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Bandwidth/SizeTransmit
light of Speed / DistancenPropagatio
SEPT, 2005 CSI 4118 20
Throughput
Number of bits per second that can be transmitted
Capacity Intuition: “width” of the pipe
SEPT, 2005 CSI 4118 21
Throughput
imeTransfer t / sizeTransfer Throughput
sizeTransfer h1/Bandwidt RTT imeTransfer t
(effective end-to-end throughput)
SEPT, 2005 CSI 4118 22
Components of Delay
Fixed (nearly constant) Propagation delay Switching delays constant
Variable Queuing delay Depends on throughput
SEPT, 2005 CSI 4118 23
Relationship BetweenDelay 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
SEPT, 2005 CSI 4118 24
Define D0 to be the propagation and switching delay U to be the utilization delay D to be the total delay
Then
High utilization known as congestion
Relationship BetweenDelay and Utilization
SEPT, 2005 CSI 4118 25
Practical Consequence
Any network that operates with a utilization approaching 100% of capacity is doomed
SEPT, 2005 CSI 4118 26
Delay-Throughput Product
Delay 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”
SEPT, 2005 CSI 4118 27
Delay x Bandwidth
Bandwidth
Delay
This product is analogous to the volume of a pipe or the number of bits it holds. It corresponds to how many bits the sender must transmit before the first bit arrives at the receiver.
Delay may be thought of as one-way latency or round-trip time (RTT) depending on the context.
SEPT, 2005 CSI 4118 28
Jitter
Network
Interpacket gap
Packetsource
Packetsink
1234 1234
Jitter is a variation (somewhat random) of the latency from packet to packet. Jitter is most often observed when packets traverse multiple hops from source to destination.
Question: What is the cause of jitter?
SEPT, 2005 CSI 4118 29
Summary
Network can be Public Private
Virtual Private Network Uses public network Connects set of private sites Addressing and routing guarantee isloation
SEPT, 2005 CSI 4118 30
Summary (continued)
Networks are Connectionless Connection-oriented
Connection types Permanent Virtual Circuit Switched Virtual Circuit
Two performance measures Delay Throughput
SEPT, 2005 CSI 4118 31
Summary (continued)
Delay and throughput interact Queuing delay increases as utilization
increases Delay x Throughput
Measured in bits Gives total data “in transit”