Download - Communication concepts (Continued)
Communication concepts(Continued)
Week 2 Lecture 2
Agenda Yesterday, we covered
Media Analogue & digital transmission Multiplexing
Today we will cover Circuit & packet switching Virtual circuits & datagrams Latency & bandwidth
Multiplexing Transmission capacity of the media is
often much greater than the needs of any one user
This capacity can be shared by allowing simultaneous transmission of multiple signals on a single data link
This technique is known as multiplexing There is one device to combine the
signals – a mux and one to separate them again – a demux – at each end of the link
Wide Area Network
Local Area Network
Home ISP
ISP & Telco (IBP)
The last kilometre
Business
WiFiNetwork
Home computer
ISPPC is in Sydney
Web server is inNew York
How do our messagesget through to New York?
Hong Kong
LondonNew York
Sydney
How do we connect two hosts across the world?
Circuit switching Packet switching
Circuit switching The resources required for a connection are
reserved for the duration of the connection. Good for voice, with low bandwidth and
relatively constant usage Set up time to establish a circuit While the circuit is reserved this does not
imply physical resources totally reserved (multiplexing)
When used by data, transfer rate is constant
Packet switching Data is bursty, often high data rates for
short periods, and low usage for other parts of the connection. Inefficient use of circuits
Packet switching breaks the message into small packets, wraps an electronic envelope with address and sends it through the network with other packets from other users
Circuit switching
Packet switching
Packet switching
Packets can be routedthrough the network
Packet switching like public road usage
Driveway Many vehicles share
the road Junctions allow cars to
change direction Map determines
which roads to take to get to destination
Telephone connects to PSTN
Data packets share the link with data from other connects
Switches allow data also to change direction
Packet address instructs switch as to which links should be taken
Multiplexing is not the same as Packet switching Multiplexing allows a physical link to
carry multiple circuits Any one circuit can carry packets for a
number of user “connections” Think of how a road can be divided into
multiple lanes One lane can carry a large number of cars
going to different destinations Another lane may be only available to buses
going from A to B.
Two types of addressing Virtual circuit –
the path is established at the beginning defining each segment the data is to be passed along for the period of the connection
Each of the switches must maintain state information about the connection
X.25, Frame relay and ATM use this form of addressing
Two types of addressing (cont) Datagram
The packet has a final address in a hierarchical form similar to that on an envelope
Switches maintain tables that map destination addresses to outbound links and use this to send the message along
IP uses datagram addressing IP can be carried over virtual circuit protocol
links like X.25 & ATM
Packets can go via anypathPackets thus can get out of sequenceAssembled in right order at end
Store & Forward Packets are routed through a number of
switches in their journey Each switch uses a store & forward mechanism The packet must be totally received and
checked before being sent on the outward link Sometimes the outward link is busy, thus the
packet must be buffered Sometimes buffers are full and the packet is
dropped, and must be sent again This delay is called latency, and the uncertain
nature of this latency is one of the problems with packet switching
Packet V Message switching Why break the message into
packets? Messages can be quite long – eg on
a file transfer In packets you get a parallel
processing through each switch Many messages fit into a packet But packets have an overhead of
data and processing
Quick quiz
You have a 10 second message to transmit through two switches. Will sending it asone whole message, or as 10x1 second packetstake longer, end to end?
10 second file transfer
A B C D
10 second file transfer
A B C D
Message switchA to B 10B to C 10C to D 10
Total transmit time is 30
Packet switch (10 x 1)
A-B B-C C-D
1 sec 1 sec 1 secA-B B-C C-D
1 sec 1 sec 1 secA-B B-C C-D
1 sec 1 sec 1 sec
P1
P2
P3
Total transmit time is 10+2= 12 seconds
Latency & Bandwidth Bandwidth is the speed at which data is
passed down a specific link. Usually expressed in bits per second – bps
Latency is the collective delay from all of the switches in the trip to the final destination
As bandwidth increases, then latency becomes more of an issue
Read Kurose pages 41 to 49 for a more complete discussion