Download l1b(commSys)

Networks are about
Communication!
•
What is a good model of communication?
A COMMUNICATION
MODEL
xmitted signal
Source
rcvd signal
Channel
noise
Destination
DATA COMMUNICATION
NETWORK
comm.net
comm net node
net station i.e
computer terminal or telephone
DATA COMMUNICATION
NETWORKS
Communication networks
switched
unswitched
DATA COMMUNICATION
NETWORKS
Switched
Circuit Switched Packet Switched
DATA COMMUNICATION
NETWORKS
Unswitched(broadcast nets)
packet radio
satellite
local
Packet switched nets are store and forward nets.
DATA COMMUNICATION
NETWORKS
ckt switched nets have dedicated communication
paths (i.e. telephone)
For example:
n o d e
n e t
s t a t i o
A Broadcast Comm. Net can be WAN, the media is unshared
DATA COMMUNICATION
NETWORKS
Dish
LAN EXAMPLES:
DATA COMMUNICATION
NETWORKS
Ethernet with a bus topology:
DATA COMMUNICATION
NETWORKS
Ring topology net:
Note: No switching nodes ,media is shared in LAN
For packet radio,every station can hear every other,
the media is shared so this is a LAN technology
DATA COMMUNICATION
NETWORKS
Agent
channel
input device
receiver
transmitter
channel
output device
This course addresses the communication systems
parts between the input and output devices.
Computer Communications
Architecture
The Three Layer Model
Application run on computer which use networks
•
•
•
network layer - xchange of data between computer
and network
transport layer - error detection and correction
application layer - file xfer etc.
Computer Communications
Architecture
A p p l i c a t i o n As p p l i c a t i o
T r a n s p o r t l T ar ya en r s p o r t
1 2 3
1 2
3
n e t wo r k
n e t wo r k
n ae ct c e s s
a c c e s sC o m
l
Protocol Data Units
•
TPDU1 = transport protocol data
unit := transport hdr, chunk1
•
•
TPDU2 = transport hdr, chunk2
NPDU = network protocol data
unit network
hdr, TPDU
Protocol Data Units
•
•
•
Application data = chunk1 +
chunk2 + ...
Destination SAP (service access
points)
TPDU has destination SAP so that
appropriate application obtains
data
Protocol Data Units
•
•
•
Sequence # TPDU’s may be sent in any
order,and by different routes and
may arrive at different times.
TPDU may have error detection or correction
codes built-in
NPDU
destination address (i.e. IP address)
facilities requires (i.e. priority)
Transmission Terminology
•
point-to-point :- guided
transmission is unshared
I.e. O(N**2) connectivity
(completely connected)
1 2
L  (N  N) = number of links
2
Transmission Terminology
Multipoint guided transmission allows shared medium
Transmission Terminology
ANSI Defs:
– simplex - one direction only
– half-duplex - two-way but not at the
same time.
– full-duplex - simultaneous two way
transmission
Transmission Terminology
•
CCITT
– CCITT vs ANSI
– simplex = half-duplex
– duplex = full-duplex
•
CCITT = international telegraph and
telephone consultive committee
Analog and Digital Data
Transmission
• data
- entities which convey meaning
•
signals
- encoding of data
•
signaling
- act of sending a signal
•
transmission
- communication of data by
propagation and processing
of signals
Analog and Digital Data
Transmission
•
digital data
- takes on discrete values, i.e. text integers.
•
analog data
- takes on continuously varying patterns of
intensity, i.e.. sound
But what is digital? What is Analog? How
do we defines these things?
Analog and Digital Data
Transmission
s(t) is continuous (i.e. analog) iff
lim s(t)  s(a)
ta
for all a
DATA and SIGNALS
•
Modem (modulator/demodulator)
modulation involves two waveforms: The
modulating signal (the message) and the carrier
wave which is altered by the modulating signal
•
The complementary process is called
demodulation
Bits <-> modem <-> analog signal <-> modem <-> bits
ISO-OSI Model
• ISO=
International
Standards Organization
• OSI = Open Systems
Interconnection
• OSI Model = a 7 layer model
used to describe most
networks today
1. Physical
Layer
•This is a physical
connection between
machines. BITS
2. Data Link
Layer
•Provides reliable
transfer of information
across the physical
links. FRAMES/Packets
or DLPDU
3. Network
Layer
• Routing
4. Transport
Layer
• error
recovery
and flow control
5. Session
Layer
• Control
for
communication, close and
open connections
6. Presentation
Layer
• Data
formatting and
character code
conversion
7. Application
Layer
• file
transfer,
• virtual terminals,
electronic mail
• etc..