Computer Networks
 Fundamental terminology
 Network standards organizations
 Standard network hardware examples
– ethernet
– token ring
– ATM
 Future issues
 Protocols
 OSI layers and internetworking
– bridging and routing
 Web-based commerce
(9.1)
Computer Networks
(9.2)
 Networks are simply a set of computers
connected by a communication medium,
communicating by established conventions
– typically bursty transmission characteristics
 Network goals
– resource sharing
» expensive equipment
» large, reliable data storage
– high reliability
» replication of services
» although network may be single point of failure
– lower overall system costs with better
performance
» a few shared high-end machines, many
individual low-end machines
Computer Networks (continued)
 Network categories
– local area networks (LAN)
» typically single transmission
– wide area networks (WAN)
» typically store and forward
– interconnected networks (internets)
(9.3)
Fundamental Terminology
(9.4)
 Circuit switched / packet switched
– circuit switched = telephone
» connection route established at start of
communication, maintained as long as
communication continues
» route is dedicated to one communication
– packet switched
» whole communication is broken down into
smaller chunks called packets
» each packet may take a different route to the
destination
» routes may be shared by other
communications
Fundamental Terminology (continued)
(9.5)
 Another categorization of packet switched
networks is
– virtual circuit
» packets all take same route, set up in advance
– datagram
» each packet routed independently, no
connection established
» typically more robust
Fundamental Terminology (continued)
 Packets typically consist of
– header
» source and destination addresses
» sequence number
– data portion
– trailer
» error detecting and correcting information
 Packets may be fixed or variable length
head
data
tail
(9.6)
Network Standards Organizations
(9.7)
 International Standards Organization (ISO)
– makes standards for many different activities
 American National Standards Institute (ANSI)
– US representative to ISO
 CCITT
– one part of the UN agency International
Telecommunications Union
– concerned with telephone and data
communication services
– US representative is the State Department
Network Standards Organizations (cont.)
(9.8)
 National Instituted of Standards and
Technology (NIST)
– standards body for US government purchases
 Institute of Electrical and Electronics
Engineers (IEEE)
– key standards for LANs
 Internet Engineering Task Force (IETF)
Local Area Network Standards
(9.9)
 Ethernet (IEEE 802.3)
– Carrier Sense Multiple Access / Collision
Detect (CSMA/CD)
» listen to cable, no one transmitting go ahead
» if collide, delay random time and retransmit
– multiple topologies
– multiple media
» coaxial cable (thick or thin)
» twisted pair wire (10BaseT)
– minimum packet size 64 bytes
» heavy overhead for one byte of data
– fast ethernet technologies allow 100 Mbps on
twisted pair
» 1000 Mbps now available on optical fiber
(9.10)
Local Area Network Standards (continued)
 Token ring (IEEE 802.5)
– token determines which host gets to transmit
» special bit pattern used for token
» when station wants to transmit
•watch network for token
•take token off ring and transmit packet
•drain packet as it comes around again
•put token back on ring when done
– single topology
– one node monitors ring to regenerate token if
necessary
(9.11)
Local Area Network Standards (continued)
 Ethernet
– faster access if light load
– degrades rapidly as load increases
» > 60% utilization is a problem
– no guaranteed access
» host can “starve”
– most popular LAN technology
(9.12)
Local Area Network Standards (continued)
 Token ring
– minimum delay to transmit, since have to wait
for token
– delivers much better performance under
heavy load
» can reach close to 100% utilization
– fair
» host can hold token for 10 msec, then must put
back on network
» each host gets token at least every 10 *
number of hosts msec
– “sick” monitor station can cause problems
Frame Relay and ATM
(9.13)
 Frame Relay
– virtual circuit organization
– uses leased or private lines
» vc set up at time subscribers are connected to
the network
– “light” protocol
» no error checking by network, just delivery
mechanism
– 64 Kbps to 45 Mbps
ATM
(9.14)
 Asynchronous Transfer Mode (ATM)
– being standardized by CCITT
– switched, virtual circuit approach
– allows reserving network capacity
– uses fixed size packets
» easier for hardware to handle
– twisted pair or fiber medium
» 100 m to few km
– 155 Mbps to 2400 Mbps
– also used for LAN interconnect as backbone
Future Issues
(9.15)
 Mobility
– mobility within an office complex is possible
now
» wireless LANs at 10 Mbps
» low power radio connecting to wired
communication
– problem with computing outside the office
– several factors force mobile computing
outside office
» cellular phone popularity
» centrality of computing
» reliance on networking and communication
– problems are
» creating enough bandwidth
•possibly need multiple channels
» not enough spectrum
•current transmission / broadcast activities
may swap
Network Protocols
 A protocol is a set of conventions for
communicating
 Concerned with issues such as
– addressing and routing
– error detection and correction
– flow control and congestion control
– higher level coordination depending on the
purpose of the communication
 International Standards Organization Open
Systems Interconnection model (ISO OSI)
– 7 layers of protocols
– defined interfaces between each layer
– lower layers typically implemented in
hardware, upper in software
(9.16)
Network Protocols (continued)
(9.17)
Network Protocols (continued)
(9.18)
 ISO layers
– application - detailed information about data
being exchanged
– presentation - conventions for representing
data
– session - management of connections
between programs
– transport - delivery (reliable or otherwise) of
sequences of packets
– network - format of individual data packets
– link - access to and control of transmission
medium
– physical - medium of transmission
 Internet protocols (IP) are a competing set for
lower layers
– network and transport
– defacto standard
Bridging
 Bridging happens at Link layer
– between LANs
– provides
» filtering - only forward packets destined offnet
» repacketization
» speed matching
(9.19)
Routing
(9.20)
 Internetworking (in WANs)
– happens at network layer
– routers connect WANs with very dissimilar
characteristics
» also called gateways
 Routing
– routers know whole network topology
» determines which network to send packet on
next from destination address
– protocol conversion
– fragmentation
» when packet too large for next network it must
pass through
Routing (continued)
(9.21)
Routing (continued)
(9.22)
(9.23)
Internet Futures
 Current IP protocol is
running out of addresses
– 32 bits = 4 billion
addresses
– Class A - 127 large
groups with 16M
machines
A
 Revised IP protocols (IPng)
currently being devised
– 128 bit addresses
– also called IPv6
24
0
B
14
1
» countries
– Class B - 16K networks
with 64K or fewer
machines
– Class C - 2M networks
with 256 machines
– each machine can only
have one network
address
7
16
0
C
21
1
1
0
Multicast
1
1
8
28
1
0
(9.24)
Web-based Commerce
 Conducting business (exchanging money)
over the Internet requires several features that
aren’t yet available
– confidentiality
» no one else listens to the transaction
encryption
– authentication
» you really are who I think you are
– data integrity
» no one increases my order enroute
– nonrepudiation
» I can’t back out after I buy
– selective application of services
digital
signatures and
certificates
» seller never sees my credit card number
» I seal it and only give name of the bank, who
authorizes payment
Web-based Commerce (continued)
(9.25)
 Public-key systems provide confidentiality
– I choose 2 keys, one public and one private
» publish public key widely
– I encode with private key
– you decode with public key
or
– you encode with public key
– I decode with private key
Web-based Commerce (continued)
(9.26)
 Digital signatures use public key algorithms,
usually on digest of message to speed up
– encode message digest (hash function)
– cleartext message also digested at receiving
end
– if my decoded digest matches calculated
one, I sent it and it hasn’t been changed
Web-based Commerce (continued)
(9.27)
 Authentication provided by authentication
entities
– I don’t send order directly to Nordstroms
– send to authenticator instead, who verifies I’m
me and sends the order to a prestored
Nordstroms address
– other variations exist
 Several proposals to modify IP protocols at
various levels to help this
– IP protocol
– Secure Sockets Layer session protocol
– secure HTTP and MIME messaging