1. Computer Networks & The Internet University of Management & Technology
Lecture 2
Imran Ahmed
University of Management & Technology

2. Agenda Network & its types What’s the Internet? What’s a protocol?
History
Network edge
Network core
Access net, physical media
Internet/ISP structure
Performance: loss, delay
Protocol layers, service models

3. A closer look at network structure:
network edge: applications and hosts
network core:
routers
network of networks
access networks, physical media: communication links

4. The network edge: end systems (hosts): client/server model
run application programs
e.g. Web,
at “edge of network”
client/server model
client host requests, receives service from always-on server
e.g. Web browser/server; client/server
peer-peer model:
minimal (or no) use of dedicated servers
e.g. Gnutella, KaZaA

5. Network edge: connection-oriented service
Goal: data transfer between end systems
Handshaking: setup (prepare for) data transfer ahead of time
Hello, hello back human protocol
Set up “state” in two communicating hosts
TCP – Transmission Control Protocol
Internet’s connection-oriented service
TCP service [RFC 793]
Reliable, in-order byte-stream data transfer
Loss: acknowledgements and retransmission
Flow control:
Sender won’t overwhelm receiver
Congestion control:
Senders “slow down sending rate”, when network congested

6. Network edge: connectionless service
Goal: data transfer between end systems
Same as before!
UDP – user Datagram Protocol [RFC768]:
Connectionless
Unreliable data transfer
No flow control
No congestion control
App’s using TCP:
HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP ( )
App’s using UDP:
Streaming media, teleconferencing, DNS, Internet telephony

7. Agenda Network & its types What’s the Internet? What’s a protocol?
History
Network edge
Network core
Access net, physical media
Internet/ISP structure
Performance: loss, delay
Protocol layers, service models

8. The Network Core mesh of interconnected routers
the fundamental question: how is data transferred through net?
circuit switching: dedicated circuit per call: telephone net
packet-switching: data sent thru net in discrete “chunks”

9. Network Core: Circuit Switching
End-end resources reserved for “call”
link bandwidth, switch capacity
dedicated resources: no sharing
circuit-like (guaranteed) performance
call setup required

10. Circuit Switching It’s the method used by the telephone network.A B
Source
Destination
It’s the method used by the telephone network.
A call has three phases:
Establish circuit from end-to-end (“dialing”),
Communicate,
Close circuit (“tear down”).
Originally, a circuit was an end-to-end physical wire.
Nowadays, a circuit is like a virtual private wire: each call has its own private, guaranteed data rate from end-to-end.

11. Circuit Switching Telephone Network
Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000 calls.
Destination
“Callee”
Source
“Caller”
Central
Office
“C.O.”
Central
Office
“C.O.”
Trunk
Exchange

12. Packet Switching It’s the method used by the Internet.
Source
Destination R1 R3 R4
It’s the method used by the Internet.
Each packet is individually routed packet-by-packet, using the router’s local routing table.
The routers maintain no per-flow state.
Different packets may take different paths.
Several packets may arrive for the same output link at the same time, therefore a packet switch has buffers.

13. Packet Switching Simple router model
Link 1, ingress
Link 1, egress
“4”
Choose
Egress
Link 2
Link 2, ingress
Choose
Egress
Link 2, egress R1
“4”
Link 1
Link 3
Link 3, ingress
Choose
Egress
Link 3, egress
Link 4
Link 4, ingress
Choose
Egress
Link 4, egress

14. Why does the Internet use packet switching?
Efficient use of expensive links:
The links are assumed to be expensive and scarce.
Packet switching allows many, bursty flows to share the same link efficiently.
“Circuit switching is rarely used for data networks, ... because of very inefficient use of the links” - Gallager
Resilience to failure of links & routers:
”For high reliability, ... [the Internet] was to be a datagram subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” - Tanenbaum
Breaking message into packets allows parallel transmission across all links, reducing network latency.
In summary the benefits that of packet switched networks can be summarized as follows:
They use the bandwidth efficiently, meaning that a trunk link uses less resources than the sum of its tributaries, as they multiplex and conserve bandwidth
They have little state in the intermediate nodes
They are robust, some claim that they were designed to withstand a nuclear attack
They do not have a central authority from whom we need permission to run experiments

15. Some Definitions Packet length, P, is the length of a packet in bits.
Link length, L, is the length of a link in meters.
Data rate, R, is the rate at which bits can be sent, in bits/second, or b/s.1
Propagation delay, PROP, is the time for one bit to travel along a link of length, L. PROP = L/c.
Transmission time, TRANSP, is the time to transmit a packet of length P. TRANSP = P/R.
Latency is the time from when the first bit begins transmission, until the last bit has been received. On a link: Latency = PROP + TRANSP.
1. Note that a kilobit/second, kb/s, is 1000 bits/second, not 1024 bits/second.

16. Packet Switching: Store-and-forward

17. Packet Switching Host A Host B A B Source Destination
TRANSP1
“Store-and-Forward” at each Router
TRANSP2 R1
PROP1
TRANSP3 R2
PROP2
TRANSP4 R3
PROP3
Host B
PROP4

18. Agenda Network & its types What’s the Internet? What’s a protocol?
History
Network edge
Network core
Access net, physical media
Internet/ISP structure
Performance: loss, delay
Protocol layers, service models

19. Access networks and physical media
Q: How to connect end systems to edge router?
residential access nets
institutional access networks (school, company)
mobile access networks
Keep in mind:
bandwidth (bits per second) of access network?
shared or dedicated?

20. Internet access technologies
Previously, most people use 56K dial-up lines to access the Internet, but a number of new access technologies are now being offered.
The main new access technologies are:
Digital Subscriber Line (DSl,ADSL)
Cable Modems
Local Area Networks
Wireless Networks

21. Access networks: DSL & ADSL
Digital Subscriber Line (DSL) is one of the most used technologies now being implemented to significantly increase the data rates over traditional telephone lines.
Historically, voice telephone circuits have had only a limited capacity for data communications because they were constrained by the 4 kHz bandwidth voice channel.
Most local loop telephone lines actually have a much higher bandwidth and can therefore carry data at much higher rates.

22. Access networks: DSL & ADSL
DSL services are relatively new and not all common carriers offer them.
Two general categories of DSL services have emerged in the marketplace.
Symmetric DSL (SDSL) provides the same transmission rates (up to 128 Kbps) in both directions on the circuits.
Asymmetric DSL (ADSL) provides different data rates to (up to 640 Kbps) and from (up to Mbps) the carrier’s end office. It also includes an analog channel for voice transmissions.

23. DSL Architecture Customer Premises Local Carrier End Office DSL Modem
Line Splitter
Main
Distribution
Frame
Voice
Telephone
Network
Local Loop
Hub
Telephone
ISP POP
ATM Switch
Computer
DSL Access
Multiplexer
Computer
ISP POP
Customer
Premises
ISP POP
ISP POP
Customer
Premises

24. Access networks: Cable modems
One potential competitor to DSL is the “cable modem” a digital service offered by cable television companies which offers an upstream rate of Mbps and a downstream rate of 2-30 Mbps.
A few cable companies offer downstream services only, with upstream communications using regular telephone lines.

25. Cable Modem Architecture
Customer Premises
Cable Company
Fiber Node
Cable Company Distribution Hub
TV Video
Network
Cable Modem
Cable Splitter
Downstream
Combiner
Optical/Electrical
Converter
Upstream
Hub TV
Router
Shared
Coax
Cable
System
Cable
Company
Fiber Node
Cable Modem
Termination
System
Computer
Computer
ISP POP
Customer
Premises
Customer
Premises
Cable Modem Architecture

26. Access networks: Local area networks
Company/univ. local area network (LAN) connects end system to edge router
Ethernet:
Shared or dedicated link, connects end systems and router
10 Mbs, 100mbs, Gigabit Ethernet etc.
Details will be available in future

27. Access networks: Wireless networks
Shared wireless access network connects end system to router via base stations aka “access point”
Wireless Lans:
802.11b (WiFi): 11 Mbps
Wide-area wireless access:
Provided by telecommunication companies
WAP/GPRS etc.
Satellite:
Up to 50 Mbps channels or multiple smaller channles

28. Physical Media Bit: propagates between transmitter/rcvr pairs
Physical link: What lies between transmitter & receiver
Guided media:
Signals propagates in solid media; copper, fiber, coax.
Unguided media:
Signals propagates freely, e.g., radio
Twisted Pair (TP)
Two insulated copper wires:
Category 3: traditional phone wires, 10 Mbps (Ethernet)
Category 5: 100 Mbps (Ethernet)

29. Physical Media Coaxial cable: Fiber optic cable:
Two concentric copper conductors
Bidirectional
Baseband:
Single channel on cable
Broadband:
Multiple channel on cable
Fiber optic cable:
Glass fiber carrying light pulses, each pulse a bit
High-speed operation:
High-speed point-to-point transmission (e.g., 5 Gps)
Low error rate: repeaters spaced far apart; immune to electromagnetic noise

30. Summary
We studied about network edge: end systems etc.
Circuit switching & packet switching.
Internet access technologies: DSL, cable modem etc.
Physical medias: Twisted pair, coaxial pair & fiber optics.