1. A Taxonomy of Communication Networks
Y. Richard Yang
1/16/2008

2. Outline
Recap
A taxonomy of communication networks
Summary

3. Recap
A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission or receipt of a message or other events.
Some implications of the past:
ARPANET is sponsored by ARPA 
The initial IMPs (routers) were made by a small company 
Many networks 
Commercialization 
design should survive failures
keep the network simple
internetworking: need a network to connect networks
architecture supporting distributed, autonomous systems

4. Recap: Current Internet Physical Infrastructure
Residential access
Cable
Fiber
DSL
Wireless
Backbone ISP
ISP
The Internet is a network of networks
Each individually administrated network is called an Autonomous System (AS)
Campus access, e.g.,
Ethernet
Wireless

5. Northern CrossRoads (NoX) Aggregation Point (AP)

6. Abilene I2 Backbone

7.
Qwest Backbone Map

8. ATT Global Backbone IP Network
From

9. AT&T USA Backbone Map
From AT&T web site.

10. Commercial Internet ISP Connectivity
Roughly hierarchical
Divided into tiers
Tier-1 ISPs are also called backbone providers, e.g., AT&T, Verizon, Sprint, Level 3, Qwest
An ISP runs (private) Points of Presence (PoP) where its customers and other ISPs connect to it
ISPs also connect at (public) Network Access Point (NAP)
called public peering

11. Network Access Point Interconnect multiple ISP’s
Example: Chicago NAP (

12. Outline
Admin. and recap
A taxonomy of communication networks
Summary

13. A Taxonomy of Communication Networks
So far we have looked at only the topology and physical connectivity of the Internet: a mesh of computers interconnected via various physical media
The fundamental question: how is data (the bits) transferred through a communication network?

14. Broadcast vs. Switched Communication Networks
switched networks
broadcast networks
Broadcast networks
nodes share a common channel; information transmitted by a node is received by all other nodes in the network
examples: TV, radio
Switched networks
information is transmitted to a small sub-set (usually only one) of the nodes

15. A Taxonomy of Switched Networks
communication networks
switched networks
broadcast networks
circuit-switching networks (e.g. telephone, GSM)
packet-switching networks (e.g. Internet)
Circuit switching: dedicated circuit per call/session:
e.g., telephone, GSM High-Speed Circuit-Switched Data (HSCSD), Integrated Services Digital Networks (ISDN)
Packet switching: data sent thru network in discrete “chunks”
e.g., Internet, General Packet Radio Service (GPRS)

16. Outline Admin. and review A taxonomy of communication networks Summary
circuit switching networks
packet switching networks
circuit switching vs. packet switching
datagram switching vs. virtual circuit switching
Summary

17. Circuit Switching Each link has a number of “circuits”
sometime we refer to a “circuit” as a channel or a line
An end-to-end connection reserves one “circuit” at each link
First commercial telephone switchboard was opened in 1878 to serve the 21 telephone customers in New Haven, Connecticut

18. Circuit Switching: Resources/Circuits (Frequency, Time and others)
Divide link resource into “circuits”
frequency division multiplexing (FDM)
time division multiplexing (TDM)
others such as code division multiplexing (CDM), color/lambda division

19. Circuit Switching: The Process
Three phases
circuit establishment
data transfer
circuit termination

20. processing delay at Node 1
Timing Diagram of Circuit Switching
Host A
Host B
Node 1
Node 2
processing delay at Node 1
propagation delay
from A to Node 1
circuit establishment
propagation delay
from B To A
DATA
data transmission
circuit termination

21. Delay Calculation in Circuit Switched Networks
Propagation delay: delay for the first bit to go from a source to a destination
Transmission delay: time to pump data onto link at reserved rate
d/s
DATA
L/R
Propagation delay:
d = length of physical link
s = propagation speed in medium (~2x105 km/sec)
propagation delay = d/s
Transmission delay:
R = reserved bandwidth (bps)
L = message length (bits)
time to send a packet into link = L/R

22. An Example Propagation delay Transmission delay
suppose the distance between A and B is 4000 km, then one-way propagation delay is:
Transmission delay
suppose we reserve a one slot HSCSD channel
a frame can transmit about 115 kbps
A frame is divided into 8 slots
each reserved one slot HSCSD has a bandwidth of about 14 Kbps (=115/8)
then the transmission delay of a message of 1 Kbits:

23. An Example (cont.)
Suppose the setup message is very small, and the total setup processing delay is 200 ms
Then the delay to transfer a message of 1 Kbits from A to B (from the beginning until host receives last bit) is:
DATA 20 20 70

24. Outline Admin. and review A taxonomy of communication networks Summary
circuit switching networks
packet switching networks
circuit switching vs. packet switching
Summary

25. Packet Switching
Each end-to-end data flow (i.e., a sender-receiver pair) divided into packets
Packets have the following structure:
header and trailer carry control information (e.g., destination address, check sum)
where is the control information for circuit switching?
any analogy in life?
At each node the entire packet is received, processed (e.g., routing), stored briefly, and then forwarded to the next node; thus packet-switched networks are also called store-and-forward networks
Header
Data
Trailer

26. Packet Switching

27. Inside a Packet Switching Router
An output queueing switch
incoming links
node
outgoing links
Memory

28. Packet Switching: Resources
Resources used as needed
On its turn, a packet uses full link bandwidth

29. Outline Admin. and review A taxonomy of communication networks Summary
circuit switching networks
packet switching networks
circuit switching vs. packet switching
Summary

30. Packet Switching vs. Circuit Switching
The early history of the Internet was a heated debate between Packet Switching and Circuit Switching
the telephone network was the dominant network
Need to compare packet switching with circuit switching

31. Circuit Switching vs. Packet Switching
resource
partitioned
not partitioned
when using resource
use a single partition bandwidth
use whole link bandwidth
reservation/setup
need reservation (setup delay)
no reservation
resource contention
busy signal (session loss)
congestion (long delay and packet losses)
header
no header
per packet header
processing
fast
per packet processing

32. Key Issue to be Settled
All the other comparisons are easy to understand, a key debating issue was whether resource partition would be inefficient.
Tool used to analyze the issue: queueing theory

33. Analysis of Circuit-Switching Blocking (Busy) Time
Assume N circuits
Session arrival:  per second
Session service rate:  per second
What is the percentage of time that a new session is blocked?
For a demo of M/M/1, see:

34. Analysis of Delay at a Link
Four types of delay at each hop
nodal processing delay: check errors & routing
queueing: time waiting for its turn at output link
transmission delay: time to pump packet onto a link at link speed
propagation delay: router to router propagation
The focus is on transmission delay and propagation delay

35. The Key Case: To Partition or not to Partition?
Assume:
R = link bandwidth (bps)
L = average packet length (bits)
a = average packet arrival rate (pkt/sec)
R/L = packet service rate (pkt/sec)
Setup: n streams; each stream has an arrival rate of a/n
Comparison: each stream reserves 1/n bandwidth or not
Case 2 (reserve): the arrivals are divided into n links with rate R/n each, what is the queueing delay + transmission delay?
Case 1 (not reserve): all arrivals into a single link with rate R, what is the queueing delay + transmission delay?

36. Analysis of Queueing + Transmission Time
Consider a single queue
Packet arrival rate:  per second
Packet service rate:  per second
What is the queueing + transmission time of each packet?

37. Delay: M/M/1 Model Assume: R = link bandwidth (bps)
L = average packet length (bits)
a = average packet arrival rate (pkt/sec)
R/L = packet service rate (pkt/sec)

38. Queueing Delay as A Function of Utilization
Assume:
R = link bandwidth (bps)
L = packet length (bits)
a = average packet arrival rate (pkt/sec)
R/L = packet service rate (pkt/sec) 
 ~ 0: average queueing delay small
 -> 1: delay becomes large
 > 1: more “work” arriving than can be serviced, average delay infinite !

39. Statistical Multiplexing Gain
Assume:
R = link bandwidth (bps)
L = average packet length (bits)
a = average packet arrival rate (pkt/sec)
R/L = packet service rate (pkt/sec)
Setup: n streams; each stream has an arrival rate of a/n
Comparison: each stream reserves 1/n bandwidth or not
Case 1 (not reserve): all arrivals into a single link with rate R
Case 2 (reserve): the arrivals are divided into n links with rate R/n each
any analogy in life?

40. Packet Switching vs. Circuit Switching
One of the major issues facing the Internet: How to provide circuit-like behavior (in terms of quality of service)?
virtual circuit switching is originally mainly for routing efficiency
bandwidth guarantees needed for audio/video apps
still an unsolved problem

41. Outline Admin. and review A taxonomy of communication networks Summary
circuit switching networks
packet switching networks
circuit switching vs. packet switching
routing in packet switching networks
Summary

42. A Taxonomy of Packet-Switched Networks According to Routing
Goal: move packets among routers from source to destination
we’ll study routing algorithms later in the course
Two types of packet switching
datagram network
each packet of a flow is switched independently
virtual circuit network:
all packets from one flow are sent along a pre-established path (= virtual circuit)

43. Datagram Packet Switching
Example: IP networks
Each packet is independently switched
each packet header contains complete destination address
receiving a packet, a router looks at the packet’s destination address and searches its current routing table to determine the possible next hops, and pick one
Discussions:
an example of datagram-style communication in daily life?
advantages of datagram switching?
potential problems of packet switching?
Disadvantages: routes may change during session
Advantage: routers do not keep any state about a flow;
thus, network architecture is
robust
scalable

44. Datagram Packet Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Host E
Node 7
Node 6
Node 4

45. Timing Diagram of Datagram Switching
Host A
Host B
Node 1
Node 2
processing and queueing delay of Packet 1 at Node 2
propagation
delay from
Host A to
Node 1
transmission
time of Packet 1
at Host A
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3

46. Virtual-Circuit Packet Switching
Example: Asynchornous Transfer Mode (ATM) networks; Multiple Label Packet Switching (MPLS) in IP networks
Hybrid of circuit switching and datagram switching
each packet carries a short tag (virtual-circuit (VC) #); tag determines next hop
fixed path determined at Virtual Circuit setup time, remains fixed thru flow
routers maintain per-flow state
what state do routers maintain for datagram switching?
Incoming Interface
Incoming VC#
Outgoing Interface
Outgoing VC# 1 12 2 22 16 3 …
What advantages do virtual circuit have over datagram?
can treat different flows differently, if flows setup its desired treatment
Guarantees in-sequence delivery of packets
However: Packets from different virtual circuits may be interleaved

47. Virtual-Circuit Switching
Host C
Host D
Host A
Node 1
Node 2
Node 3
Node 5
Host B
Host E
Node 7
Node 6
Node 4

48. Virtual-Circuit Packet Switching
Three phases
VC establishment
Data transfer
VC disconnect

49. Timing Diagram of Virtual-Circuit Switching
Host 1
Host 2
Node 1
Node 2
propagation delay
between Host 1
and Node 1 VC
establishment
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
data
transfer
Packet 1
Packet 2
Packet 3 VC
termination

50. Discussion: Datagram Switching vs. Virtual Circuit Switching
What are the benefits of datagram switching over virtual circuit switching?
What are the benefits of virtual circuit switching over datagram switching?

51. Summary of the Taxonomy of Communication Networks
circuit-switched network
communication network
switched network
broadcast communication
packet-switched network
datagram network
virtual circuit network

52. Backup Slides

53. Packet Switching
Packet-switching:
store and forward behavior

54. Connection-Oriented Service
Goal: data transfer between end sys.
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 retransmissions
flow control:
sender won’t overwhelm receiver
congestion control:
senders “slow down sending rate” when network congested

55. Connectionless Service
Goal: data transfer between end systems
same as before!
UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service
unreliable data transfer
no flow control
no congestion control
App’s using TCP:
HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP ( )
App’s using UDP:
streaming media, teleconferencing, Internet telephony

56. Relationship Between Switching Techniques and End Host Services
End hosts determine end host services: connection-oriented or connectionless
what an application wants
Network service providers determine network services: circuit-switching, packet switching, datagram switching, or virtual circuit switching
how the ISP builds their networks

57. Bandwidth division into “pieces”
Packet Switching: Resources
Resources used as needed
On its turn, a packet uses full link bandwidth
Aggregated resource demand can exceed amount available
congestion: packets queue, wait for link use
buffer overflow: packet losses
Bandwidth division into “pieces”
Resource reservation

58. Packet Switching vs. Circuit Switching
The early history of the Internet was a heated debate between Packet Switching and Circuit Switching
Advantages of packet switching over circuit switching
most important advantage of packet-switching over circuit switching is statistical multiplexing, and therefore efficient bandwidth usage
no call setup (for datagram switching only)
no per-flow state information (for datagram switching only)
simple to implement
Disadvantages of packet switching
potential congestion: packet delay and high loss
protocols needed for reliable data transfer, congestion control
packet header overhead
per packet processing overhead
Questions: why does the current Internet use datagram?

59. Statistical Multiplexing
Assume:
R = link bandwidth (bps)
L = average packet length (bits)
a = average packet arrival rate (pkt/sec)
R/L = packet service rate (pkt/sec)
Setup: N flows; each flow has an arrival rate of a/n
Comparison: each flow reserves 1/n bandwidth or not
Case 2 (reserve): the arrivals are divided into n links with rate R/n each, what is the queueing delay + transmission delay?
Case 1 (not reserve): all arrivals into a single link with rate R, what is the queueing delay + transmission delay?

60. Queueing Delay: Kendall Notation
Arrival/Service/NumberServers
E.g., M/M/1, M/D/1
Arrival process: the inter-arrival time
D: fixed/deterministic inter-arrival time
M: Markov, i.e., in each  unit of time, where  small, the probability of exactly one arrival is , where  is called arrival rate
Service process: the service time
D: fixed/deterministic service time
M: Markov, i.e., in each  unit of time, where  small, the probability of finish serving is , where  is called service rate
For a demo of M/M/1, see:

61. Queueing Delay: M/M/1 Model
Assume:
R = link bandwidth (bps)
L = average packet length (bits)
a = average packet arrival rate (pkt/sec)
R/L = packet service rate (pkt/sec)
A more realistic model of modern routers: M/D/1