The document discusses packet switching and circuit switching in computer networks. It defines packet switching as dividing data into discrete chunks or packets that are sent through the network and shared across network resources. In circuit switching, dedicated circuits are established for each connection, reserving bandwidth for the duration even if not in use. The document also outlines the hierarchical structure of the Internet, made up of tier 1, 2, and 3 ISPs that connect end users and pass traffic through multiple networks of routers and switches.

1. Network Core
Packet Switching and Circuit Switching
Engr. Abbas Abbasi

2. Important Terms Review
 DSL
 LAN
 DSLAM
 3G
 HFC
 LTE
 CMTS
 WiFi
 FTTH
 Physical Medium Types
 AON
 UTP
 PON
 Shared Medium
 ONT
 OC (Optical Carrier)
 OLT
 Geostationary Satellite
 LEO Satellite

3. 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”
 Routers Vs Switches
 Message length L, Transmission
rate R, and transmission delay

4. Network Core: Circuit Switching
network resources (e.g.,
bandwidth) divided into
“pieces”


 dividing link bandwidth into
“pieces”
 frequency division
pieces allocated to calls
 time division (TDM)
resource piece idle if not used by
 code division (CDM)
owning call (no sharing)

5. Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time

6. Numerical example
 How long does it take to send a file of 640,000 bits from host
A to host B over a circuit-switched network?
 All links are 1.536 Mbps
 Each link uses TDM with 24 slots/sec
 500 msec to establish end-to-end circuit
Let’s work it out!
t = (0.500 s)+ (640,000 bits)/((1,536,000 bits/s)/ (24 channels))
= 0.5 + 10.0 = 10.5 seconds until last bit leaves Host A
but how long until last bit arrives at Host B?

7. Network Core: Packet Switching
each end-end data stream divided into
packets
 user A, B packets share network
resources
 each packet uses full link bandwidth
(for short time)
 resources used as needed
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
resource contention:
 aggregate resource demand
can exceed amount available
 congestion: packets queue,
wait for link use
 store and forward: packets
move one hop at a time
 Node receives complete
packet before forwarding

8. Packet Switching: Statistical Multiplexing
100 Mb/s
Ethernet
A
B
C
statistical multiplexing
1.5 Mb/s
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed pattern, shared on demand
statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.

9. Packet-switching: store-and-forward
L
R
R
 Takes L/R seconds to transmit
(push out) packet of L bits on
to link or R bps
 Entire packet must arrive at
router before it can be
transmitted on next link: store
and forward
 delay = 3L/R (assuming zero
propagation delay)
R
Example:
 L = 7.5 Mbits
 R = 1.5 Mbps
 delay = 15 sec
Note: local connection so that
(a) propagation time is
negligible, and (b) no delay
due to congestion.
more on delay shortly …

10. Packet switching versus circuit switching
Packet switching allows more users to use network!
 1 Mb/s link
 each user:
 100 kb/s when “active”
 active 10% of time
1 Mbps link
 circuit-switching:
 10 users
 packet switching:
 with 35 users, probability > 10
active less than .0004
N users
Q: how did we get value 0.0004?
P(n,N) =(N!) ( p^n) (1-p)^(N-n)
(n!) (N-n)!
P(11,35) =(35!) ( 0.1^11) (0.9^24)
(11!) (24!)
= 0.0003
P(12,35) = 0.0001

11. Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
 Great for bursty data
resource sharing
simpler, no call setup
 If excessive congestion: packet delay and loss.
protocols needed for reliable data transfer, congestion control
(TCP does a good job)
 Q: How to provide circuit-like behavior?
bandwidth guarantees needed for audio/video apps
still an unsolved problem (chapter 7)

12. Internet structure: network of networks
 roughly hierarchical
 at center: “tier-1” ISPs (e.g., MCI, Sprint, AT&T, Cable and Wireless),
national/international coverage
 treat each other as equals
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
NAP
Tier 1 ISP
Tier-1 providers
also interconnect
at public network
access points
(NAPs)

13. Tier-1 ISP: e.g., Sprint
Sprint US backbone network
Seattle
Tacoma
DS3 (45 Mbps)
OC3 (155 Mbps)
OC12 (622 Mbps)
OC48 (2.4 Gbps)
POP: point-of-presence
to/from backbone
Stockton
peering
…
…
.
Kansas City
…
…
Anaheim
…
San Jose
Cheyenne
New York
Pennsauken
Relay
Wash. DC
Chicago
Roachdale
Atlanta
to/from customers
Fort Worth
Introduction
Orlando

14. Internet structure: network of networks
 “Tier-2” ISPs: smaller (often regional) ISPs
 Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
 tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
NAP
Tier 1 ISP
Tier-2 ISP
* NAP - Network Access Point
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP*
Tier-2 ISP

15. Internet structure: network of networks
 “Tier-3” ISPs and local ISPs
 last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP

16. Internet structure: network of networks
 a packet passes through many networks!
local
ISP
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP

17. IXPs and Content Provider Networks
IXP: Internet Exchange Point

18. Important Terms
 Store and Forward







Transmission
Messages and Packets
Packet Switch
Transmission Delay
Queuing Buffer or Output
Buffer
Packet Loss
Forwarding Table
Routing Protocols
 Circuit
 Circuit Switching
 End to End Connection
 FDM
 TDM
 Silent Period
 Tier 1,2,3 ISPs
 Access ISP
 IXP