Circuit switching and packet switching

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