1. Comp 410 AOS Packet Switching
These slides derived from Computer Networking: A Top Down Approach , 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.
Introduction

2. 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”
Resources reserved
Resources allocated on demand.
Core Resources: buffers, link transmission rate
Introduction

3. The Network Core The internet is packet switched
Telephone network is circuit switched
Introduction

4. 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
State maintained
Data transferred at a guaranteed rate
Introduction

5. Network Core: Circuit Switching
End-end resources reserved for “call”
Links between circuit switches
Each link can support n circuits
There can be n simultaneous connections.
Each circuit thus gets 1/n of the link’s bandwidth.
Introduction

6. Network Core: Circuit Switching
network resources (e.g., bandwidth) divided into “pieces”
pieces allocated to calls
resource piece idle if not used by owning call (no sharing)
When two hosts want to communicate network establishes a dedicated end-to-end connection between the hosts.
dividing link bandwidth into “pieces”
frequency division
time division
Introduction

7. Circuit switching: analysis
Disadvantages:
Network resources are wasted during “silent” times (when no one is talking but still connected)
Establishing end-to-end circuits and reserving end-to-end bandwidth is complicated and requires complex signaling software.
Advantage:
Guaranteed bandwith and transmission time
Necessary for some applications (streamed music/video)
Introduction

8. Network Core: Packet Switching
transmission:
Each end-end data stream divided into packets
Each packet travels through communication links
Links connected by packet switches
Routers
Or link-level switches.
Switches use store-and-forward transmission
Switch receives entire packet before it transmits any of it again
Introduction

9. Network Core: Packet Switching
transmission:
Introduction

10. 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
resources used as needed
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
Introduction

11. Packet-switching: store-and-forwardL R R R
This Figure:
takes L/R seconds to transmit (push out) packet of L bits on to link at R bps
store and forward: entire packet must arrive at router before it can be transmitted on next link
delay = 3L/R (assuming zero propagation delay)
Example:
L = 7.5 Mbits
R = 1.5 Mbps
transmission delay = ??
15 sec
more on delay shortly …
Introduction

12. Network Core: Packet Switching
Switching delays:
Each switch has multiple links
Each link has buffer
If packet arrives and another packet is already being transmitted on that link, must wait in queue
Called queuing delay.
Varies depending on network congestion
Packet loss: queue is full when packet arrives.
Introduction

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

14. Packet switching versus circuit switching
Packet switching allows more users to use network!
1 Mb/s link
For each user assume:
100 kb/s when “active”
active 10% of time
circuit-switching:
10 users
packet switching:
with 35 users, probability > 10 active at same time is less than .0004
N users
1 Mbps link
Q: how did we get value ?
Introduction

15. Routing
How do packets make their way through packet-switched Networks?
Introduction

16. Internet structure: network of networks
roughly hierarchical
at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage
treat each other as equals
Tier 1 ISP
Tier-1 providers interconnect (peer) privately
Tier 1 ISP
Tier 1 ISP
Introduction

17. Internet structure: network of networks
“tier-1” ISPs
Form their own network
Each tier-1 ISP is connected directly to each of the other tier-1 ISPs
Each tier-1 ISP is connected to many tier-2 ISPs
Tier 1 ISP
Tier-1 providers know as the internet backbone
No group officially sanctions tier-1 status!
Tier 1 ISP
Tier 1 ISP
Introduction

18. Tier-1 ISP: e.g., Sprint … …. to/from backbone peering
to/from customers
peering
to/from backbone ….
POP: point-of-presence
Introduction

19. 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 ISPs also peer privately with each other.
Tier-2 ISP
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet
tier-2 ISP is customer of
tier-1 provider
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Introduction

20. Internet structure: network of networks
“Tier-3” ISPs and local ISPs
last hop (“access”) network (closest to end systems)
local
ISP
Tier 3
Local and tier- 3 ISPs are customers of
higher tier ISPs
connecting them to rest of Internet
Tier-2 ISP
Points of Presence (POP): a link or the group of routers in an ISP where other ISPs or customers connect.
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Introduction

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