Chapter 1 Introduction
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
❑ If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)
❑ If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2007
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach , 6th edition.
Jim Kurose, Keith Ross
Introduction 1-1
Chapter 1: Introduction
Our goal:
❑ get “feel” and terminology
Overview:
❑ what’s the Internet?
❑ network edge; hosts, access
net, physical media
❑ network core: packet/circuit switching, Internet structure
❑ performance: loss, delay, throughput
❑ protocol layers, service models Introduction 1-2
❑ more depth, detail later in course
❑ approach:
❖ use Internet as example
Chapter 1: roadmap
1.1 What is the Internet? 1.2 Network edge
❑ end systems, access networks, links 1.3 Network core
❑ circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched
networks
1.5 Protocol layers, service models
Introduction 1-3
What’s the Internet: “nuts and bolts” view
PC server
wireless laptop
❑ millions of connected computing devices: hosts = end systems
Mobile network
Global ISP
Home network
Regional ISP
Institutional network
❖ running network apps
❑ communication links
❖ fiber, copper, radio, satellite
❖ transmission rate = bandwidth
cellular handheld
access points
wired links
router
❑ routers: forward packets (chunks of data)
Introduction 1-4
What’s the Internet: “nuts and bolts” view
❑ applications
❑ protocols control sending, receiving of msgs
❖ e.g., TCP, IP, HTTP, Skype, Ethernet
❑ Internet: “network of networks”
❖ loosely hierarchical
❖ public Internet versus
private intranet
❑ Internet standards
❖ RFC: Request for comments
❖ IETF: Internet Engineering Task Force
Mobile network
Global ISP
Home network
Regional ISP
Institutional network
Introduction 1-5
What’s a protocol?
human protocols:
❑ “what’s the time?” ❑ “I have a question” ❑ introductions
… specific msgs sent
… specific actions taken when msgs received, or other events
network protocols:
❑ machines rather than humans
❑ all communication activity in Internet governed by protocols
protocols define format, order of msgs sent and
received among network entities, and actions taken on msg transmission, receipt
Introduction 1-6
What’s a protocol?
a human protocol and a computer network protocol:
Hi
Hi
Got the time?
2:00
TCP connection request
TCP connection response
Get http://www.awl.com/kurose-ross
time
Q: Other human protocols?
Introduction 1-7
Chapter 1: roadmap 1.1 What is the Internet?
1.2 Network edge
❑ end systems, access networks, links
1.3 Network core
❑ circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks
1.5 Protocol layers, service models
Introduction 1-8
A closer look at network structure:
❑network edge: applications and hosts
❑access networks, physical media: wired, wireless communication links
❑network core: ❖ interconnected
routers
❖ network of networks
Introduction 1-9
The network edge:
❑end systems (hosts):
❖ run application programs ❖ e.g. Web, email
❖ at “edge of network”
❑client/server model
❖ client host requests, receives
service from always-on server
❖ e.g. Web browser/server; email client/server
client/server
Introduction 1-10
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?
Introduction 1-11
Residential access: point to point access
❑ Dialup via modem
❖ up to 56Kbps direct access to
router (often less)
❖ Can’t surf and phone at same
time: can’t be “always on”
❑ DSL: digital subscriber line
❖ deployment: telephone company (typically)
❖ up to 1 Mbps upstream (today typically < 256 kbps) ❖ up to 8 Mbps downstream (today typically < 1 Mbps) ❖ dedicated physical line to telephone central office
Introduction 1-12
Residential access: cable modems
❑ HFC: hybrid fiber coax
❖ asymmetric: up to 30Mbps downstream, 2
Mbps upstream
❑ network of cable and fiber attaches homes to ISP router
❖ homes share access to router
❑ deployment: available via cable TV companies
Introduction 1-13
Cable Network Architecture: Overview
cable headend
cable distribution network (simplified)
Typically 500 to 5,000 homes
home
Introduction 1-14
Cable Network Architecture: Overview
server(s)
cable headend
cable distribution network
home
Introduction 1-15
Cable Network Architecture: Overview
cable headend
cable distribution network (simplified)
home
Introduction 1-16
Cable Network Architecture: Overview
FDM (more shortly):
cable headend
cable distribution network
C
O VVVVVV N IIIIIIDDT DDDDDDAAR EEEEEETTO OOOOOOAAL
123456789 Channels
home
Introduction 1-17
Company access: local area networks
❑ company/univ local area network (LAN) connects end system to edge router
❑ Ethernet:
❖ 10 Mbs, 100Mbps,
1Gbps, 10Gbps Ethernet
❖ modern configuration: end systems connect into Ethernet switch
❑LANs: chapter5
Introduction 1-18
Wireless access networks
❑ shared wireless access network connects end system to router
❖ via base station aka “access point”
❑ wireless LANs:
❖ 802.11b/g (WiFi): 11 or 54 Mbps
router
base station
❑ wider-area wireless access
❖ provided by telco operator
❖ ~1Mbps over cellular system (EDGE, HSPA+, LTE)
mobile hosts
Introduction 1-19
Home networks: a combined example
Typical home network components:
❑ DSL or cable modem ❑ router/firewall/NAT ❑ Ethernet
❑ wireless access
point
to/from cable headend
cable
modem firewall
Ethernet
wireless laptops
wireless access
point
router/
Introduction 1-20
Physical Media
❑ Bit: propagates between transmitter/rcvr pairs
❑ physical link: what lies between transmitter & receiver
❑ guided media:
❖ signals propagate in solid
media: copper, fiber, coax
❑ unguided media:
❖ signals propagate freely, e.g., radio
Twisted Pair (TP)
❑ two insulated copper wires
❖
❖
Category 3: traditional phone wires, 10 Mbps Ethernet
Category 5: 100Mbps Ethernet
Introduction 1-21
Physical Media: coax, fiber
Coaxial cable:
❑ two concentric copper conductors
❑ bidirectional
❑ baseband:
❖ single channel on cable ❖ legacy Ethernet
Fiber optic cable:
❑ glass fiber carrying light pulses, each pulse a bit
❑ high-speed operation:
❖ high-speed point-to-point transmission (e.g., 10’s- 100’s Gps)
❑ low error rate: repeaters spaced far apart ; immune to electromagnetic noise
❑ broadband:
❖ multiple channels on
cable ❖ HFC
Introduction 1-22
Physical media: radio
❑ signal carried in electromagnetic spectrum
❑ no physical “wire”
❑ bidirectional
❑ propagation environment effects:
❖ reflection
❖ obstruction by objects ❖ interference
Radio link types: ❑terrestrial microwave
❖ e.g. up to 45 Mbps channels ❑ LAN (e.g., Wifi)
❖ 11Mbps, 54 Mbps
❑ wide-area (e.g., cellular)
❖ 3G cellular: ~ 1 Mbps ❑ satellite
❖ Kbps to 45Mbps channel (or multiple smaller channels)
❖ 270 msec end-end delay
❖ geosynchronous versus low altitude
Introduction 1-23
Chapter 1: roadmap 1.1 What is the Internet?
1.2 Network edge
❑ end systems, access networks, links
1.3 Network core
❑ circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks
1.5 Protocol layers, service models
Introduction 1-24
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 providers interconnect (peer) privately
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Introduction 1-25
Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone peering
...
... .
to/from customers
Introduction 1-26
...
... ...
From AT&T web site.
27
ATT Global Backbone IP Network
From http://www.business.att.com
28
China Education and Research Network
From http://www.edu.cn/20040326/3102431.shtml
29
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-2 ISPs also peer privately with each other.
Tier 1 ISP
Tier-2 ISP
Tier 1 ISP
Tier-2 ISP
Tier-2 ISP
Introduction 1-30
Internet structure: network of networks
❑ “Tier-3” ISPs and local ISPs
❖ last hop (“access”) network (closest to end systems)
local ISP
Tier 3 ISP
Tier-2 ISP
local local local ISP ISP ISP
Tier-2 ISP
Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local ISP
Tier-2 ISP
local ISP
Introduction 1-31
local Tier-2 ISP
ISP local ISP
Internet structure: network of networks
❑ a packet passes through many networks!
local ISP
Tier-2 ISP
local local ISP ISP
Tier-2 ISP
Tier 3 ISP
local ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local ISP
Tier 1 ISP
Tier-2 ISP
local ISP
Introduction 1-32
local Tier-2 ISP
ISP local ISP
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”
Introduction 1-33
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
Introduction 1-34
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)
❑ dividing link bandwidth into “pieces”
❖ frequency division ❖ time division
Introduction 1-35
Circuit Switching: FDM and TDM
FDM frequency
TDM frequency
Example:
4 users
time
time
Introduction 1-36
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
Let’s work it out!
Introduction 1-37
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
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
Introduction 1-38
Packet Switching: Statistical Multiplexing
A
Ethernet
100 Mb/s statistical multiplexing
1.5 Mb/s
C
B
queue of packets waiting for output link
D
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 1-39
E
Packet-switching: store-and-forward
RR
L
R
❑ 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 1-40
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
N users
1 Mbps link Q: how did we get value 0.0004?
Introduction 1-41
❑ circuit-switching: ❖ 10 users
❑ packet switching:
❖ with 35 users, probability > 10 active at same time is less than .0004
Chapter 1: roadmap 1.1 What is the Internet?
1.2 Network edge
❑ end systems, access networks, links
1.3 Network core
❑ circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks
1.5 Protocol layers, service models
Introduction 1-42
How do loss and delay occur?
packets queue in router buffers
❑ packet arrival rate to link exceeds output link
capacity
❑ packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets dropped (loss) if no free buffers
Introduction 1-43
Four sources of packet delay
❑ 1. nodal processing: ❖ check bit errors
❖ determine output link
❑ 2. queueing
❖ time waiting at output
link for transmission
❖ depends on congestion level of router
A
transmission
propagation
B
nodal processing
queueing
Introduction 1-44
Delay in packet-switched networks
3. Transmission delay:
❑ R=link bandwidth (bps)
❑ L=packet length (bits)
❑ time to send bits into link = L/R
4. Propagation delay:
❑ d = length of physical link ❑ s = propagation speed in
medium (~2×108 m/sec) ❑ propagation delay = d/s
Note: s and R are very different quantities!
A
transmission
propagation
B
nodal processing
queueing
Introduction 1-45
Delay in packet-switched networks
Transmission delay vs. Propagation delay
Think a water pipe From PQ708 to PQ707
From Hong Kong to Rio de Janeiro Brazil
Introduction 1-46
Nodal delay
dnodal = dproc + dqueue + dtrans + dprop
❑ dproc = processing delay
❖ typically a few microsecs or less
❑ dqueue = queuing delay ❖ depends on congestion
❑ dtrans = transmission delay
❖ = L/R, significant for low-speed links
❑ dprop = propagation delay
❖ a few microsecs to hundreds of msecs
Introduction 1-47
Queueing delay (revisited)
❑ R=link bandwidth (bps)
❑ L=packet length (bits)
❑ a=average packet arrival rate
traffic intensity = La/R
❑ La/R ~ 0: average queueing delay small
❑ La/R -> 1: delays become large
❑ La/R > 1: more “work” arriving than can be serviced, average delay infinite!
Introduction 1-48
Packet loss
❑queue (aka buffer) preceding link in buffer has finite capacity
❑packet arriving to full queue dropped (aka lost) ❑lost packet may be retransmitted by previous
node, by source end system, or not at all
A
B
buffer (waiting area)
packet being transmitted
packet arriving to full buffer is lost
Introduction 1-49
Throughput
❑throughput: rate (bits/time unit) at which bits transferred between sender/receiver
❖ instantaneous: rate at given point in time ❖ average: rate over long(er) period of time
server, with
link capacity pipe that can carry
link capacity
pipe that can carry
server sends bits
R bits/sec
R bits/sec
file of F bits
fluid at rate s
c
fluid at rate
(fluid) into pipe
Rs bits/sec)
to send to client
Rc bits/sec)
Introduction 1-50
Throughput (more)
❑Rs < Rc What is average end-end throughput?
Rs bits/sec
Rc bits/sec
❑Rs > Rc What is average end-end throughput?
Rc bits/sec
bottleneck link
link on end-end path that constrains end-end throughput
Rs bits/sec
Introduction 1-51
Throughput: Internet scenario
❑ per-connection end-end
throughput: min(Rc,Rs,R/10)
❑in practice: Rc or Rc Rs is often
bottleneck
Rs
R
Rc Rc
Rs Rs
10 connections (fairly) share backbone bottleneck link R bits/sec
Introduction 1-52
Chapter 1: roadmap 1.1 What is the Internet?
1.2 Network edge
❑ end systems, access networks, links
1.3 Network core
❑ circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks
1.5 Protocol layers, service models
Introduction 1-53
Protocol “Layers”
Networks are complex!
❑ many “pieces”:
❖ hosts
❖ routers
❖ links of various
media
❖ applications
❖ protocols
❖ hardware, software
Question:
Is there any hope of organizing structure of
network?
Or at least our discussion of networks?
Introduction 1-54
Organization of air travel
ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing
ticket (complain) baggage (claim) gates (unload) runway landing airplane routing
airplane routing ❑a series of steps
Introduction 1-55
Layering of airline functionality
ticket (purchase) ticket (complain) ticket
baggage (check) baggage (claim baggage
gates (load) gates (unload) gate
runway (takeoff) runway (land) takeoff/landing
airplane routing airplane routing airplane routing airplane routing airplane routing
departure intermediate air-traffic arrival airport control centers airport
Layers: each layer implements a service
❖ via its own internal-layer actions
❖ relying on services provided by layer below
Introduction 1-56
Why layering?
Dealing with complex systems:
❑ explicit structure allows identification, relationship of complex system’s pieces
❖ layered reference model for discussion
❑ modularization eases maintenance, updating of
system
❖ change of implementation of layer’s service transparent to rest of system
❖ e.g., change in gate procedure doesn’t affect rest of system
Introduction 1-57
Internet protocol stack
❑ application: supporting network applications
❖ FTP, SMTP, HTTP
❑ transport: process-process data transfer
❖ TCP, UDP
❑ network: routing of datagrams from source to destination
❖ IP, routing protocols
❑ link: data transfer between neighboring network elements
❖ PPP, Ethernet
❑ physical: bits “on the wire”
Introduction 1-58
application
transport
network
link
physical
ISO/OSI reference model
❑ presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine- specific conventions
❑ session: synchronization, checkpointing, recovery of data exchange
❑ Internet stack “missing” these layers!
❖ these services, if needed, must be implemented in application
❖ needed?
application
presentation
session
transport
network
link
physical
Introduction 1-59
Introduction: Summary
Covered a “ton” of material!
❑ Internet overview
❑ network edge, core, access
❖ packet-switching versus circuit-switching
You now have:
❑ context, overview, “feel” of networking
❑ more depth, detail to follow!
network
❖ Internet structure
❑ performance: loss, delay,
throughput
❑ layering, service models
Introduction 1-60