Transcript Introduction to the Internet

The Internet - overview
Context and feel of the Internet
How it is organised & managed
(Adapted from Kurose & Ross)
Roadmap
What is ‘The Internet’
Network Edge, Network Core
Circuit & packet switching
Access Networks
Internet/ISP Structure
Protocol Layers, Service Models
 Internet: “network of
networks”
 loosely hierarchical
 public Internet plus
private intranets
 Standards
 IETF: Internet
Engineering Task Force
 RFC: Request for
comments
 protocols control
sending, receiving of
msgs
 e.g., TCP, IP, HTTP,
FTP, PPP
What’s the Internet?
A closer look at the Internet:
network edge:
applications and hosts
access networks
dialup, ADSL, switches,
LANs
network core:
Routers
physical media:
communication links
The Network Core
 mesh of interconnected
routers
 the fundamental question:
how is data transferred
through the net?
 circuit switching:
dedicated circuit per call:
telephone net
 packet-switching: data
sent thru net in discrete
“chunks”
Core Networks - Taxonomy
Core networks
Circuit-switched
networks
FDM
TDM
Packet-switched
networks
Networks
with VCs
Datagram
Networks
Internet provides both connection-oriented (TCP) and connectionless
services (UDP) to applications. IP is packet switched, connectionless.
Circuit Switching
End-end resources
reserved for “call”
 link bandwidth, switch
capacity
 dedicated resources: no
sharing – TDM, FDM
 circuit-like (guaranteed)
performance
 call setup required
Network Core: Packet Switching
Data stream divided into packets; Packets are sent into the
network and are routed to the correct destination.
 packets from different sources share network resources
 each packet uses full link bandwidth
 resources used as needed
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
* transmit over link
* wait turn at next link
Packet Switching: Statistical Multiplexing
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
E
Sequence of A & B packets does not have fixed pattern.
A kind of statistical multiplexing (bandwidth is shared, and total demand
from A and B can exceed capacity of outgoing link).
In TDM (by contrast) each host gets same slot in revolving TDM frame.
Packet switching versus circuit switching
Packet switching allows more users to use the network!
 1 Mbit link
 each user:
 100 kbps when “active”
 active 10% of time
N users
 circuit-switching:
 10 users
 packet switching:
 with 35 users, probability >
10 active less than .0004
1 Mbps link
Packet switching vs circuit switching
Packet switching is
 Suited for bursty data
 Efficient resource sharing
 Simpler, no call setup, route determination, BW
reservation
 Flexible routing
 Excessive congestion  causes packet delay and loss
 protocols needed for reliable data transfer, congestion
control
 Q: How to provide circuit-like behavior?
 bandwidth guarantees needed for audio/video apps
 still a research problem
Packet-switching: store-and-forward
L
R
R
Takes L/R seconds to transmit
(push out) packet of L bits on to
link of R bps
Entire packet must arrive at
router before it can be
transmitted on next link:
store and forward
Delay (in the 3-hop example
network shown above) = 3L/R
R
Example for single hop:
 L = 7.5 Mbits
 R = 1.5 Mbps
 delay = 15 sec
Message Segmenting – breaking into smaller packets
Consider breaking up the 7.5 Mbit
message into 5000 packets
 Each packet is 1,500 bits
 1500 / 1500000 = 1 mS to
transmit one packet on one link
(packet-time = 1mS)
 pipelining: links work in parallel
Number of packet-times = number
of packets + (number of links -1)
In this example 5000 + 2 = 5002
5002 * 1mS = 5.002 Seconds
 Delay reduced from 15S to
5.002S
Another example
A
S1
S2
B
4000 byte data, 20 byte header;
Link speed is enough to send 1 byte in t seconds
 1 packet: 4020 x 3 = 12060 t
 2 packets: 2020 x 4 = 8080 t
 10 packets: 420 x 12 = 5040 t
 20 packets: 220 x 22 = 4840 t
 40 packets: 120 x 42 = 5040 t
 100 packets: 60 x 102 = 6120 t
Optimal number of packets
≈ SQRT [ data_bytes x (#hops-1)/hdr_size)]
Two types of packet networks
 datagram network:
 destination address in packet determines next
hop
 routes may change during session
 analogy: driving, asking directions
 virtual circuit network:
 each packet carries tag (virtual circuit ID)
which determines next hop
 Route decided at call setup time, remains fixed
 routers maintain per-call state
Virtual Circuit v Datagram
Virtual circuits
 Provides sequencing and error control
 Faster forwarding as no routing decisions
 Failure of a node loses all circuits through that
node
Datagram
 No call setup phase, better if few packets
 More flexible routing to avoid congested parts
Event Timing
Circuit switching; Connection oriented; Connectionless
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?
Residential access: point to point access
 Dialup via modem
Up to 56Kbps direct access to router
(often less)
Can’t surf and phone at the same
time: not “always on”
 ADSL: asymmetric digital subscriber line
 up to 1 Mbps upstream
 up to 8 Mbps downstream
 FDM:
50 kHz - 1 MHz for downstream
4 kHz - 50 kHz for upstream
0 kHz - 4 kHz for ordinary telephone
Residential access: cable modems
 HFC: hybrid fiber coax
asymmetric: up to 1 Mbps upstream, 10 Mbps downstream
 Network of cable and fiber attaches homes to ISP router
 shared access to router among homes
 issues: congestion, dimensioning
 Deployment: available via cable companies,
e.g. Virgin Media (from merger of NTL, Telewest), Sky, BT.
Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
Cable Network Architecture: Overview
FDM:
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Channels
cable headend
cable distribution
network
home
Company access: local area networks
 company/univ local area network
(LAN) connects end system to
edge router
 Ethernet:
 shared or dedicated link
connects end system and router
 10 Mbs, 100Mbps, Gigabit
Ethernet, 10 Gigabit Ethernet
 deployment: used to be only
institutions, now home LANs
Wireless access networks
shared wireless access network
connects end system to router
via base station aka “access point”
router
wireless LANs:
base
802.11 (WiFi): up to 150 Mbps (n)
station
wider-area wireless access
 provided by cellular operator
 3G ~ 384 kbps
 WAP/GPRS
mobile
hosts
Hierarchy of the Internet
Categories of ISPs & examples
A Tier-1 network provider: UUNET (Verizon)
Internet structure: network of networks
A packet crosses many networks. Opportunity for loss, delay.
local
ISP
Tier 3
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Protocol “Layers” - 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
 layering considered harmful/inefficient?
Application
Presentation
Session
Transport
Network
Datalink
Physical
Provides network services to the
end-users and applications.
Data compression, encryption and
representation.
Establishes and manages sessions
(conversations) end-to-end.
Reliable, efficient end-to-end
transmission of data.
Routing and end-to-end
addressing.
Defines the format of data
(frames) on the physical link.;
sub-layers: LLC, MAC
Defines the cable or physical
medium itself.
Comparison between the ISO OSI seven-layer model and the TCP/IP model
ISO OSI seven-layer model
TCP / IP model
Application
Presentation
Application
Session
Transport
Transport
Network
Internet
Data-Link
Physical
Host-tonetwork
IEEE 802.x Standards
http://www.technicom.net/clanci/pdf/802overview.pdf
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Sponsor Exec. Committee IEEE LANs &
MANs, overview and architecture……
High Level interface HILI standard
Logical Link Control LLC
CSMA/CD Ethernet
Token Bus
Token Ring
Metropolitan Area Network MAN
Broadband BBTAG
Fibre Optics FOTAG
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Integrated Services LAN
Standards for Interoperable LAN Security
SILS
Wireless LANs
Demand Priority VGAnyLAN
Cable TV Broadband
Wireless PAN (Bluetooth)
BB Wireless Access
Resilient Packet Ring Group RPRG
Radio Regulatory TAG
Co-existence Advisory Group
Mobile Wireless Access Working Group