Chapter 1 Notes (Slides)

Uses of Computer Networks

• Computer networks are collections of autonomous computers, like the Internet.

• They have many uses:

  • Business Applications

  • Home Applications

  • Mobile Users

• These uses raise social issues.

Business Applications

• Companies use networks and computers for resource sharing with the client-server model.

• Other popular uses are communication (e.g., email, VoIP) and e-commerce.

Home Applications

• Homes contain many networked devices (e.g., computers, TVs) connected to the Internet via cable, DSL, wireless, etc.

• Home users communicate (e.g., social networks), consume content (e.g., video), and transact (e.g., auctions).

• Some applications use the peer-to-peer model, where there are no fixed clients and servers.

Mobile Users and Ubiquitous Computing

• Tablets, laptops, and smartphones are popular devices.

• WiFi hotspots and 4G cellular provide wireless connectivity.

• Mobile users communicate (e.g., voice and texts), consume content (e.g., video and Web), and use sensors (e.g., GPS).

• Wireless and mobile are related but different.

Social/Ethical Issues

• Network neutrality: no network restrictions.

• Content ownership, e.g., DMCA takedowns.

• Anonymity and censorship.

• Privacy, e.g., Web tracking and profiling.

• Theft, e.g., botnets and phishing.

Network Hardware

• Networks can be classified by their scale:

  • PAN (Personal Area Network): Vicinity (1m-10m)

  • LAN (Local Area Network): Building (10m-1km)

  • MAN (Metropolitan Area Network): City (10km)

  • WAN (Wide Area Network): Country (100km-1000km)

  • The Internet: Planet (10,000km) - network of all networks

Personal Area Network (PAN)

• Connects devices over the range of a person.

• Example: Bluetooth (wireless) PAN.

Local Area Networks (LAN)

• Connects devices in a home or office building.

• Called enterprise network in a company.

• Uses Wireless LAN with 802.11 and Wired LAN with switched Ethernet.

Metropolitan Area Networks (MAN)

• Connects devices over a metropolitan area.

• Example: MAN based on cable TV.

Wide Area Networks (WAN)

• Connects devices over a country.

• An ISP (Internet Service Provider) network is also a WAN. Customers buy connectivity from the ISP to use it.

• A VPN (Virtual Private Network) is a WAN built from virtual links that run on top of the Internet.

Sizes of Networks

• PAN: Square meter (Around person)

• LAN: 10m, 100m, 1km (Room, Building, Campus)

• MAN: 10km (City)

• WAN: 100km, 1000km (Country, Continent)

Network Software

• Protocol layers

• Design issues for the layers

• Connection-oriented vs. connectionless service

• Service primitives

• Relationship of services to protocols

Protocol Layers

• Protocol layering is the main structuring method used to divide up network functionality.

• Each protocol instance talks virtually to its peer.

• Each layer communicates only by using the one below.

• Lower layer services are accessed by an interface.

• At the bottom, messages are carried by the medium.

• Each lower layer adds its own header (with control information) to the message to transmit and removes it on receive. Layers may also split and join messages, etc.

Design Issues for the Layers

• Each layer solves a particular problem but must include mechanisms to address a set of recurring design issues:

  • Reliability despite failures: Codes for error detection/correction ($\S3.2, 3.3$ ), Routing around failures ($\S5.2$)

  • Network growth and evolution: Addressing ($\S5.6$) and naming ($\S7.1$), Protocol layering ($\S1.3$)

  • Allocation of resources like bandwidth: Multiple access ($\S4.2$), Congestion control ($\S5.3, 6.3$)

  • Security against various threats: Confidentiality of messages ($\S8.2, 8.6$), Authentication of communicating parties ($\S8.7$)

Connection-Oriented vs. Connectionless Service

• Service provided by a layer may be either:

  • Connection-oriented: Must be set up for ongoing use (and torn down after use), e.g., phone call.

  • Connectionless: Messages are handled separately, e.g., postal delivery.

Service Primitives

• A service is provided to the layer above as primitives.

• Hypothetical example of service primitives that may provide a reliable byte stream (connection-oriented) service.

Relationship of Services to Protocols

• A layer provides a service to the one above [vertical].

• A layer talks to its peer using a protocol [horizontal].

Reference Models

• Reference models describe the layers in a network architecture:

  • OSI reference model

  • TCP/IP reference model

  • Model used for this text

  • Critique of OSI and TCP/IP

OSI Reference Model

• A principled, international standard, seven-layer model to connect different systems.

• Provides functions needed by users.

• Converts different representations.

• Manages task dialogs.

• Provides end-to-end delivery.

• Sends packets over multiple links.

• Sends frames of information.

• Sends bits as signals.

TCP/IP Reference Model

• A four-layer model derived from experimentation; omits some OSI layers and uses IP as the network layer.

• IP is the “narrow waist” of the Internet.

• Protocols are shown in their respective layers.

Model Used in this Book

• Based on the TCP/IP model but calls out the physical layer and looks beyond Internet protocols.

Critique of OSI & TCP/IP

• OSI:

  • (+) Very influential model with clear concepts

  • (-) Models, protocols, and adoption all bogged down by politics and complexity

• TCP/IP:

  • (+) Very successful protocols that worked well and thrived

  • (-) Weak model derived after the fact from protocols

Example Networks

• The Internet

• 3G mobile phone networks

• Wireless LANs

• RFID and sensor networks

Internet

• Before the Internet was the ARPANET, a decentralized, packet-switched network based on Baran’s ideas.

• The early Internet used NSFNET (1985-1995) as its backbone; universities connected to get on the Internet. NSFNET topology in 1988 used T1 links ($1.5$ Mbps).

• The modern Internet is more complex:

  • ISP networks serve as the Internet backbone.

  • ISPs connect or peer to exchange traffic at IXPs.

  • Within each network, routers switch packets.

  • Between networks, traffic exchange is set by business agreements.

  • Customers connect at the edge by many means: Cable, DSL, Fiber-to-the-Home, 3G/4G wireless, dialup.

  • Data centers concentrate many servers (“the cloud”).

  • Most traffic is content from data centers (especially video).

  • The architecture continues to evolve.

Architecture of the Internet

• Includes elements like Fiber (FTTH), Dialup, DSL, Peering at IXP, DSLAM, POP, DSL modem, Data center, Tier 1 ISP, other ISPs, Backbone - Router, 3G mobile phone, Cable, Cable modem, Data path, and CMTS.

3G Mobile Phone Networks

• 3G network is based on spatial cells; each cell provides wireless service to mobiles within it via a base station.

• Base stations connect to the core network to find other mobiles and send data to the phone network and Internet.

• As mobiles move, base stations hand them off from one cell to the next, and the network tracks their location (Handover).

Wireless LANs

• In 802.11, clients communicate via an AP (Access Point) that is wired to the rest of the network.

• Signals in the 2.4GHz ISM band vary in strength due to many effects, such as multipath fading due to reflections -- requires complex transmission schemes, e.g., OFDM.

• Radio broadcasts interfere with each other, and radio ranges may incompletely overlap -- CSMA (Carrier Sense Multiple Access) designs are used.

RFID and Sensor Networks

• Passive UHF RFID networks everyday objects: Tags (stickers with not even a battery) are placed on objects. Readers send signals that the tags reflect to communicate.

• Sensor networks spread small devices over an area: Devices send sensed data to a collector via wireless hops.

Network Standardization

• Standards define what is needed for interoperability.

• Some of the many standards bodies:

  • ITU (Telecommunications): G.992 (ADSL), H.264 (MPEG4)

  • IEEE (Communications): 802.3 (Ethernet), 802.11 (WiFi)

  • IETF (Internet): RFC 2616 (HTTP/1.1), RFC 1034/1035 (DNS)

  • W3C (Web): HTML5 standard, CSS standard

Metric Units

• Use powers of 10 for rates, powers of 2 for storage.

  • E.g., 1 Mbps = $1,000,000$ bps, 1 KB = $1024$ bytes.

• “B” is for bytes, “b” is for bits.

• The Main Prefixes

  • K(ilo) = $10^3$ , m(illi) = $10^{-3}$

  • M(ega) = $10^6$, μ(micro) = $10^{-6}$

  • G(iga) = $10^9$, n(ano) = $10^{-9}$