Network Architecture Basics

Introduction to Computer Communications and Networks

• The 20th and 21st centuries are defined by the technology of information gathering, processing, and distribution.
• There has been rapid technological progress in both computers and communications.

Evolution of Computers

• Computers were previously centralized and large, serving numerous users via mainframes.
• Computers are currently more powerful and smaller due to Moore's Law:
  • The number of transistors on an integrated circuit (IC) doubles approximately every 18 months without increasing the cost per IC.
  • Cost per transistor on an IC reduces by a factor of 2 every 18 months.

Communications Advancements

• Communications handle various forms of data including voice and video, transitioning from analog to digital formats.
• High-speed communication links exist through optical fibers.
• Wireless communications utilize modulation, detection, and estimation techniques.

Computer Networks

• Combining computer and communication technologies is what lead to computer networks
• Organizations have moved away from the old model of a single computer serving all users.
  • The current structure is with numerous separate but interconnected computers doing the job.
• Computer networks is used to describe autonomous computers interconnected by a communication medium.

Applications of Computer Networks

• Business applications employ resource sharing for programs, databases, files, and printers accessible to network users,
• Servers store software such as programs, files, and records.
• Hardware like printers, scanners, and CD burners are connected, supporting the client-server model.

Home Applications of Computer Networks

• Home applications include e-mail, instant messaging (ex: Twitter), and streaming multimedia.
• Peer-to-peer communications are utilized for file sharing like Gnutella and BitTorrent.
• Social network applications (ex: Facebook) and e-commerce (online banking, online shopping) have become prominent.

Network Hardware - Transmission Technology

• Broadcast links: a single communication channel shared among multiple machines.
• Point-to-point links: a dedicated communication channel connecting only two machines.
• Network classification is thus based on network topology (structure).

Network Classification based on Topology

• Broadcast networks share a single communication channel among all machines on the network.
  • Topologies include:
    • bus
    • ring
• Any machine sends messages that are received by all.
• Some broadcast systems support transmission from one machine to a subset of others, known as multicasting.

Point-to-Point Networks

• Point-to-point networks consist of links connecting individual pairs of machines.
• Connection to every machine is not required.
• Messages may visit intermediate machines to reach their destination.
  • Topologies include:
    • star
    • mesh
    • fully connected

Local Area Networks (LAN)

• Local Area Networks (LAN) cover a localized area, office building, university campus, up to a few kilometers.
• Typically (but not always), LANs use broadcast topologies.
• A core challenge in LANs is handling multiple access to avoid conflicts in transmission
• Static channel allocation entails dividing time into slots, using a round-robin approach for each machine to transmit in its slot (TDMA).
  • This can be capacity-inefficient, where a machine has nothing to send during its allocated slot.
• Dynamic channel allocation can be centralized (a central controller that decides who transmits next) or decentralized.

LAN Examples

• Classic Ethernet was developed by Xerox researcher Bob Metcalfe in 1976, and has a bus topology.
• Wireless LAN uses IEEE 802.11.
• Switched Ethernet is also used, which is a wired LAN built from point-to-point links.
• Wireless LANs have a broadcast topology.

Metropolitan Area Networks (MAN)

• Metropolitan Area Networks (MAN) covers a city (10's of kilometers across)s
• A common example is cable TV networks available in many cities.

Wide Area Networks (WAN)

• Wide Area Networks (WAN) cover a country or continent at 100's or 1000's of kilometers across.
• One example is the WAN that connects hosts in Perth, Melbourne, and Brisbane, Australia.

WAN Components

• Hosts: network users running application problems.
• Subnet: responsible for host-to-host message delivery.
• Switching elements: specialized computers for the subnet (routers are now commonly used).
• Transmission lines/links: move information between machines.

Internetworks

• Many networks exist using different hardware and software.
• Users on one network may want to communicate with users on a different network.
• Using gateways, different networks enable connecting possibly incompatible networks..
• A collection of interconnected networks is known as an internetwork (example: the Internet).

Switching Methods

• Circuit Switching (originally designed for the telephone networks) establishes a dedicated, continuous path for the duration of a communication session like a phone call.

• Packet Switching involves dividing communication "messages"into smaller units called packets.

  • Packets transmit individually through possibly different routes in the network without dedicated paths.
  • Switching elements store arriving packets then forwarding them to a certain route.
  • Subnets using packet switching are called store-and-forward subnets.
  • Packets belonging to the same session may take different paths.

• Message Switching - same as packet switching but communication units or data packets are larger: messages

Network Architecture

• Network functionality is organized as a stack of layers.
• Each layer carries out a communication service from the higher layer to the next.
• Protocol is a set of rules that govern the information exchange between peer layers.
• A protocol consists of several layers (i.e., layer n on one machine to layer n on another machine)
• Interface facilitates communication between successive layers on the same machine.
• A set of layers, protocols, and interfaces is known as a network architecture.

Layer Communications

• A message M is created by an application running in layer 5 of the source machine
• M is given to layer 4 for transmission.
• Layer 4 adds a header (message sequence information) and passes the message to layer 3.
• Layer 3 breaks message M into small packets, and adds a layer 3 header (to determine the packet sequence) to each packet.
• Layer 3 decides the outgoing link for each packet and passes the packets to layer 2.
• Layer 2 adds a header and trailer to each packet and delivers the resulting unit to layer 1 for physical transmission.
• At the receiving machine, the message forwards upwards from layer to layer with headers and trailers stripped off.

Connection Vs Connectionless Services

• Networks layers provide two types of service: connection-oriented and connectionless.
• Connection-oriented service
  • A service user establishes a connection, uses a connection, then releases the connection.
  • Negotiation between sender and receiver may be required before the connection.
  • Messages and packets usually arrive in order.
• Connectionless Service
  • Each message and packet carries full destination address information and is routed through the network independently.
  • Messages and packets arrive out of order.

The OSI Reference Model

• The Open Systems Interconnections (OSI) model was developed by the International Standards Organization (ISO)

Physical Layer

• Transmits raw bits over a communication channel.
• Design issues: Transmitted 1 is received as intended and not as a 0.

Data Link Layer

• Transforms an unreliable raw bit pipe into a communication channel for transmitting frames error free reliably.
• Performs error detection and error correction.
• Performs flow control to stop a fast transmitter from overwhelming a slow receiver.
• In broadcast networks, handles multiple accesses with a special sub-layer which is called the medium access control (MAC).

Network Layer

• Provides routing to provide a virtual channel for end-to-end packets via static or dynamic routing.
• Utilizes Congestion control to avoid link bottle necks.
• Broadcast network routing is simple or non-existent making the network layer is often thin or non-existing.

Transport Layer

• Breaks larger "messages" into smaller packets at Transmit side, and assembles packets into larger "messages" at the Receive side.
• Corrects out of order delivery and lost packets.
• Performs end-to-end flow and congestion control and may perform end-to-end error control.

Session Layer

• Allows users from different machines to establish sessions between them.

Presentation Layer

• Works with the syntax and semantics of the information transmitted.
  • Performs:
    • Encryption
    • Data compression
    • File format transfer

Application Layer

• Consists of any application program of the end user.

The TCP/IP Reference Model

• Used in the grandparent of all computer networks, the ARPANET (Advanced Research Projects Agency) & it successor the Internet.

The Internet Layer

• Is connection-less, enabling hosts to inject packets into the network and routed independently to the destination - may arrive out of order.
• Defines the packet format in a protocol called the Internet Protocol (IP).
• The internet layer is very similar to the network layer in the OSI model.

The Transport Layer

• Allows source and destination hosts to carry on a conversation similar to the OSI transport layer and is an end-to-end layer
• It provides two end-to-end protocols
  • TCP (Transport Control Protocol): This is a reliable connection-oriented protocol comprised of a byte stream into discrete messages and handles flow and congestion control.
  • UDP (User Datagram Protocol): An unreliable, connectionless protocol for applications with prompt over accurate delivery where TCP's flow control is unneeded, ex: voice and video.

The Application Layer

• Contains all the higher-level protocols
  • File transfer (FTP)
  • E-mail (SMTP, Simple Mail Transfer Protocol)
  • Domain Name System (DNS) for mapping host names onto their network addresses
  • HTTP (the protocol for creating pages on the World Wide Web)

The Link Layer

• Describes what links such as serial lines and Ethernet must do to meet the needs of the connectionless Internet layer.
• It is not really a layer at all, in the normal sense of the term, but rather an interface between hosts and transmission links (allows hosts to inject packets into the network).

Example Network: ARPANET

• In the 1950's US military communications used the general telephone network.
• The US Department of Defence wanted a network that could survive failures.
• ARPA (Advanced Research Projects Agency) was created to solve the issue.

Original ARPANET Design

• Subnet made up of IMP’s (Interface Message Processors)
• IMP's are connected by 56 Kbps transmission lines (leased lines from telephone companies).
• Every IMP connected to at least 2 other IMP’s (for reliability)

Growth of ARPANET

• In its early stages had universities such as:
  • UCLA
  • UCSB
• SRI (Stanford Research Institute)
• The University of Utah who had a considerable amount of ARPA contracts.

NSFNET

• In the 1970's, NSF i(US National Science Foundation) recognized impact the ARPANET was making regarding university research.
• Access to ARPANET required universities to have a research contract with the Department of Defense.
• NSF’s initial response was to fund the Computer Science Network (CSNET) in 1981.
• Connected Computer Science departments and industrial research labs to ARPANET.
• During 1980s, NSF decided to design a successor to ARPANET: the NSFNET comprised a backbone and various regional networks.
• It enabled users at thousands of universities, research labs, libraries and museums to be connected.

Current Internet Configuration

• Clients at home call their ISP over a dial-up telephone line.
• The modem card within the PC converts the digital signals (produced by the PC) into signals for telephone system.
• Signals transfer to the ISP's Point of Presence (POP), removed from the telephone system, and injected into the ISP's regional network.
• The ISP's Network routes include interconnected routers in cities the ISP serves.
• The packet travels on a dedicated host, otherwise, the packet forwards to the ISP’s backbone operator.
• Backbone operators operates large international backbone networks with thousands of routers connected by optical fibers.
• The differences of interconnected backbones happen within Network Access Points (NAPs).
• In NAPs the router forwards from backbone to backbone using a LAN.

Understand the basics of network architecture. Key concepts covered include layers, protocols, interfaces, and network models like client-server and peer-to-peer. Explore network topologies and communication types.