Introduction to Computer NetworksPresentation Transcript
Computer Networks Chapter 1 Introduction Abdul Ghaffar CS-4304
1.2 Network Hardware
1.3 Network Software
1.4 Reference Models
1.5 Example Networks
1.6 Example Data Communication Services
1.7 Network Standardization
What You Will Learn
Media and signals
Binary numbering system
Data transmission characteristics
asynchronous and synchronous communication
serial and parallel transmission
bandwidth, throughput and noise
What You Will Learn
Networking and Network Technologies
Packet Switching, Circuit Switching
Protocols and Layering
Interconnection (bridges, switches, routers)
Topologies (star, ring, bus, mesh)
Flow, Error and Congestion Control
Introduction The Big Picture: Where are We Now? Some of the overall issues we’ll be dealing with in this course. 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization
What is a network ? A large number of separate but interconnected computers to do the job is called COMPUTER NETWORK.
File sever, www
Sharing of Resources
email, text and voice chatting,video phone/conference
High reliability ( alternate resources)
Introduction Motivation for Networks
Introduction What A Network Includes
NIC, MODEM, Other transmitter
Intermediate hardware devices
Hub, Repeaters, Amplifier, Switch, router
encodes and formats data
detects and corrects problems
Provides communication that is
From one application to another
Automatically detects and corrects
Automatically finds optimal path from source to destination
Introduction What A Network Does
Introduction Where are we going: This chapter is basically an overview of the course. Before we go in details, we want to get a general idea of what this whole course is about. Here are the kind of issues we’ll be dealing with. Broadcast Networks - Single channel with multiple machines connected to it. Point-to-point - Individual connections between pairs of machines. Packets - Messages - the "chunk" of data transmitted from one machine to the next. Addressing - One to one: Packet contains specific target address. Broadcasting: All machines on the network receive and process the packet. Multicasting: A subset of machines receive and process the packet. Lingo
Introduction Can also view messaging in terms of the distance of transmission: 100 meter Building -- Local area network 1 kilometer Campus 10 kilometer City -- Metropolitan area network 100 kilometer Country 1K kilometer Continent -- Wide area network 10K kilometer Planet -- Internet Serial: Parallel: Lingo
route fixed at setup
idle capacity wasted
network state(connection Establishment)
Circuit Switching Lingo Packet Switching
Network Hardware The Big Picture: Where are We Now? The various possible hardware structures that are possible. LANs, MANs, WANs and all that. 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization
LOCAL AREA NETWORKS
Privately owned. Can be up to several kilometers long;
Run at speeds of 10, 100, or more Mbps.
Low delay. High reliability.
Bus based broadcast network with decentralized control at 10 or 100 Mbps.
Ring based broadcast network with token arbitration at 4 or 16 Mbps.
METROPOLITAN AREA NETWORKS: MANS are: Larger version of LAN ("city" wide). Public or private / data or voice. Broadcast - no switches. Can be distinguished from LANs based on wiring mechanism. Network Hardware
WIDE AREA NETWORKS WANS are: Networks spanning large distances. Hosts or End Systems - Machines running user applications. (Communication) Subnet - Connections between hosts - transmission lines + switches. A "locality" understanding each other's addresses. Circuits/Channels - Transmission lines move the bits. Packet switching nodes/Intermediate systems/Data switching exchanges Specialized computers moving data between several inputs to several outputs. Network Hardware
WIDE AREA NETWORKS Router - Generic term for switching computers. Point-to-point/Store-and-forward/Packet-switched - Moving through a series of routers, packets are received at a router, stored there, then forwarded to the next router. Topologies - Network Hardware
Light and mirrors
What are they: Used where computer is mobile or far away from wires.
Network Software The Big Picture: Where are We Now? This covers the way the software is put together. It talks about the philosophy of connecting together two entities. “ Layering” is the key word. 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization
Network Software Layers - The concept that network software is organized functionally into levels. A level on one host talks to the same level on another host (its peer). Protocol - The protocol is the convention or standard that a layer uses to talk to the other layer. An agreement or standard on the conversation. Physical Medium - Underneath the layers is the wire or fiber or whatever. Interface - Defines the services that one layer offers another (either up or down.) Important to keep this simple and clean. Important that each layer perform specific actions. PROTOCOL HIERARCHIES
Network architecture -
A set of layers and protocols. It contains details on what happens in the layer and what the layers says to its peer.
Functional interfaces and implementation details are not part of the spec, since that's not visible outside the machine.
Protocol stack -
A list of protocols used by a system, one protocol per layer.
Information flow -
"Send_to_peer" rather than "call_next_layer_down".
Network Software Both Directions Simultaneous Simplex No No Half duplex Yes No Full duplex Yes Yes o Number of logical channels per connection (for priority purposes) o Error control. (garbled or missing.) o Preservation of message ordering. o Flow control. o Breaking up messages into a smaller chunks (and reassembly.) o Multiplexing messages on same connection. o Routing - how to get from one host to another. DESIGN ISSUES FOR THE LAYERS That word “Multiplexing”:
INTERFACES AND SERVICES Purpose of each layer is to provide services to the layer above it. Entities / Peer entities - Active element in each layer (process/IO chip). Peer entity = layer N entity <--> layer N entity. Layer N entity provides service for layer N + 1. Service providers and users - Layer N is a provider for user N + 1. SAPs (Service Access Points) - Entry points in N that layer N + 1 can access. Has an address that uniquely identifies it. IDUs (Interface Data Unit) - The information from N + 1 provided at the SAP. Made up of SDU + control information. Network Software
INTERFACES AND SERVICES Purpose of each layer is to provide services to the layer above it. SDUs (Service Data Unit) - The portion of the IDU that will be passed up to the peer entity. PDUs (Protocol Data Unit) - The SDU may be broken up into PDUs, that being the chunk size for further transmission. Rolled up in this figure: Network Software
CONNECTION-ORIENTED / CONNECTIONLESS SERVICES:
Connection oriented service -
Like the phone system. The system establishes a connection, uses it, and closes it. Acts like a tube. Data comes out the other end in the same order as it goes in.
Connectionless service -
Like the post office. Each message has the entire address on it. Each
message may follow a different route to its destination. Ordering not
CONNECTION-ORIENTED / CONNECTIONLESS SERVICES: Quality of service - Will the message arrive?? A reliable connection-oriented service guarantees success. o Message sequence - message boundaries and order are maintained. o Byte streams - messages are broken up or combined; flow is bytes. Can pair mechanism with upper-layer requirements. Network Software
CONNECTION-ORIENTED / CONNECTIONLESS SERVICES: Datagram Service - Like junk mail. It's not worth the cost to determine if it actually arrived. Needs a high probability of arrival, but 100% not required. Connectionless, no acknowledgment. Acknowledged datagram service - As above, but improved reliability via acknowledgment. Request-reply service - Acknowledgment is in the form of a reply. Summarized in this Table. Network Software
SERVICE PRIMATIVES: Primitives are: The operations available to an entity. Possibilities include: Request -- An entity want some work done. Indication -- An entity is told about an event. Response -- An entity wants to respond to an event. Confirm -- Response to earlier request has come back. Example of Connectionless Protocol: Unitdata.request |--------->| Unitdata.indication Unitdata.indication |<---------| Unitdata.request Example of Connection-oriented Protocol: Connect.request |--------->| Connect.indication Connect.confirm |<---------| Data.request |--------->| Data.indication Data.indication |<---------| Data.request Disconnect.request |--------->| Disconnect.indication Disconnect.confirm |<---------| Network Software
SERVICE PRIMATIVES THE RELATIONSHIP OF SERVICES TO PROTOCOLS: Services are primitives that a layer provides for the layer above it. Protocols are rules governing the meaning of frames/packets/messages exchanged with the peer entity. Network Software
Reference Models The Big Picture: Where are We Now? There are two competing models for how the software is layered. These are the OSI and the TCP models. We talk about each of these here. 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization
Reference Models Layering
Reference Models Headers, Data, and Trailers Encapsulation
Reference Models OSI == Open Systems Interconnection Developed by ISO == International Standards Organization Principles used to develop OSI Layering: 1. Need a layer for each different level of abstraction. 2. Each layer performs a well defined function. 3. Each layer should be standard-izable. 4. Layer boundaries should minimize data flow across those boundaries. 5. The right number of layers - don't put too many functions together, but not too many layers either. THE OSI REFERENCE MODEL
Reference Models Physical Layer - Purpose -- Transmits raw bits across a medium. Electrical -- Concerns are voltage, timing, duplexing, connectors, etc. Where Taught -- Differentiates an EE course from a CS course. Data Link Layer - Framing -- Breaks apart messages into frames. Reassembles frames into messages. Error handling -- solves damaged, lost, and duplicate frames. Flow control -- keeps a fast transmitter from flooding a slow receiver. Gaining Access -- if many hosts have usage of the medium, how is access arbitrated. THE OSI REFERENCE MODEL
Reference Models Network Layer - Routing -- What path is followed by packets from source to destination. Can be based on a static table, when the connection is created, or when each packet is sent. Congestion -- Controls the number packets in the subnet. Accounting -- Counts packets/bytes for billing purposes. Heterogeneity -- Interfacing so one type of network can talk to another. THE OSI REFERENCE MODEL
Reference Models Transport Layer - Reliability -- Ensures that packets arrive at their destination. Reassembles out of order messages. Hides network -- Allows details of the network to be hidden from higher level layers. Service Decisions -- What type of service to provide; error-free point to point, datagram, etc. Mapping -- Determines which messages belong to which connections. Naming -- "Send to node xyzzy" must be translated into an internal address and route. Flow control -- keeps a fast transmitter from flooding a slow receiver. THE OSI REFERENCE MODEL
Reference Models Session Layer - Sessions -- Provides services that span a particular message. For instance, a login session could be logged. Synchronization -- Provide way to subdivide a long mechanism for reliability. Presentation Layer - Prettiness -- Syntax and semantics of information transmitted. Understands the nature of the data being transmitted. Converts ASCII/EBCDIC, big endian/little endian Application Layer - Interfacing -- Terminal type translation. File transfer -- Programs able to understand directory structures and naming conventions and map them onto various systems. THE OSI REFERENCE MODEL
Reference Models Data Transmission in the OSI Model - THE OSI REFERENCE MODEL
Reference Models Used in the Arpanet and in the Internet. Common mechanism that is gaining on/surpassing the OSI Model. THE TCP/IP REFERENCE MODEL Host to Network Layer - This lowest level is not defined in this model. Various mechanisms abound. Internet Layer - Connector -- Provides packet switched connectionless service. Routing -- The IP (Internet Protocol) does delivery and congestion control.
Reference Models Transport Layer - Allows peer entities to communicate. TCP -- Transmission Control Protocol provides a reliable connection oriented protocol that delivers a byte stream from one node to another. Guarantees delivery and provides flow control. UDP -- User Datagram Protocol provides an unreliable connection-less protocol for applications that provide their own. Application Layer - Terminal -- Telnet File transfer -- FTP The Web -- HTTP THE TCP/IP REFERENCE MODEL
Reference Models OSI has good definition of service, interface, and protocol as discussed before. Fits well with object oriented programming concepts. Protocols are better hidden. With TCP, the protocols came first; model was just a description of the protocols. But then the model isn't good for any other protocols. LAYERS Transport Network OSI Connectionless Connection-Oriented Connection-Oriented TCP Connectionless Connectionless Connection-Oriented Allows connectionless applications! COMPARISON OF REFERENCE MODELS
A CRITIQUE OF OSI:
Bad Timing -
TCP already in use by the time OSI came along.
Bad Technology -
Layers don't match reality . Chosen because IBM's SNA has seven layers.
Dominated by phone company mentality.
Bad Implementation -
Huge, unwieldy, slow.
A CRITIQUE OF TCP/IP:
Doesn't separate spec from implementation.
Model is only good for describing TCP.
Doesn't specify physical and data link layers.
THE HYBRID REFERENCE MODEL:
2 Data Link
THE TCP/IP REFERENCE MODEL
Example Networks The Big Picture: Where are We Now? This section talks about some real networks. 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization
Heavily used in PC world.
Proprietary protocol stack
Network layer -- IPX - unreliable connectionless
Transport layer -- NCP (Network Core Protocol)
- connection oriented
- other services
Naming and Addressing --
SAP (Service Advertising Protocol) - Servers advertise their address to router machines. Clients, when booted, ask for location of nearest server.
1968 Originally intended as reliable network, with multiple routing.
Used TCP/IP precursor, which got built into early UNIX.
Example Networks NSFNET: Late 1970s - Many other folks wanted to get on the net, but Arpanet was essentially limited to military contractors. NSF set up another network to handle this need. Started at 448 Kbps and by 80's upgraded to 1.5 Mbps. 1990 Formed ANS (Advanced Networks and Services) -- MERIT, MCI, IBM took over from the government running at 45 Mbps. 1995 ANSNET sold to AOL, who now runs it. THE INTERNET: Growing exponentially. All nodes run TCP/IP. Means that all nodes have an IP address by which they can be contacted. Services provided include: o e-mail o news o remote login o file transfer o the web GIGABIT TESTBEDS: Testing higher speed long distance nets. Typically 622 Mbps. While throughput improves, latency remains limited by the speed of light.
Example Data Communications These are public networks owned by the phone companies and offered to the public - thus called public networks. 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization
Example Data Communications These are subnets , often owned by the phone companies, offered to subscribers. This is a public network. SMDS - SWITCHED MULTIMEGABIT DATA SERVICE Interconnects the LANs operating within a single company. Connects diverse branches. Public Networks
Example Data Communications
Developed during 70's. Interface between public packet-switched networks and customers.
Operate at 64 Kpbs, so are very slow and becoming outdated. However there are still many of them in operation.
Connection oriented. Uses:
Switched Virtual Circuit - established when the first packet is sent. Circuit remains for duration of session providing in-order delivery, and flow control.
Permanent Virtual Circuit - established by agreement between the customer and the carrier. Like a leased line.
PAD ( Packet Assembler Disassembler ) allows non-X.25-speaking terminals to connect to X.25.
Example Data Communications
Takes advantage of modern high-speed reliable digital phone lines.
This allows simple protocols with work done by user computers rather than by the network. Runs at 1.5 Mbps with few features.
Customer leases a permanent virtual circuit between two points. This "virtual leased line" means that the wire is shared with other users at a great price reduction. Each is allowed some bursti-ness, but doesn't get the line all day.
Example Data Communications BROADBAND ISDN AND ATM: The above networks, together with a multitude of old versions, mean a major headache. Need a way to provide all of these, and many new services with a single high speed network. ISDN (Integrated Services Digital Network) will offer cable, video on demand, e-mail, etc. etc. etc. ATM (Asynchronous Transfer Mode) is underlying mechanism. Transmits in small fixed-size cells. 5 48 +---------+---------------------------------------+ | Header | User Data | +---------+---------------------------------------+ Public Networks
Example Data Communications ATM (Continued) Packet (cell) switching is dramatic change for phone companies. ATM is connection oriented; make connecting request first; then all cells follow the same path. Target is 155 Mbps and 622 Mbps. Allows TV transmission. See the ISDN ATM Reference Model below: Public Networks
Example Data Communications ATM (Continued) Comparisons to other models. Public Networks
ATM promises much but will take a while to deliver.
Moving from circuit switching will take a long time.
Requires replacement of existing wires.
Example Data Communications COMPARISONS: Here’s an overview of the properties of these Public Networks Public Networks
Standards The Big Picture: Where are We Now? Networks have more standards than any other area of the computer industry. The reason is because hardware/software from different manufactures must all play together - and you can’t do that unless the vendors get together and establish agreements about the behavior of this equipment. 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization
Standards Issues Include: de facto and de jure. Phone companies and the governments that regulate them. ISO (International Standards Organization) ANSI (American National Standards Institute) NIST (National Institute of Standards and Technology) IEEE (Institute of Electrical and Electronics Engineering) IAB (Internet Architecture Board)
Summary We (and the text) use the Internet and ATM as examples. Architecture (not hardware) of physical Layer. Data Link Layer. MAC (Medium Access Layer) - part of Data Link Layer. Network Layer Transport Layer Application Layer (naming, mail, news, web, etc.)