Data Communications Chapter 1 – Data Communications, Data Networks, and the Internet

Data Communications, Data Networks, and the Internet  The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point - The Mathematical Theory of Communication, Claude Shannon

A Communications Model

Key Communications Tasks Transmission system utilizationAddressing InterfacingRouting Signal generationRecovery SynchronizationMessage formatting Exchange managementSecurity Error detection and correctionNetwork management Flow control

Data Communications Model

Transmission Medium  selection is a basic choice internal use entirely up to business internal use entirely up to business long-distance links made by carrier long-distance links made by carrier  rapid technology advances affects choice fiber optic – high capacity, getting cheaper fiber optic – high capacity, getting cheaper Wireless - mobility Wireless - mobility  transmission cost is still high  hence interest in efficiency improvements – multiplexing and compression

Networking  growth of number & power of computers is driving need for interconnection  also seeing rapid integration of voice, data, image & video technologies  two broad categories of communications networks: Local Area Network (LAN) Local Area Network (LAN) Wide Area Network (WAN) Wide Area Network (WAN)

Wide Area Networks  span a large geographical area  rely in part on common carrier circuits  alternative WAN technologies used include: circuit switching circuit switching packet switching packet switching frame relay frame relay Asynchronous Transfer Mode (ATM) Asynchronous Transfer Mode (ATM)

Circuit Switching  uses a dedicated communications path established for duration of communication  comprising a sequence of physical links  with a dedicated logical channel  eg. telephone network

Packet Switching  data sent out in sequence  small chunks (packets) of data at a time  packets passed from node to node between source and destination  used for terminal to computer and computer to computer communications

Frame Relay  packet switching systems have large overheads to compensate for errors  modern systems are more reliable  errors can be caught in end system  Frame Relay provides higher speeds  with most error control overhead removed

Asynchronous Transfer Mode  ATM - evolution of frame relay  fixed packet (called cell) length  with little overhead for error control  anything from 10Mbps to Gbps  constant data rate using packet switching technique with multiple virtual circuits

Local Area Networks  smaller scope Building or small campus Building or small campus  usually owned by same organization as the attached devices  data rates much higher within  switched LANs, eg Ethernet  wireless LANs

Metropolitan Area Networks  MAN  middle ground between LAN and WAN  private or public network  high speed  large area – city or metro

1.15 Types of connections: point-to-point and multipoint

1.16 Categories of topology

1.17 A fully connected mesh topology (five devices)

1.18 A star topology connecting four stations

1.19 A bus topology connecting three stations

1.20 A ring topology connecting six stations

1.21 A hybrid topology: a star backbone with three bus networks

The Internet  Internet evolved from ARPANET first operational packet network first operational packet network applied to tactical radio & satellite nets also applied to tactical radio & satellite nets also had a need for interoperability had a need for interoperability led to standardized TCP/IP protocols led to standardized TCP/IP protocols

Internet Elements

 The purpose of the Internet, of course, is to interconnect end systems, called hosts; these include PCs, workstations, servers, mainframes, and so on. Most hosts that use the Internet are connected to a network, such as a local area network or a wide area network (WAN).

 These networks are in turn connected by routers. Each router attaches to two or more networks. Some hosts, such as mainframes or servers, connect directly to a router rather than through a network.

 the Internet operates as follows. A host may send data to another host anywhere on the Internet. The source host breaks the data to be sent into a sequence of packets, called IP datagrams or IP packets.  Each packet includes a unique numeric address of the destination host. This address is referred to as an IP address, because the address is carried in an IP packet.

 Based on this destination address, each packet travels through a series of routers and networks from source to destination. Each router, as it receives a packet, makes a routing decision and forwards the packet along its way to the destination.

Internet Architecture

 The Internet today is made up of thousands of overlapping hierarchical networks.  Individual hosts and LANs are connected to an Internet service provider (ISP) through a point of presence (POP).

 The connection is made in a series of  steps starting with the customer premises equipment (CPE). The CPE is the communications equipment located onsite with the host.  the CPE was traditionally a 56-kbps modem.

Summary  introduced data communications needs  communications model  defined data communications  overview of networks  introduce Internet