1.2 The Network Edge

• Host

  • Clients

    • Tend to be desktop and mobile PCs, smartphones, and so on

  • Servers

    • Tend to be more powerful machines that store and distribute Web pages, stream video, relay e-mail and so on

    • Most of the servers reside in large data centers

1.2.1 Access Networks

• Access network: the network that physically connects an end system to the first router ("edge router") on a path from the end system to any other distant end system.

• Home Access: DSL, Cable, FTTH, Dial-Up, and Satellite

  • Two most prevalent types of broadband residential access: digital subscriber line (DSL) and cable

  • DSLAM: digital subscriber line access multiplexer

  • Hybrid fiber coax (HFC): both fiber and coaxial cable are employed

  • Fiber to the home (FTTH)

  • Optical network terminator (ONT)

• Access in the Enterprise (and the Home): Ethernet and WiFi

  • Local area network (LAN)

    • Ethernet

  • Wireless LAN

    • Access based on IEEE 802.11 technology, more colloquially known as WiFi

• Wide-Area Wireless Access: 3G and LTE

  • Unlike WiFi, a user need only be within a few tens of kilometers (as opposed to a few tens of meters) of the base station

1.2.2 Physical Media

• E.x. HFC uses a combo of fiber cable and coaxial cable. DSL and Ethernet use copper wire. Mobile access networks use the radio spectrum.

• For each transmitter-receiver pair, the bit is sent by propagating electromagnetic waves or optical pulses across a physical medium.

  • Can take many shapes and forms and does not have to be of the same type for each transmitter-receiver pair along the path

  • E.x. twisted-pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum

  • Categories

    • guided media: the waves are guided along a solid medium, such as a fiber-optic cable, a twisted-pair copper wire, or a coaxial cable

    • unguided media: the waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel

  • Cost of physical link is relatively minor compared with other networking costs

• Twisted-Paire Copper Wire

  • Unshielded twisted pair (UTP)

  • Rate: 10 Mbps to 10 Gbps

  • Dominant solution for high-speed LAN networking

• Coaxial Cable

  • Can be used as a guided shared medium

    • A number of end systems can be connected directly to the cable, with each of the end systems receiving whatever is sent by the other end systems

• Fiber Optics

  • Thin, flexible medium that conducts pulses of light, with each pulse representing a bit

  • Preferred for oversea links

• Terrestrial Radio Channels

  • Radio channels carry signals in the electromagnetic spectrum.

  • Classified: short distance, local areas, span from ten to a few hundred meters, wide area, spanning tens of kilometers

• Satellite Radio Channels

  • Links two or more Earth-based microwave transmitter/receivers, known as ground stations. The satellite receives transmissions on on frequency band, regenerates the signal using a repeater, and transmits the signal on another frequency.

  • geostationary satellites

  • low-earth orbiting (LEO) satellites