> For the complete documentation index, see [llms.txt](https://sliu583.gitbook.io/blog/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sliu583.gitbook.io/blog/networking/index/computer-networking-a-top-down-approach/chapter-1.-computer-network-and-the-internet/1.2-the-network-edge.md).

# 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. 

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* 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) 

![](https://2097630930-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MVORxAomcgtzVVUqmws%2F-MZfMbmAR0-EbMUxivlI%2F-MZfOv4bgoygNuvCeoWH%2Fimage.png?alt=media\&token=dffc06b1-7dd0-4d85-ab48-64a2930d3527)

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* 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 

![](https://2097630930-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MVORxAomcgtzVVUqmws%2F-MZfMbmAR0-EbMUxivlI%2F-MZi4ZOKrCK0IhxLHin4%2Fimage.png?alt=media\&token=3dfa502f-c9d0-4732-8764-b38c8e75ce3f)

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* 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