Computer Networking 002

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Computer Networking 002

Access Networks

The network that Physically connects an end system to the first router (also known as the “edge router”) on a path from the end system to any other distant end system

Different types of access networks:

• Residential access nets -- private individual devices
• Institutional access networks (school, company) -- business devices
• Mobile access networks (WiFi, 4G/5G)

DSL 数字用户线路

AKA. Digital Subscriber Line -- in residential

Mechanism

• DSL modem (Residential): takes digital data and translates it to high-­frequency tones for transmission over telephone wires to the CO
• Splitter: splits telephone and internet data by frequency coming from ISP, then pass the different information to phone and DSL modem
• DSLAM (ISP): Recives analog signals from DSL modem, translated back into digital format,  then splits telephone and internet data by frequency, and pass them on to different places

Telephone line 有多个不同 frequency 的信号同时传播，用于区分电话信号和网络 data, AKA. frequency division multiplexing (FDM)

Transmission rate

• "Stated" rate: 12 Mbps downstream and 1.8 Mbp upstream -- Asymmetric Access
• Actural rate: less than "state" rate
  • Provider limit the rate purposefully
  • Rate reduced by home-CO distance, thickness of the twisted-pair line and the degree of electrical interference

Cable Internet Access (HFC) 光纤同轴混合网络

Get Internet access from TV cable instead of telephone line. AKA hybrid fiber-coaxial access network (HFC). *Fiber = Optical Fiber = 光缆, Coaxial = 同轴电缆

• Fiber cable: Connects cable head end to neighborhood-level junctions 
• Coaxial cable: Connects neighborhood-level junctions to individual houses and apartments
• Cable modem: Translates analog signal to digital signal at cable head ends; divides HFC into up/down streams (asymmetric, downstream higher)

A Shared Network: Downstream and Upstream signal rates are divided among homes (if downloading the same file or accessing the same webpage at the same time)

FTTH (Fiber To The Home) 光缆到家

Optical fiber path from the CO goes directly to the home

• Faster than HFC

LAN (Local Area Network) -- Enterprise Network

Connects an end system to the edge router. e.g. Ethernet

Ethernet

End users uses twisted-pair copper wire to connect to an Ethernet switch, Ethernet switch connects into the larger Internet

• Wired access at 100Mbps, 1Gbps, 10Gbps

WIFI

Wireless LAN setting, end user connects to a local access point (base station) with IEEE's WiFi technology, which is then connected to internet.

• Wireless access points at 11, 54, 450 Mbps

3G or LTE

Similar to WIFI, but allow end users to be farther away from base station

• Provided by mobile, cellular network operator

Summarization of a Home Network

• Wireles device --> Base station --> Router --> DSL modem --> Cable Head End
• Wried Ethernet device --> Router --> DSL modem --> Cable Head End

Physical Media

Bits propagates between transmitter/receiver pairs

Physical link is what lies between transmitter & receiver

Bit is sent by propagating electromagnetic waves or optical pulses across a physical medium.

Examples of physical media include twisted-pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum.

Guided Media

Waves are guided along a solid medium, such as a fiber-optic cable, a twisted-pair copper wire, or a coaxial cable.

Unguided Media

Waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel.

Twisted-Pair Copper Wire

2 insulated copper wires wisted together in a way to reduce the electrical interference from similar pairs close by.

• Guided
• Largely used by telephones
• Least expensive
• Widely used for LAN networking
• Speed
  • Category 5:100Mbps,1Gbps Ethernet
  • Category 6:10 Gbps Ethernet

Coaxial Cable -- HFC

2 concentric copper conductors

• Guided
• Largely used by TV
• Bidirectional (传输交流电，电流方向可逆转)
• Broadband
  • Multiple frequency channels on cable
  • 100’s Mbps per channel

Optical Fiber

Glass fiber carrying light pulses, each pulse a bit

• Guided
• High-speed (for long-haul transmission)
• High cost
• Immune to electromagnetic interference
• Signal declays slowly

Wireless radio

Examples of wireless radio include:

WIFI 3G/ LTE Bluetooth Terrestrial Radio Channels 地波

Signal carried in various “bands” in electromagnetic spectrum.

• Unguided -- no physical wires
• Can go long distance
• Can penetrate objects
• Performance affected by the environment
  • Can be interferenced by other signals
  • Can be impeded by objects
  • Can be reflected by surfaces

Satellite 卫星

Receives signal from earth and transmit it to another location (ground location) on earth.

• In coming frequency different from outgoing frequency
• Uses repeater to regenerate the signal

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