Network(2)

The Network Edge

End systems/hosts include desktop computers, servers, and mobile devices. host = end system. hosts include clients and servers. Today, most of the servers reside in large data centers.

Access network: 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.

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

Today, the two most prevalent types of broadband residential access are digital subscriber line (DSL) and cable.

DSL

When DSL is used, a customer’s telco is also its ISP. DSL modems use the existing telephone line to exchange data with a digital subscriber line access multiplexer (DSLAM) located in the telco’s local central office (CO). The home’s DSL modem takes digital data and translates it to high-frequency tones for transmission over telephone wires to the CO.

The residential telephone line carries both data and traditional telephone signals simultaneously, which are encoded at different frequencies.

• A high-speed downstream channel, in the 50 kHz to 1 MHz band
• A medium-speed upstream channel, in the 4 kHz to 50 kHz band
• An ordinary two-way telephone channel, in the 0 to 4 kHz band

A splitter separates the data and telephone signals.

Because the downstream and upstream rates are different, the access is said to be asymmetric (ADSL). Different prices, different rates. The maximum rate is alse limited by the distance between the home and the CO, the gauge of the twisted-pair line and the degree of electrical interference.

DSL is for short distance network, within 5 to 10 miles.

Cable

cable Internet access makes use of the cable television company’s existing cable television infrastructure.

Because both fiber and coaxial cable are employed in this system, it is often referred to as hybrid fiber coax (HFC).

Cable internet access requires special modems, called cable modems, which connect to the home PC through an Ethernet port.

The cable modem termination system (CMTS) turns the analog signal sent from the cable modems in many downstream homes back into digital format.

Cable modems divide the HFC network into two channels, a downstream and an upstream channel, asymmetrically.

Cable Internet access is a shared broadcast medium. Share rate. A distributed multiple access protocol is needed to coordinate transmissions and avoid collisions.

Fiber to the home

The FTTH provide an optical fiber path from the CO directly to the home. Technologies: direct fiber, shared fiber. Splitting fiber: active optical networks (AONs), and passive optical networks (PONs).

PON: Each home has an optical network terminator (ONT), connects to optical splitter.The splitter combines homes onto a single shared optical fiber, which connects to an optical line terminator(OLT) in the telco’s CO. The OLT provides conversion between optical and electrical signals.

Satellite link

Dial-up access

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

On corporate and university campuses, a local area network (LAN) is used to connect and end system to the edge router.

Ethernet

Ethernet users use twisted-pair copper wire to connect to an Ethernet switch.

WiFi

In a wireless LAN setting, wireless users transmit/receive packets to/from an access point that is connected into the enterprise’s network.

Home LAN

4G

LTE, Long-Term Evolution

Physical Media

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

A bit is sent by propagating electromagnetic waves or optical pulses across a physial medium, including twisted-pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum.

Two categories: guided media and unguided media.

Twisted-Pair Copper Wire

Unshielded twisted pair (UTP) is commonly used for computer networks. 10 Mbps to 10 Gbps. The data rates depend on the thickness of the wire and the distance between transmetter and receiver.

Coaxial Cable

Can be used as a guided shared medium.

Satellite Radio Channels

Two types of satellites: geostationary satellites and low-earth orbiting (LEO) satellites