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0x350 Physical and Link

1. Network Elements

1.1. Guided Media

Guided media means the wave are guided along a solid medium.

1.1.1. Twisted-Pair Copper Wire

The least expensive and most twisted pair, the UTP (Unshielded twisted pair) is still common for LAN cables.


The ISP is usally the local wired telephone companies (telco). Dial-Up

Traditionally, most of the Internet users connect Internet over ordinary analog telephone line using a dial-up modem. Phone call and Internet access cannot be done at the same time. DSL

Usually ADSL (Asymmetric Digital Subscriber Line), with which download speed is much faster than the upload speed. For business, there is the SDSL (symmetric) version.

DSL uses different frequencies to encode phone call and network access. For example, one configuration can be as follows:

  • phone: 0 ~ 4kHz
  • upload: 4kHz - 50kHz
  • download 50kHz - 1MHz

1.1.2. Coaxial Cable

Coaxial Cable is quite common in cable television systems, so the ISP is usually the television company. The disadvantage is the cable is shared by neighborhood, therefore requires some protocols to coordinate transmissions and avoid collisions. (e.g: CSMA/CD)

1.1.3. Fiber


Submarine cables

1.2. Unguided Media

Unguided media, the waves propagate in the atmosphere and in outer space. Sorry Michelson, no ether is guiding!

Depending on the wave lengths, we have roughly 3 types here

1.2.1. Infrared

Frequency 300GHz - 400THz Used for very short distance communication such as TV remotes, wireless mouse. Cannot penetrate obstacles.

1.2.2. Microwave

Frequency 1GHz - 300GHz Its propagation is a type of line-of-sight propagation, so sender/receiver anthenas should be properly aligned.

It is used for mobile phone communication and television distribution. Wifi is usually in this range: 900 MHz, 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz, 5.9 GHz and 60 GHz bands

1.2.3. Radiowave

Frequency 3KHz - 1GHz Easy to generate and penetrate, anthenas do not need to be aligned. Usage is AM/FM radios Terrestrial Radio Channels

carry signals in the elctromagnetic spectrum, spanning from 10~100m (e.g: wifi) and ~10km (e.g: cellular access) Satellite Radio Channels

Two types aof satellite are used - geostationary satellites: stationary over the same spot on Earth (about 36,000 km), siginificant communication delay. - low-earth orbiting satellites: much closer (2,000 km), rotate around Earch and might communicate with each other. (SpaceX's StarLink is this type)

Note GPS is around 20,000km, ISS is around 400 km


There are two fundamental approaches to moving data through a network of links and switches: circuit switch and packet switch.

Telephone network is an example of circuit-switched networks and Internet is a packet-switched network.


circuit switching: resources along a path (e.g: buffer, link transmission rate) are reserved for the duration. the connection is called a circuit. delivery is guaranteed

The circuit in a link is implemented with either frequency-division multiplexing (FDM) or time-division multiplexing (TDM)


packet switching: resources are not reserved. it use the resources on demand. delivery is not guaranteed (it only makes best effort)

2. Physical Protocol (1)

2.1. Ethernet (IEEE 802.3)

2.2. Wireless LAN (IEEE 802.11)

This layer is the protocol layer that transfers data between nodes within a local network segment

3.1. MAC

  • unique 48 bit for each hardware
  • each mac address is associated with NIC (network interface card)
  • OUI (Organizationally Unique Identifier) is the first three octats

3.2. ARP (Address Resolution Protocol)

ARP is a IPv4 Protocol defined at RFC0826, it is used to translate 32bit IP address into 48bit MAC address.RFC0826


reference: from TCP/IP Illustrated, Volume 1: The protocols

3.2.1. Protocol

ARP can be either dynamic or static. User can specify IP/mac mapping with arp -s, otherwise it can be requested dynamically and stored in cache:

  • ARP cache: store arp records in cache, TTL is typically 20 min.
  • ARP request: If not in cache or expired, broadcast an ethernet frame under the subnet mask to request a MAC address corresponding to an IP.
  • ARP reply: NIC with the MAC address reply to the request with unicast. Other NICs will ignore the requests (although they can see the broadcast request)

ARP can also be

3.2.2. Command

  • arp -a: retrieval local arp cache
  • arp -s: register static IP/mac pair

3.2.3. Security

  • arp spoofing: man in the middle attack (ettercap)
  • mac flooding

3.3. NDP (Neighbor Discovery Protocol)

  • IPv6 Protocol as a replacement of ARP

3.4. Hardware

L2 Switch

4. Reference

[1] TCP/IP Illustrated, Volume 1: The protocols Chapter 4