Day 2 - Communication Methods and Network Technologies

📌 Key Points

• There are two switching methods — circuit switching (telephone network, PSTN) and packet switching (data network, PSDN).
• Packet switching splits further into datagram (connectionless) and virtual circuit (connection-oriented).
• LAN access methods — Ethernet uses CSMA/CD, wireless LAN uses CSMA/CA, Token Ring and FDDI use token passing.
• The WAN technology order is X.25 → Frame Relay → ATM. An ATM cell is a fixed 53 bytes.
• IEEE standards — 802.3 Ethernet · 802.5 Token Ring · 802.6 DQDB · 802.11 wireless LAN.

Classification by Scale

• The MAN standard is DQDB (IEEE 802.6). Questions ask about the middle one of the three scales.

Circuit Switching and Packet Switching

   Circuit Switching                      Packet Switching
 
   A ══════════════════════ B            A ─┐ ┌─ Packet 1 ─┐ ┌─ B
     A dedicated path is held                 └─┼─ Packet 2 ─┼─┘
     for the whole call                         └─ Packet 3 ─┘
   Telephone network (PSTN)                Data network (PSDN)

• It is easy to remember that in the name, T (telephone) means circuit switching, D (data) means packet switching.

The difference seen in the order of operation

   Circuit switching   ① Set up circuit  →  ② Transfer data  →  ③ Release circuit
                       (Reserve the path first)  (Flows only along it)   (Give the path back)
 
   Packet switching    ① Cut the data into packets  →  ② Attach the destination address to each packet
                       →  ③ Intermediate devices receive and hold it, then pass it on  →  ④ Reassemble at the destination

• Circuit switching takes connection setup time before the call. In exchange, once connected you have the path to yourself, so the delay is constant. The ring tone while you place a phone call is step ①.
• Step ③ of packet switching is called store-and-forward. An intermediate device stores the packet first and sends it out when the path is free. That lets many share one line, but delay grows when it is congested.
• Because an address must be attached to every packet, there is header overhead. Circuit switching has no such overhead.

The two packet-switching methods

• A virtual circuit is "packet switching that looks like circuit switching." It resembles circuit switching in that the path is decided in advance, but it does not hold that path exclusively and shares it with other communications. So it is classified as packet switching all the same.
• In a datagram, each packet carries a complete destination address and finds its own way. If one intermediate path breaks, it can go around by another route, so it is resilient to failures. The Internet (IP) works this way.
• Tie the three words together in one line: "virtual circuit = connection-oriented = order preserved," "datagram = connectionless = order may change."

LAN Access Control Methods

These are the rules that decide who sends and when when several computers share one wire.

CSMA/CD

   1  Listen to see whether the line is idle (Carrier Sense)
   2  If it is idle, send
   3  If a collision happens, detect it (Collision Detection)
   4  Wait a random time and send again

• It is a "send first, and retry if there is a collision" method. It is the standard for wired Ethernet and is IEEE 802.3.
• In wireless, signals are weak, so collisions are hard to detect. That is why it uses CSMA/CA, a method that avoids them in advance (Avoidance).
• CD is wired, CA is wireless — this one sentence is the exam point.

Token passing

• A signal called a token is passed around, and only the holder transmits. There are no collisions at all.
• Token Ring (IEEE 802.5) and FDDI use this method.

The three methods side by side

• Spelling out the abbreviations gives the meaning directly — CS (Carrier Sense, listening to the line) · MA (Multiple Access, many use it together) · CD (collision detection) / CA (collision avoidance).
• The reason CSMA/CD waits a "random time" after a collision is that if both waited the same time, they would send at the same moment again and collide again.
• The reason collisions are hard to detect in wireless is that the device's own transmitted radio wave is much stronger, so it cannot pick out a weak signal arriving from far away at the same moment.
• The CSMA family is "contention," token passing is "taking turns." If a question asks for "the method in which collisions do not occur," it is token passing.

WAN Technologies

   X.25  ────►  Frame Relay  ────►  ATM
   (1970s)       (1990s)            (1990s)
   Slow but      Less error         Fixed 53-byte cells
   robust to     checking, so       Voice and video together
   errors        faster

ATM's 53 bytes is tested as a literal value. It is a 5-byte header + 48 bytes of data. The decisive difference from the other methods is that the length is fixed.

Remember FDDI as fiber-optic cable + dual ring + token passing together. If one ring breaks, traffic detours through the opposite ring.

Why it changed in this order

• When X.25 appeared, lines were noisy. So every intermediate device checked for errors and received the data again if it was wrong. It is safe, but slow because of checking at every hop. It is a standard set by ITU-T, and the X in the name is the series letter of ITU-T recommendations.
• When fiber-optic cable was laid and line errors dropped sharply, per-hop checking became wasteful. Frame Relay removed that checking and left error recovery to the devices at both ends, raising speed.
• ATM went one step further and cut data into cells that are all the same size. When sizes are constant, equipment can predict processing time, so delay-sensitive voice and video can be carried on the same network as data. That is why ATM's switching method is also called cell switching.
• The A (Asynchronous) in ATM means that cells are filled and sent only when there is something to send. It shares only its spelling with the bank ATM (cash machine).

• All three are the same in being packet switching with virtual circuits. What differs is the length of the transfer unit and how much error control is done.

Picking the technology from one keyword

• FDDI and DQDB are confusing because both say "dual." FDDI is a dual ring (fiber-optic cable), DQDB is a dual bus (MAN).
• Token Ring and FDDI both use token passing. If it is a copper ring, it is Token Ring (802.5); if it is a dual fiber-optic ring, it is FDDI.

IEEE 802 Standards

• 802.3 (Ethernet) and 802.11 (wireless LAN) are the representative ones.

Related International Organizations

• IANA is the place that hands out addresses and port numbers. The port assignments in /etc/services also come from here.
• Mobile communication standards such as 5G are made by 3GPP.

• ITU and 3GPP have split roles in 5G. The International Telecommunication Union (ITU) set the requirements 5G must meet under the name IMT-2020, and the detailed technical specifications that satisfy those requirements are made by 3GPP.
• IANA and ICANN are the ones that "hand out numbers and names", while IETF, IEEE, ISO, and ITU-T are the ones that "set the rules." First check whether the question asks about allocation or about standard-setting.
• IEEE and IETF, whose names are similar, cover different layers. IEEE is closer to network cards and cables, the lower layers (LAN), while IETF covers Internet protocols such as IP, TCP, and HTTP.

Tomorrow we will calculate IP addresses and subnet masks by hand.

📖 Terms

• Circuit switching : A method that reserves a dedicated path for the whole communication. The telephone network (PSTN) is the typical example.
• Packet switching : A method that breaks data into pieces and sends each one. The data network (PSDN) is the typical example.
• Datagram / virtual circuit : The connectionless method / connection-oriented method of packet switching.
• CSMA/CD : The wired Ethernet access control method that detects collisions and retransmits.
• CSMA/CA : The wireless LAN access control method that avoids collisions in advance.
• ATM : An integrated transmission method that uses fixed-length 53-byte cells.
• FDDI : A network technology that works by token passing on a dual fiber-optic ring.
• IANA : The organization that manages allocation of Internet resources such as IP addresses and port numbers.