Computer Networks – Objective Revision Notes

Wireless LAN Architectures

• Two basic Basic-Service-Set (BSS) deployment modes are repeatedly contrasted:
  • $\textbf{Infrastructure BSS}$: one Access Point (AP) centralises all coordination; this is the answer hinted at by Q.1.
  • $\textbf{Ad-hoc BSS}$ (also called Independent BSS, IBSS): stations communicate peer-to-peer with no AP (Q.25).
• $\textbf{ESS – Extended Service Set}$: multiple infrastructure BSSs interconnected through a Distribution System (wired or wireless).
• $\textbf{NAV – Network Allocation Vector}$: a timer each station maintains to implement virtual carrier sensing (collision avoidance) in IEEE 802.11 CSMA/CA.

Throughput vs. Load

• Ideal performance curve: throughput increases linearly with offered load until the channel capacity is reached.
• After capacity:
  • In well-controlled systems (e.g., token passing) throughput can plateau.
  • In contention-based LANs (e.g., Ethernet, WLAN) collisions cause throughput to $\textit{decline}$; Q.2 specifically cites “declines sharply.”

Physical & Logical Topologies

• $\textbf{Star}$ (Q.3): requires a central hub/switch.
• $\textbf{Bus, Ring, Mesh}$ do not need a hub; each has unique pros/cons (fault tolerance, cabling length, complexity).

Inter-Networking Devices

• $\textbf{Repeater}$ – regenerates signals at the physical layer.
• $\textbf{Hub}$ – multi-port repeater, single collision domain.
• $\textbf{Bridge}$ – data-link device; forwards frames based on MAC address (Q.11 & Q.4).
• $\textbf{Switch}$ – multi-port bridge with micro-segmented links.
• $\textbf{Router}$ – network-layer device; forwards packets via IP address.
• $\textbf{Gateway}$ – protocol-translation device operating across all $7$ OSI layers (Q.32).

Multiplexing Techniques

• $\textbf{FDM}$ (Frequency Division) – analogue.
• $\textbf{TDM}$ (Time Division) – digital (correct for Q.5).
• $\textbf{WDM}$ (Wavelength Division) – optical fibre.

IPv4 Subnetting Scenario

• Given Class-A $10.0.0.0/8$, already $40$ subnets; need $60$ more => $100$ total.
• Formula: $2^{n} \ge 100 \Rightarrow n = 7\ (128\ subnets)$.
• New subnet mask: $/15 = 255.254.0.0$ (choice d) – maximises host bits (Q.6).

IPv4 Header Essentials

• Size ranges $20 \text{–} 60$ bytes (Q.7). Extra bytes come from options field.
• Header Length field (IHL) is $4$ bits (Q.8) → value × $4$ gives size in bytes.

IPv6 Highlights

• Hop Limit field supersedes TTL; controls diagram lifetime (Q.9).
• Next Header = demultiplexing to TCP/UDP/ICMPv6, etc.

Cyclic Redundancy Check (CRC)

• Sender treats data as polynomial; appends $\textit{remainder}$ of division by generator polynomial; receiver re-computes. Remainder is called CRC (Q.10).

Routing Protocols

• $\textbf{RIP}$ uses distance-vector algorithm with metric = hop-count, max $15$ (Q.12).
• $\textbf{OSPF}$ (link-state), $\textbf{BGP}$ (path-vector).
• $\textbf{Spanning Tree Protocol (STP)}$: constructs loop-free subset of bridges (Q.14).

WLAN MAC Behaviour

• Collision Detection (CD) impossible in radio; WLAN relies on CSMA/CA with ACKs (Q.13 lists "collision detection" as impossible; "collision avoidance" is actually used).

Addressing & Ports

• $\textbf{Port number}$ identifies application process (Q.15), logical host = IP, physical = MAC.
• $\textbf{SSH}$ default TCP port $22$ (Q.18).
• $\textbf{FTP}$: control $21$, data $20$ (Q.19). One control connection + one data connection per file ⇒ $1 + 10 = 11$ TCP connections for $10$ files (Q.22).
• $\textbf{Ephemeral ports}$: $1024 – 65535$ (Q.20).
• $\textbf{DNS}$ can use UDP or TCP port $53$ (Q.21).

QoS Models

• IntServ (per-flow reservations, RSVP).
• DiffServ (class-based, DS field in IP header) – answer to Q.16.

Guard Space in TDMA

• Silent interval inserted between consecutive bursts to compensate clock drift & propagation (Q.17).

Network Availability Metric

• Availability $A = \dfrac{\text{Total Time Available} - \text{Downtime}}{\text{Total Time Available}}$ (choice b vs d wording in Q.23; standard definition yields fraction between $0$ and $1$).

Bluetooth Piconet

• One primary (master) + up to $7$ active secondaries (Q.24 choice d).
• Additional devices may park/sleep.

IEEE 802 Family

• $802.3$ → CSMA/CD Ethernet (Q.26).
• $802.2$ LLC, $802.4$ Token Bus, $802.5$ Token Ring.

DHCP Lease

• Allocates IP address for limited lease period, renewable (Q.27).

HTTP Request Methods

• $GET$ retrieves resource, $PUT$ uploads/creates, $POST$ submit data, $DELETE$ removes. Q.29 answer is $GET$.

SMS & Mobile Handoffs

• SMS length: $160$ 7-bit ASCII chars or $140\,\text{bytes}$ (Q.30).
• $\textbf{Hard handoff}$: break–before-make vs $\textbf{Soft}$: make-before-break using RAKE receivers (CDMA). Q.31 describes soft handoff.

OSI / Internet Stack Mapping

• Physical, Data-Link, Network = $\textbf{network support layers}$ (Q.34).
• Network layer adds IP header with logical addresses (Q.33).

Transmission Media Speed Order

• Slowest → fastest: Twisted-pair < Coaxial < Microwave (variable) < Fibre-optic. Q.35 marks twisted-pair as slowest.