Computer Networks

4% of exam

Routing & switching, topologies, LAN/WAN, OSI & TCP/IP models, network security.

3 concepts

Exam areas

A. Routing and switching

Routing and Switching

Layer-2 switching on MAC addresses versus Layer-3 routing on IP, IPv4 addressing and CIDR subnetting math, basic routing behavior, and the LAN/WAN/MAN/PAN scale ladder.

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D. Network models

OSI and TCP/IP Models (with Network Topologies)

The seven-layer OSI stack, the four-layer TCP/IP stack, encapsulation into PDUs, and protocols per layer (FE subarea D), plus the star/bus/ring/mesh physical topologies that belong to FE subarea B.

Every networking question on the FE rests on one idea: a network is built in layers, and each layer talks only to its peer on the far machine while leaning on the layer below it. The FE Reference Handbook (Electrical and Computer Engineering — Computer Networks) presents two layered models side by side: the seven-layer OSI reference model and the four-layer TCP/IP model that actually runs the internet. If you can name the layers in order, state what each one does, and trace how a message gets wrapped in headers on the way down and unwrapped on the way up, you will pick up most of the points in this area without any arithmetic at all. Scope note: the layered-model material here is FE Computer Networks subarea D (Network models). The final two sections cover physical topologies, which the NCEES FE ECE specification lists as a separate subarea, B (Network topologies); they are bundled here for convenience and are clearly flagged where they begin.

Why layering exists

Layering is divide-and-conquer for communication. Each layer hides its internal machinery behind a clean interface, so the designer of an email program never thinks about voltage levels and the designer of a fiber link never thinks about email. A layer adds exactly one job's worth of information — an address, a sequence number, an error check — and trusts the layer below to carry the result. This is the same modular-abstraction principle the handbook describes for encapsulation: logically separate functions are abstracted from their underlying structure by information hiding within higher-level objects. The payoff is that you can swap Wi-Fi for Ethernet at the bottom, or swap a browser for a video player at the top, without touching anything in between.

The seven OSI layers

From top to bottom the OSI layers are Application, Presentation, Session, Transport, Network, Data Link, and Physical. The Application layer is closest to the user; Presentation handles formatting, encoding, and encryption of data; Session opens, manages, and closes conversations (full-duplex, half-duplex, or simplex). Transport gives process-to-process delivery with optional reliability and flow control; Network finds routes and forwards packets between hosts; Data Link moves frames between two devices on the same link and does medium access and error detection; Physical defines the bits on the wire — modulation, coding, data rate. A classic mnemonic top-down is "All People Seem To Need Data Processing."

\underbrace{\text{App}\;\text{Pres}\;\text{Sess}}_{\text{TCP/IP Application}}\;\underbrace{\text{Transport}}_{\text{Transport}}\;\underbrace{\text{Network}}_{\text{Internet}}\;\underbrace{\text{Data Link}\;\text{Physical}}_{\text{Network Interface}}

The four TCP/IP layers and how they map

The TCP/IP model collapses OSI into four layers: Application, Transport, Internet, and Network Interface (sometimes called Link). The handbook states the mapping explicitly: the TCP/IP Application layer covers OSI's Application, Presentation, and Session; TCP/IP Transport and Internet correspond one-to-one with OSI Transport and Network; and the TCP/IP Network Interface layer covers OSI's Data Link and Physical. TCP/IP is the model that matters in practice — OSI is the reference vocabulary everyone uses to describe it.

Encapsulation and PDUs

As application data travels down the stack, each layer prepends its own header (and the link layer often adds a trailer too) — this wrapping is encapsulation. The handbook names the Protocol Data Units precisely: the application produces a message; the transport layer adds a TCP/UDP header to make a segment; the network layer adds an IP header to make a packet (datagram); the link layer adds a frame header to make a frame; the physical layer sends bits. On receipt the process reverses — each layer strips its own header and hands the rest up. A handy PDU mnemonic for the lower layers is "Some People Fear Birthdays": Segment, Packet, Frame, Bits.

\text{Data} \to [\text{TCP/UDP}\,|\,\text{Data}] \to [\text{IP}\,|\,\text{TCP/UDP}\,|\,\text{Data}] \to [\text{Frame}\,|\,\text{IP}\,|\,\text{TCP/UDP}\,|\,\text{Data}]

Protocols and devices by layer

Match each protocol to its layer. Application: HTTP/HTTPS, FTP, SMTP, DNS, DHCP. Transport: TCP (connection-oriented, reliable, ordered, with retransmission) and UDP (connectionless, no delivery guarantee, lower overhead). Internet/Network: IP (IPv4 and IPv6), plus the supporting ICMP used for error and diagnostic messages such as the Time Exceeded reply that traceroute relies on. Network Interface/Data Link: Ethernet and Wi-Fi (802.11), ARP. Devices map the same way: a hub or repeater is purely Physical, a switch or bridge is Data Link (Layer 2), and a router is Network (Layer 3). The handbook's host/switch/router figure makes the point that a switch runs only the bottom two layers while a router runs three.

\text{TCP} \Rightarrow \text{reliable, ordered}, \qquad \text{UDP} \Rightarrow \text{fast, best-effort}

Physical topologies (FE subarea B: Network topologies)

Cross-reference: this section and the worked mesh example below cover FE Computer Networks subarea B (Network topologies — e.g., mesh, ring, star), which NCEES lists separately from the subarea D network models above. Topology is the geometric arrangement of nodes and links, distinct from the logical layering. In a star, every node connects to a central hub or switch — a single point of failure, but easy to manage; this is the dominant modern LAN form. In a bus, all nodes share one backbone cable; cheap but a break or collision affects everyone. In a ring, each node connects to two neighbors forming a loop, so traffic circulates (often with a token). In a mesh, nodes have many redundant links — a full mesh of

n

nodes needs

n(n-1)/2

links, giving maximum fault tolerance at maximum cost, which is why it is reserved for backbones.

\text{Full-mesh links} = \dfrac{n(n-1)}{2}

Exam strategy

Most questions here are recognition, not calculation. Memorize the OSI layers in order both ways and the OSI-to-TCP/IP mapping; the handbook gives you the figure but you save time knowing it cold. When asked where a protocol or device 'lives', anchor on three facts: switch = Layer 2 (MAC), router = Layer 3 (IP), and the Application layer absorbs OSI's top three. For encapsulation questions, walk the PDU chain message

\to

segment

\to

packet

\to

frame

\to

bits and remember headers are added going down, stripped going up. For topology questions (subarea B), reason from failure modes: star fails if the hub dies, bus fails if the backbone breaks, mesh survives almost anything but costs

n(n-1)/2

links.

Key equations

OSI layer order (top to bottom)

\text{App} \to \text{Pres} \to \text{Sess} \to \text{Trans} \to \text{Net} \to \text{Link} \to \text{Phys}

The seven OSI layers, layer 7 down to layer 1. Mnemonic: All People Seem To Need Data Processing.

TCP/IP layer order

\text{Application} \to \text{Transport} \to \text{Internet} \to \text{Network Interface}

The four TCP/IP layers, top to bottom; the practical model used on the internet.

OSI to TCP/IP mapping

\{7,6,5\}\!\to\!\text{App},\;4\!\to\!\text{Trans},\;3\!\to\!\text{Internet},\;\{2,1\}\!\to\!\text{Net.\ Interface}

TCP/IP Application = OSI App+Pres+Sess; Network Interface = OSI Data Link+Physical; Transport and Internet map one-to-one.

PDU by layer

\text{message} \to \text{segment} \to \text{packet} \to \text{frame} \to \text{bits}

Protocol Data Unit at Application, Transport, Network, Data Link, and Physical respectively. Packet = datagram.

Encapsulation rule

\text{PDU}_{n} = \text{Header}_{n} \,\|\, \text{PDU}_{n+1}

Each layer n prepends its header to the PDU from the layer above; $\|$ denotes concatenation. Reversed on receipt (decapsulation).

Device-to-layer map

\text{hub}\!=\!L1,\quad \text{switch/bridge}\!=\!L2,\quad \text{router}\!=\!L3

Physical, Data Link, and Network layers respectively; the layer a device reads to make forwarding decisions.

Full-mesh link count (subarea B)

L = \dfrac{n(n-1)}{2}

Topology metric (FE subarea B, Network topologies): number of direct links in a full mesh of n nodes; quantifies the cost of full redundancy.

TCP vs UDP

\text{TCP: connection-oriented, reliable};\;\; \text{UDP: connectionless, best-effort}

TCP detects loss/duplication/reordering and corrects; UDP has less overhead but no delivery guarantee.

Worked examples

Place the PDU and the device

Problem. A browser sends an HTTPS request. (a) At which OSI layer is the unit called a 'segment'? (b) A device on the path reads only the destination MAC address to forward the unit out a port — is it a hub, switch, or router? (c) Which TCP/IP layer absorbs the OSI Presentation layer that encrypts the request?

Solution. (a) The 'segment' is the Transport-layer PDU — OSI layer 4 — where the TCP header is added. (b) Reading the MAC (link-layer) address is a Layer 2 action, so the device is a switch (a hub is Layer 1 and forwards bits blindly; a router reads the Layer 3 IP address). (c) The TCP/IP Application layer corresponds to OSI Application + Presentation + Session, so Presentation is absorbed into the TCP/IP Application layer. Sanity check: encryption/formatting at Presentation, sessions at Session, and the app itself all live in the single TCP/IP Application layer — consistent with the handbook mapping.

\text{segment}\!\Rightarrow\!L4,\quad \text{MAC forwarding}\!\Rightarrow\!L2\ (\text{switch}),\quad \text{Pres}\!\subset\!\text{TCP/IP App}

Mesh link count and redundancy (subarea B)

Problem. Topology problem (FE subarea B). A backbone is to fully mesh 8 core routers so every router has a direct link to every other. How many physical links are required, and how many links would be added by growing to 10 routers?

Solution. Full mesh links for n=8:

L_8 = \dfrac{8(8-1)}{2} = \dfrac{56}{2} = 28

links. For n=10:

L_{10} = \dfrac{10\cdot 9}{2} = 45

links. Added links going from 8 to 10:

45 - 28 = 17

. Sanity check: each new router connects to all existing ones, so the 9th adds 8 and the 10th adds 9, total

8+9=17

— matches.

L = \dfrac{n(n-1)}{2};\quad L_8 = 28,\; L_{10} = 45,\; \Delta = 17

Common pitfalls

•Reversing the OSI order: layer 1 is Physical (bottom), layer 7 is Application (top). 'Top three' for the TCP/IP mapping means App+Pres+Sess, not Transport.
•Calling the network-layer PDU a 'packet' but the transport-layer PDU also a 'packet' — on the FE, Transport = segment, Network = packet (datagram). Keep them distinct.
•Placing a router at Layer 2 or a switch at Layer 3. Switch/bridge = Layer 2 (MAC), router = Layer 3 (IP). A hub is Layer 1.
•Putting TCP/UDP at the Network layer. They are Transport (Layer 4); IP and ICMP are Network (Layer 3).
•Assuming encapsulation only adds headers going up. Headers are added going DOWN the sender's stack and stripped going UP the receiver's stack.
•Confusing physical topology (FE subarea B) with logical layering (subarea D) — a star-wired LAN can still behave as a logical bus (classic Ethernet) or be switched; topology is geometry, not protocol.
•Forgetting UDP is connectionless with no delivery guarantee — do not attribute TCP's reliability/ordering to UDP just because both are Transport-layer.

References

NCEES FE Reference Handbook — Electrical and Computer Engineering: Computer Networks (OSI and TCP/IP models, encapsulation)
NCEES FE Electrical and Computer Exam Specification — Computer Networks subareas B (Network topologies: mesh, ring, star) and D (Network models: OSI, TCP/IP) — Confirms topologies (subarea B) are a separate official subarea from network models (subarea D); the topology section, full-mesh equation, and mesh example here are subarea-B coverage cross-referenced into this models concept.
ISO/IEC 7498-1 — OSI Basic Reference Model — Authoritative definition of the seven layers.
Tanenbaum & Wetherall, Computer Networks — Layering, encapsulation, and topologies; the handbook figure is adapted from Tanenbaum.

F. Security

Network Security

The CIA triad, symmetric versus asymmetric encryption and key-scaling math, authentication and hashing, firewalls and IDS/IPS placement, and common attacks with their mitigations.

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