Computer Networks (CS-602) Unit 2 Notes - RGPV

How unit 2 is examined

The unit covers layer 2: services, framing, flow and error control, sliding-window ARQ, protocol modelling and address resolution. Go-Back-N/Selective Repeat and ARP carry the most marks, then flow control, framing and error control.

Data Link Layer: Need

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. The data link layer is layer 2 of OSI; it turns the raw, error-prone bit pipe of the physical layer into a reliable link for node-to-node delivery of frames.

Key points.

The physical layer only moves bits and does not know where a message starts or whether it arrived correctly, so a layer above it must add structure and reliability.
The layer groups bits into frames and adds physical (MAC) addresses so that the correct neighbouring node picks up each frame.
It detects and corrects transmission errors and stops a fast sender from swamping a slow receiver.
It shares one medium among many stations through the MAC sublayer.

Services Provided

Definition. The data link layer provides services to the network layer above it: framing, physical addressing, error control, flow control and access control over a single hop.

Key points.

Framing packs network-layer packets into frames with header and trailer so the receiver can find frame boundaries.
Physical addressing puts the sender and receiver MAC addresses in the frame header.
Error control detects damaged, lost or duplicate frames and recovers them by retransmission.
Flow control keeps the sender no faster than the receiver can accept.
Access control (MAC) decides which station may use a shared channel. Services may be unacknowledged connectionless, acknowledged connectionless or acknowledged connection-oriented.

Asked: [7 marks] (May 2023) What is the data link layer and what services does it provide?

Framing

Definition. Framing divides the bit stream into frames, each with header, data and trailer, so the receiver can detect where every frame starts and ends.

Key points.

Functions of the data link layer are framing, physical addressing, error control, flow control and access control; state these first in a "functions" question.
Character count puts the frame length in the header's first field; it fails if that count is corrupted, because the receiver loses sync.
Byte stuffing marks frames with a FLAG byte; a FLAG or ESC byte appearing inside the data is preceded by an ESC byte, and the receiver removes the ESC.
Bit stuffing uses the flag 01111110; the sender inserts a 0 after every five consecutive 1s in the data, and the receiver deletes a 0 that follows five 1s.
Physical layer coding violation uses signal patterns that are illegal for data (for example, Manchester code with no mid-bit transition) as delimiters; it works only where the encoding has spare signals.

Example. Bit stuffing: data 0110111110111111 becomes 011011111001111101 (a 0 added after each run of five 1s), placed between two 01111110 flags. Byte stuffing: data A FLAG B is sent as FLAG A ESC FLAG B FLAG; data A ESC B becomes A ESC ESC B.

<mark>Stuffing lets a frame carry any data pattern without the flag being mistaken for a frame boundary.</mark>

Answer frame. Open with the definition of framing; list DLL functions; then character count, byte stuffing with the ESC example, bit stuffing with the 01111110 example, coding violation; close with the note that stuffing keeps the data transparent.

Asked: [7 marks] (May 2022) What is bit and byte stuffing? Explain with example. Asked: [7 marks] (Jun 2026) Explain the functions of the Data Link Layer. Discuss framing techniques in detail.

Flow Control

Definition. Flow control is the set of procedures that stops a fast sender from overwhelming a slow receiver's buffer; the receiver tells the sender when to send.

Key points.

Stop-and-Wait: the sender sends one frame and waits for its ACK before the next; a timeout triggers retransmission and sequence numbers 0/1 separate a new frame from a duplicate.
Stop-and-Wait wastes the link because the sender idles for a round trip per frame; efficiency is $\eta = \frac{1}{1+2a}$ with $a = T_p/T_t$.
Sliding window lets the sender transmit up to $W$ frames without waiting, using sequence numbers; the receiver's window shows which frames it will accept.
The window slides forward as ACKs arrive, which keeps the pipe full; efficiency is $\eta = \frac{W}{1+2a}$ (at most 1).
Piggybacking attaches the ACK of received data to an outgoing data frame in the reverse direction. Advantages: saves bandwidth and reduces frame overhead. Disadvantages: more complex, and the ACK is delayed while waiting for a data frame, which can cause a timeout, so an ACK timer is needed.

Example. Window $W=4$, frames 0-7: the sender sends 0,1,2,3; ACK 1 arrives and the window slides to 1-4, so frame 4 may now be sent.

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Point	Stop-and-Wait	Sliding Window
Frames in flight	1	Up to W
Sequence numbers	0, 1	0 to 2^m - 1
Efficiency	1/(1+2a), low	W/(1+2a), high
Buffers	1 frame	W frames
Best for	Short links	Long, fast links

<mark>Flow control regulates the sender's speed to the receiver's capacity; sliding window improves on stop-and-wait by pipelining.</mark>

Answer frame. Open with the definition; draw the sender/receiver window diagram or the stop-and-wait timeline; develop stop-and-wait, then sliding window with the W=4 example, then the table; close with piggybacking if asked.

Asked: [7 marks] (Dec 2020) What is meant by Piggybacking? What are its advantages and disadvantages? Asked: [7 marks] (Dec 2020) Explain Flow control and Error control. Asked: [7 marks] (May 2022) Describe the stop and wait flow control technique. Asked: [7 marks] (May 2023) What is the mechanism of sliding window flow control? Explain with an example. Asked: [7 marks] (Jun 2026) Describe flow control techniques. Compare Stop-and-Wait and Sliding Window Protocols.

Error control

Definition. Error control detects damaged or lost frames and corrects them, either by retransmission (ARQ) or by forward error correction (FEC).

Key points.

Error detection only finds that bits were corrupted; error correction also locates and repairs them.
Detection methods are parity check, checksum and CRC; Hamming code corrects single-bit errors.
ARQ (Stop-and-Wait, Go-Back-N, Selective Repeat) uses ACKs, NAKs and timeouts to retransmit bad or lost frames.
Flow control and error control together give reliable delivery over a noisy channel: flow control paces the sender, error control repairs the damage.

Formula. Hamming: $2^r \ge m + r + 1$; parity bits sit at positions $1,2,4,8,\dots$ and each checks the positions whose binary index contains that power of 2.

Example. Data 10101111, $m=8$, so $r=4$ ($2^4=16\ge13$), 12-bit code. Data goes to positions 3,5,6,7,9,10,11,12 = 1,0,1,0,1,1,1,1.

Parity	Positions checked	Data 1s	Bit
P1	1,3,5,7,9,11	3 (odd)	1
P2	2,3,6,7,10,11	4	0
P4	4,5,6,7,12	2	0
P8	8,9,10,11,12	4	0

Encoded word (positions 1 to 12) = 101001001111

Answer frame. Open by defining error detection versus correction; list the methods; for the numerical, give $r$, the table and the final word; close with the flow/error control link.

Asked: [7 marks] (Dec 2024) Explain in detail about the error and flow control mechanisms employed at data link layer. Asked: [7 marks] (Jun 2025) What do you mean by Error detection? Write down the name of methods. An 8-bit with binary value 10101111 is to be encoded using an even-parity Hamming code. What is the binary value after encoding?

Elementary and Sliding Window protocol: 1-bit

Definition. The elementary protocols are the simplex stop-and-wait ones; the 1-bit sliding window protocol has window size 1 and a 1-bit sequence number (0 or 1).

Key points.

The sender sends a frame carrying sequence bit 0 and does not send the next until the matching ACK arrives.
The 1-bit sequence number lets the receiver tell a new frame from a retransmitted duplicate.
It works in both directions with piggybacked ACKs, but only one frame is outstanding, so utilisation is low on long links.

Go-Back-N

Definition. Sliding window ARQ protocols let the sender keep up to $N$ frames outstanding; Go-Back-N (GBN) has sender window $N$ and receiver window 1, and on an error the sender retransmits the damaged frame and all frames after it.

Key points.

The sender window size is at most $2^m - 1$ for $m$-bit sequence numbers; larger windows make old and new frames indistinguishable.
To fill the pipe, $N$ is chosen from the bandwidth-delay product: $N = 1 + 2a$, where $a = T_p/T_t$, bounded by $2^m-1$.
The receiver accepts frames only in order and discards out-of-order frames, so it needs no buffering.
ACKs are cumulative: ACK $n$ confirms all frames up to $n-1$.
A lost or damaged frame is detected when later frames arrive out of order (or on sender timeout); the sender goes back and resends from that frame onward.
A lost ACK is harmless if a later cumulative ACK arrives before the timer expires; otherwise the timeout resends the window.
Advantage: simple receiver and high efficiency compared with stop-and-wait. Disadvantage: wasted retransmissions on a noisy link.

Example. $N=4$: frames 0-3 sent, frame 1 lost. Receiver gets 0 (ACK), discards 2 and 3; sender times out and resends 1,2,3.

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Point	Go-Back-N	Selective Repeat
Sender window	up to 2^m - 1	up to 2^(m-1)
Receiver window	1	Same as sender
Retransmission	Damaged frame and all after it	Only the damaged frame
Receiver buffer	None for out-of-order	Buffers out-of-order frames
ACK	Cumulative	Individual, plus NAK
Complexity	Low	High
Efficiency on noisy link	Lower	Higher

<mark>In Go-Back-N the receiver window is 1, so one error forces the sender to retransmit that frame and every frame after it.</mark>

Answer frame. Open by defining sliding window ARQ with window $N$; draw the sender-receiver timeline (stop-and-wait, GBN, SR as asked) with a lost frame; explain GBN, then SR (see next topic), then the table; close by stating which suits noisy long links. For the "how is n determined" question, give $2^m-1$ and $1+2a$, then lost frame, lost ACK and timeout.

Asked: [7 marks] (May 2022, Dec 2024, Jun 2026) Describe Go Back N and Selective Repeat protocol. Asked: [7 marks] (Dec 2020) What is Go back N and selective repeat protocol? Asked: [14 marks] (May 2024) With the help of suitable diagrams explain the following protocols of data link layer: a) Stop and wait b) Go back c) Selective repeat. Asked: [14 marks] (Jun 2025) What is the advantage of Go-back-n protocol? How is the parameter 'n' determined? Explain how the protocol works at transport layer in case of errors, lost packet or Acks?

Selective Repeat

Definition. Selective Repeat (SR) is a sliding window ARQ in which the receiver accepts and buffers out-of-order frames and only the damaged or lost frame is retransmitted.

Key points.

Both sender and receiver windows are $2^{m-1}$ for $m$-bit sequence numbers.
Each frame is acknowledged individually, and a NAK may ask for a missing frame at once.
The receiver buffers out-of-order frames and delivers them in order after the gap is filled, so it needs more memory.
It gives better throughput on noisy links but is more complex than GBN. Stop-and-Wait is the case $N=1$.

Hybrid ARQ

Definition. Hybrid ARQ (HARQ) combines forward error correction with ARQ retransmission: the receiver tries to correct errors, and requests a retransmission only if correction fails.

Key points.

Each transmission carries error-correcting code plus an error-detecting check such as CRC.
In Type I, failed frames are discarded and resent; in Type II (incremental redundancy) the receiver combines earlier and retransmitted copies.
It gives fewer retransmissions than plain ARQ and is used in wireless systems such as LTE.

Protocol verification: Finite State Machine Models & Petri net models

Definition. A finite state machine (FSM) models a protocol as a set of states, events (frame arrival, timeout) and transitions between states, with an action on each transition.

Key points.

A protocol machine is drawn as a state graph, for example the sender states "waiting for ACK" and "ready to send" of stop-and-wait.
Analysing all reachable states finds deadlock, livelock, missing transitions and unspecified receptions.
An FSM gives an unambiguous specification, so it is used to specify and verify protocols before implementing them.
A Petri net models the same with places, transitions and tokens; token movement shows concurrency and synchronisation.

Asked: [7 marks] (May 2023) What is finite state machine model? How finite state machines are used in the study of network protocols? Explain.

ARP

Definition. The Address Resolution Protocol (ARP) maps a known 32-bit IP address to the 48-bit MAC address of a host on the same network, because frames need a MAC address.

Key points.

A host needing a MAC broadcasts an ARP request ("Who has IP X?") in a frame with destination FF:FF:FF:FF:FF:FF.
Every host on the LAN receives it, but only the host owning IP X replies.
The reply is a unicast ARP reply carrying its MAC address.
The sender stores the pair in its ARP cache with a timeout, so repeat sends need no request.
For a destination outside the LAN, the host resolves the MAC of the default router instead.
The ARP packet has hardware type, protocol type, operation (1 request, 2 reply), and sender and target MAC and IP; it sits directly on Ethernet at the network layer boundary.

Example. A (10.0.0.1) wants B (10.0.0.2): A broadcasts "Who has 10.0.0.2?"; B replies "10.0.0.2 is at MAC-B"; A caches it and sends the frame.

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RARP. Reverse ARP finds an IP address from a known MAC address: a diskless host broadcasts its MAC, and a RARP server replies with its IP. Limitation: it needs a server on every network, and gives only an IP, so BOOTP and DHCP replaced it.

Other terms asked with ARP. FDDI: fibre-optic dual counter-rotating token ring at 100 Mbps, data link/physical layers. ICMP: network layer protocol for error and control messages such as ping (echo request/reply). UDP: connectionless transport layer protocol with an 8-byte header (source port, destination port, length, checksum), used in DNS and streaming.

<mark>ARP resolves IP to MAC by broadcast request and unicast reply, and caches the result.</mark>

Answer frame. Open with the definition; draw the broadcast-request and unicast-reply diagram; develop steps 1-4 then cache and router case; add RARP with its limitation; close with the line that ARP works inside one network only.

Asked: [14 marks] (May 2022) Explain the following terms with example: i) FDDI ii) ARP iii) ICMP iv) UDP Asked: [7 marks] (Dec 2024) Explain the working of ARP and RARP.

RARP

Definition. Reverse ARP (RARP) finds the IP address of a host from its known MAC address.

Key points.

A diskless host broadcasts a RARP request containing its own MAC address.
A RARP server holding a MAC-to-IP table replies with the IP address.
A server is needed on every network because the request is a broadcast that routers do not forward.
It is obsolete, replaced by BOOTP and DHCP.

GARP

Definition. Gratuitous ARP is an ARP request or reply a host sends for its own IP address without being asked.

Key points.

It announces or updates the host's IP-to-MAC mapping in other hosts' ARP caches, for example after a NIC change or failover.
It detects duplicate IP addresses: if another host replies, the address is already in use.
The sender and target IP fields hold the same address, and it is broadcast.

Last-minute revision

Data link layer = layer 2; frames, MAC addresses, error and flow control, access control.
Bit stuffing: flag 01111110, insert 0 after five 1s; byte stuffing: ESC before FLAG or ESC.
Stop-and-Wait efficiency $=1/(1+2a)$; sliding window $=W/(1+2a)$.
Piggybacking = ACK carried in a data frame going the other way.
Hamming: $2^r \ge m+r+1$; parity bits at positions 1, 2, 4, 8.
10101111 even-parity Hamming = 101001001111.
GBN: sender window $2^m-1$, receiver window 1, cumulative ACK, resend from error.
SR: windows $2^{m-1}$, individual ACK/NAK, buffers out-of-order frames.
HARQ = FEC plus ARQ.
ARP: IP to MAC, broadcast request, unicast reply, cached; RARP: MAC to IP; GARP: unsolicited announcement.

Memory hooks

GBN "goes back" and resends everything; SR resends "selectively" the one frame.
ARP asks everyone (broadcast), only one answers (unicast).
Hamming parity positions are powers of two: 1, 2, 4, 8.
Stuffing: five 1s means insert a 0.
FSM = states + events + transitions.

Coverage checklist

Data Link Layer: Need - definition, purpose.
Services Provided - Asked: What is the data link layer and its services (May 2023).
Framing - Asked: bit and byte stuffing (May 2022); DLL functions and framing (Jun 2026).
Flow Control - Asked: piggybacking, flow and error control, stop-and-wait, sliding window, compare (Dec 2020, May 2022, May 2023, Jun 2026).
Error control - Asked: error and flow control mechanisms (Dec 2024); error detection and Hamming numerical (Jun 2025).
Data Link Layer Protocol: Elementary & Sliding Window protocol: 1-bit - definition and key points.
Go-Back-N - Asked: GBN and SR (Dec 2020, May 2022, May 2024, Dec 2024, Jun 2025, Jun 2026).
Selective Repeat - definition and key points.
Hybrid ARQ - definition and key points.
Protocol verification: Finite State Machine Models & Petri net models - Asked: FSM model (May 2023).
ARP - Asked: FDDI, ARP, ICMP, UDP (May 2022); ARP and RARP (Dec 2024).
RARP - definition and key points.
GARP - definition and key points.

Account

Computer Networks (CS-602) - Unit 2 Short Notes

How unit 2 is examined

Data Link Layer: Need

Services Provided

Framing

Flow Control

Error control

Elementary and Sliding Window protocol: 1-bit

Go-Back-N

Selective Repeat

Hybrid ARQ

Protocol verification: Finite State Machine Models & Petri net models

ARP

RARP

GARP

Last-minute revision

Memory hooks

Coverage checklist