Computer Networks (CS-602) Unit 3 Notes - RGPV

How unit 3 is examined

The MAC sub-layer decides who may use a shared channel and how a frame is addressed; the marks are in ALOHA and slotted ALOHA (derivation and numericals) and in CSMA, CSMA/CD and CSMA/CA (explain and compare), with MAC addressing next.

MAC Addressing

<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">Medium weight</span>

Definition. <mark>A MAC (data link layer) address is a 48-bit, globally unique hardware address burned into the network interface card, used to deliver a frame to the correct station on the same LAN.</mark>

Key points.

The address is 48 bits, written as six hexadecimal bytes such as 00:1A:2B:3C:4D:5E (or 00-1A-2B-3C-4D-5E).
The first 24 bits are the OUI (Organisationally Unique Identifier) assigned by the IEEE to the manufacturer, and the last 24 bits are a serial number chosen by the manufacturer, so no two cards share an address.
It is a flat address with no network part, unlike a hierarchical IP address, so it identifies the device but not its location.
In the first byte, the least significant bit tells the type: 0 is unicast (one station) and 1 is multicast; FF:FF:FF:FF:FF:FF is the broadcast address received by all stations.
The second least significant bit of the first byte marks a globally administered (0) or locally administered (1) address.
Each frame carries destination and source MAC addresses in its header; the NIC accepts a frame only if the destination matches its own address, the broadcast address or a multicast group it joined.
MAC addresses work only inside one LAN: at every router the frame is rebuilt with new MAC addresses, while the IP addresses stay end to end (ARP maps IP to MAC).

Answer frame. Open with the definition; give one example address and split it into OUI and serial; then develop points 3-7; close with the note that IP is end-to-end and MAC is hop-to-hop. For the joint question, add the BEB section after point 7.

Asked: [7 marks] (May 2022) Explain data link layer address with examples. Asked: [7 marks] (Jun 2026) Explain MAC addressing and Binary Exponential Backoff (BEB) algorithm.

Binary Exponential Back-off (BEB) Algorithm

Definition. <mark>Binary exponential back-off is the CSMA/CD retransmission rule in which, after the $i$-th successive collision, a station waits a random number $k$ of slot times, chosen from $0$ to $2^i-1$, before trying again.</mark>

Key points.

After a collision two stations that retry at once would collide again, so each must wait a different random time.
After the first collision the station picks $k$ from {0, 1}; after the second from {0, 1, 2, 3}; after the third from {0..7}; the waiting time is $k \times$ slot time (51.2 microseconds in 10 Mbps Ethernet).
In Ethernet the exponent stops growing at 10 (range 0-1023) and the frame is dropped with an error after 16 attempts.

Example. A and B collide (attempt 1): each picks 0 or 1; if both pick 1 they collide again (attempt 2) and pick from 0-3, so the chance of a repeat falls from 1/2 to 1/4, then 1/8.

Answer frame. Open with why back-off is needed after a collision; write the range rule and formula $k \times$ slot; show the A and B example; close with the limits 10 and 16 and the stabilising effect.

Asked: [7 marks] (Dec 2024) Explain Binary Exponential Back-off (BEB) with suitable example.

Distributed Random Access Schemes/Contention Schemes: for Data Services

Definition. Random access (contention) schemes let every station transmit whenever it has data, with no central controller and no fixed turn.

Two or more simultaneous frames collide and are lost, so each protocol needs a way to detect the loss and retransmit.
They give low delay at light load but throughput collapses at heavy load because of repeated collisions.
Examples are ALOHA, slotted ALOHA and the CSMA family, which suit bursty data traffic.

ALOHA and Slotted-ALOHA

Definition. <mark>Pure ALOHA lets a station transmit a frame whenever it is ready and retransmit after a random wait if no acknowledgement arrives; slotted ALOHA divides time into slots equal to one frame time and allows transmission only at a slot start.</mark>

Diagram. Slotted ALOHA: three stations, slots of length $T$; a frame is sent only at a slot boundary.

Slot	1	2	3	4	5
Stations sending	A	none	B, C	C	A
Result	success	idle	collision	success	success

Key points.

In pure ALOHA a frame sent at time $t_0$ collides if any other frame starts in $(t_0-T, t_0+T)$, so the vulnerable time is $2T$.
In slotted ALOHA stations are synchronised, so frames either overlap completely or not at all, and the vulnerable time falls to $T$.
Collided frames are retransmitted after a random delay, which prevents the same stations colliding forever.
Let $G$ be the mean number of frames offered per frame time (offered load) and $S$ the throughput (successful frames per frame time); arrivals are Poisson.

Derivation. Frames offered in a time $t$ follow the Poisson law $P(k) = \dfrac{(Gt/T)^k e^{-Gt/T}}{k!}$. A frame succeeds if no other frame starts in its vulnerable time.

	Pure ALOHA	Slotted ALOHA
Vulnerable time	$2T$	$T$
P(no other frame)	$e^{-2G}$	$e^{-G}$
Throughput	$S = G e^{-2G}$	$S = G e^{-G}$
Maximise: $dS/dG = 0$	$G = 1/2$	$G = 1$
Maximum $S$	$1/(2e) \approx 0.184$	$1/e \approx 0.368$

Pure ALOHA: $dS/dG = e^{-2G}(1-2G) = 0$ gives $G = 0.5$. Slotted: $dS/dG = e^{-G}(1-G) = 0$ gives $G = 1$. So $S_{max}$ of slotted ALOHA is $0.368/0.184 = 2$ times that of pure ALOHA, because the vulnerable time is halved. <mark>Maximum efficiency is 18.4% for pure ALOHA and 36.8% for slotted ALOHA.</mark>

Example (May 2022). Given: channel 14.44 kbps, packet 100 bits, slotted ALOHA.

Frame time $T = 100 / 14440 = 6.93$ ms, so the channel carries $14440/100 = 144.4$ frames/s.
Maximum throughput $S_{max} = 0.368$ of capacity, at $G = 1$.
$0.368 \times 144.4 \approx 53.1$ frames/s, and $0.368 \times 14.44 = 5.31$ kbps.

Maximum throughput = 5.31 kbps (about 53 frames/s).

Example (May 2023). Given: pure ALOHA, 56 kbps, each station sends 1000 bits every 100 s.

Load per station $= 1000/100 = 10$ bps.
Usable capacity $= 0.184 \times 56000 \approx 10300$ bps.
$N \times 10 \le 10300$, so $N \le 1030$.

Maximum N is about 1030 stations (1031 if 0.184 is rounded).

Answer frame. For "explain slotted ALOHA": open with the definition, draw the slot table with success, idle and collision slots, develop points 2, 5, 3, and close with 36.8% against 18.4%. For "derive": define both, state the vulnerable times, derive $S$ for each, differentiate to get the maxima, and close that slotted is twice as efficient. For numericals: write Given, $S_{max}$, substitution and the boxed answer.

Pitfall: Using $S_{max}=0.368$ for pure ALOHA or forgetting to multiply by the channel rate gives a wrong answer.

Asked: [7 marks] (Dec 2020) How throughput is improved in slotted ALOHA over Pure ALOHA? Asked: [7 marks] (May 2022) A radio station is using 14.44 kbps channel and packets are 100 bits long. Calculate the maximum throughput using slotted ALOHA. Asked: [7 marks] (May 2023) Explain the working principle of slotted ALOHA with suitable sketch. Asked: [7 marks] (May 2023) N stations share a 56 Kbps pure ALOHA channel; each outputs a 1000-bit frame once every 100 sec. What is the max value of N? Asked: [7 marks] (May 2024) What is the function of MAC layer? Explain (i) Pure and slotted ALOHA (ii) CSMA. Asked: [7 marks] (Jun 2025) What is Pure ALOHA and slotted ALOHA? How is the efficiency of slotted ALOHA twice that of pure ALOHA? Derive it.

for Local-Area Networks (CSMA, CSMA/CD, CSMA/CA)

Definition. <mark>CSMA (Carrier Sense Multiple Access) makes a station listen to the channel before transmitting, so it sends only when the channel is idle; CSMA/CD adds collision detection with abort, and CSMA/CA adds collision avoidance for wireless.</mark>

MAC layer function (May 2024). The MAC sub-layer controls access to the shared medium, frames the data, and adds source and destination MAC addresses.

Key points: persistence methods.

1-persistent CSMA senses continuously and transmits at once, with probability 1, when the channel becomes idle; delay is low but two waiting stations always collide.
Non-persistent CSMA, on finding the channel busy, waits a random time and senses again; collisions are fewer and the channel is used better under heavy load, but delay is longer.
p-persistent CSMA (slotted channels) transmits with probability $p$ when idle and defers to the next slot with probability $1-p$, balancing the two.

CSMA/CD (Ethernet, wired).

Step 1: Sense the channel; if busy, wait (1-persistent).
Step 2: If idle, transmit the frame and keep listening.
Step 3: If a collision is detected, stop at once and send a 32-bit jam signal.
Step 4: Wait a back-off time k x slot, k chosen from 0 to 2^i - 1.
Step 5: Retry from Step 1; give up after 16 attempts.

Because the sender aborts on detecting a collision, the channel is not wasted for the whole frame time, which is the CD improvement over plain CSMA (Dec 2020).
To detect every collision, frame transmission time must be at least $2 T_{prop}$ (round trip), which is why Ethernet has a minimum frame of 64 bytes (512 bits, 51.2 microseconds at 10 Mbps).
Efficiency $\approx \dfrac{1}{1+6.44a}$ with $a = T_{prop}/T_{trans}$, so it falls for long cables or short frames.

CSMA/CA (Wi-Fi 802.11, wireless). A wireless sender cannot hear a collision while sending (hidden terminal problem), so it avoids collisions:

Interframe space: a station waits a DIFS after the channel goes idle; short SIFS is used only for ACK and CTS, giving them priority.
Random back-off: it then counts down a random number of slots, freezing the counter whenever the channel turns busy.
RTS/CTS: the sender sends a short RTS and the receiver answers CTS; hidden stations hear the CTS and stay silent.
Virtual carrier sensing: the RTS/CTS carry a duration and every station sets its NAV (Network Allocation Vector) timer and defers until it expires.
Acknowledgement: the receiver sends an ACK after SIFS, and no ACK means retransmission with a larger back-off window.

Point	CSMA/CD	CSMA/CA
Approach	Detect a collision and abort	Avoid the collision before sending
Medium	Wired (IEEE 802.3 Ethernet)	Wireless (IEEE 802.11)
Collision handling	Jam signal, then BEB	Back-off before sending, ACK after
RTS/CTS and NAV	Not used	Used for hidden terminals
Reason	Sender can listen while sending	Sender cannot listen while sending
Limitation	Distance limit from $2T_{prop}$	Overhead of waits and control frames

Answer frame. For CSMA/CD: define it, draw the flow as the 5 steps, then jam, BEB and the 64-byte rule; close with efficiency. For CSMA/CA: define it, say why wireless needs avoidance, list the five strategies; close with NAV. For persistence: define each method, compare delay against collision chance. For comparisons: use the table.

Pitfall: CSMA/CA does not detect collisions; do not write "collision detection" for Wi-Fi.

Asked: [7 marks] (Dec 2020) How performance is improved in CSMA/CD protocol compared to CSMA protocol? Asked: [7 marks] (Dec 2024) Explain briefly about the Persistent and Non-persistent CSMA protocols. Asked: [7 marks] (Jun 2025) Discuss the different strategies used to avoid collisions in CSMA/CA. Asked: [7 marks] (Jun 2025) Explain CSMA/CD protocol in detail. Asked: [7 marks] (Jun 2026) Compare CSMA/CD and CSMA/CA protocols. Asked: [14 marks] (May 2023) Short note on CSMA/CD (any two of four).

Collision Free Protocols: Basic Bit Map

Definition. A collision-free protocol reserves the channel before data is sent, so no collision occurs; in the basic bit-map protocol, a contention period of $N$ slots lets each of $N$ stations announce that it has a frame.

Station $j$ sets bit $j$ to 1 in its own slot of the reservation period; after that all stations know who wants to send.
Data frames are then sent in numerical order of station.
Efficiency for $d$ data bits: low load $d/(N+d)$, high load $d/(d+1)$, near 100%.
Drawback: a low-numbered station waits less than a high-numbered one, and overhead is high when the load is low.

BRAP

Definition. BRAP (Broadcast Recognition with Alternating Priorities) is a collision-free, bit-map style protocol in which stations transmit in a fixed cyclic order, and the priority alternates so that no station is always first.

Each station has its own turn; a station with a frame sends it in its turn, and others recognise the broadcast.
After each transmission the turn moves to the next station, giving every station equal access.

Binary Count Down

Definition. <mark>Binary countdown is a collision-free protocol in which every station has a binary address of the same length, and stations that want to send broadcast their address bit by bit, highest bit first, and the highest address wins.</mark>

Key points.

A collision-free protocol needs no retransmissions because the channel is reserved (or the winner is decided) before a frame is sent.
All contending stations broadcast bit 1 (MSB) together, and the channel gives the Boolean OR of the bits.
A station that sent 0 but sees 1 withdraws, so only the higher addresses continue to the next bit.
It avoids the overhead of the bit-map, since channel efficiency is $d/(d+\ln N)$, but it favours high-numbered stations.

Example. Stations 0010, 0100, 1001, 1010 contend.

Bit	Sent (0010, 0100, 1001, 1010)	OR	Left
1st	0, 0, 1, 1	1	1001, 1010
2nd	0, 0	0	1001, 1010
3rd	0, 1	1	1010
4th	0	0	1010

Station 1010 wins and sends its frame.

Answer frame. Define collision-free and reservation; explain the OR rule; show the table above; close with the advantage over contention protocols.

Asked: [7 marks] (Dec 2024) What do you understand by collision free protocol? Explain the Binary Count Down Protocol with suitable example.

MLMA Limited Contention Protocols: Adaptive Tree Walk

Definition. <mark>Limited-contention protocols use contention at light load (low delay) and switch to collision-free behaviour at heavy load, by letting only a small group of stations contend in a slot; adaptive tree walk does this by binary search on a tree of stations.</mark>

Diagram. Stations 8-15 are leaves; ready stations 10, 12, 13 are highlighted.

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Key points.

In slot 0 all stations may contend (node 1); if one station sends, it succeeds and the walk ends.
On a collision, the next slots are given to the left subtree (node 2) and then the right (node 3), and the search goes deeper on each further collision.
When a slot is idle or has a single sender, that subtree is finished.
Under high load the search starts lower in the tree, not at the root, to avoid wasting slots.

Slot	Node probed	Result
0	1	collision (10, 12, 13)
1	2	success (10)
2	3	collision (12, 13)
3	6	collision (12, 13)
4	12	success (12)
5	13	success (13)
6	7	idle

Answer frame. Define limited contention; draw the tree with ready stations; walk the slot table; close with light-load and high-load behaviour.

Asked: [7 marks] (May 2023) What is Limited-Contention Protocols? Explain the working principle of Adaptive Tree Walk Protocol with suitable example.

Performance Measuring Metrics

Definition. <mark>Network performance is measured in two fundamental ways: bandwidth (throughput), the amount of data carried per second, and latency (delay), the time a message takes to travel from source to destination.</mark>

Key points.

Bandwidth or throughput is measured in bits per second; bandwidth is the capacity of the link and throughput is what is actually achieved.
Latency $=$ propagation time $+$ transmission time $+$ queuing time $+$ processing time, where transmission time $= \text{message size}/\text{bandwidth}$ and propagation time $= \text{distance}/\text{speed}$.
The two are related by the bandwidth-delay product, $\text{bandwidth} \times \text{delay}$, which is the number of bits that fill the link.
In MAC protocols the measures are throughput $S$ and offered load $G$; e.g. 1 Mbps link and 1 ms delay hold 1000 bits.

Asked: [7 marks] (Dec 2020) What are the two fundamental ways by which network performance is measured?

IEEE Standards 802 series & their variant

Definition. <mark>IEEE 802 is the family of standards for LANs and MANs that defines the lower two layers (data link and physical) for each network type.</mark>

Diagram. IEEE 802 layering.

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Key points.

802.1 covers architecture and bridging (spanning tree, VLAN), and 802.2 defines the LLC (Logical Link Control), which gives a common interface to the network layer.
802.3 is Ethernet (CSMA/CD); 802.4 is token bus and 802.5 token ring; 802.11 is wireless LAN (Wi-Fi, CSMA/CA); 802.15 is personal area network (Bluetooth); 802.16 is WiMAX.
The data link layer is split into the LLC sub-layer (flow and error control, same for all LANs) and the MAC sub-layer (framing, addressing, channel access, specific to each LAN).
LAN architecture: stations with NICs connect through a hub or switch in star, bus or ring topology, using cable or wireless.

Asked: [7 marks] (Jun 2026) Describe IEEE 802 LAN standards and explain the architecture of Local Area Networks. Asked: [14 marks] (May 2023) Short note on IEEE Standards 802 series (any two of four).

Last-minute revision

MAC address is 48 bits: 24-bit OUI plus 24-bit serial; broadcast is FF:FF:FF:FF:FF:FF.
BEB: after the $i$-th collision wait $k \times$ slot, $k$ in $0..2^i-1$; cap at 10, drop after 16 tries.
Pure ALOHA: $S = G e^{-2G}$, vulnerable time $2T$, max 0.184 at $G=0.5$.
Slotted ALOHA: $S = G e^{-G}$, vulnerable time $T$, max 0.368 at $G=1$.
14.44 kbps, 100 bits, slotted: $0.368 \times 14.44 = 5.31$ kbps.
56 kbps pure ALOHA, 10 bps per station: $N \le 1030$.
1-persistent sends at once; non-persistent waits a random time; p-persistent sends with probability $p$.
CSMA/CD: detect, abort, jam, BEB; minimum frame 64 bytes ($2T_{prop}$).
CSMA/CA: DIFS, back-off, RTS/CTS, NAV, ACK; used in 802.11.
Binary countdown: OR of address bits, highest address wins.
Adaptive tree walk: on collision, search the left then right subtree.
Performance: bandwidth (throughput) and latency; 802.3 Ethernet, 802.11 Wi-Fi, 802.15 Bluetooth.

Memory hooks

ALOHA: pure has "2" in it, $e^{-2G}$ and $2T$, so half of slotted's 0.368 is 0.184.
CD is for wired (detect, then Drop); CA is for air (Avoid, then Acknowledge).
Binary countdown: highest address wins, bits OR together.
802 numbers: 3 = Ethernet, 11 = Wi-Fi, 15 = Bluetooth.

Coverage checklist

MAC Addressing: May 2022 data link address, Jun 2026 MAC and BEB.
Binary Exponential Back-off (BEB) Algorithm: Dec 2024 BEB with example, Jun 2026.
Distributed Random Access Schemes/Contention Schemes: for Data Services: definition and key points (unasked).
ALOHA and Slotted-ALOHA: Dec 2020, May 2022, two in May 2023, May 2024, Jun 2025.
for Local-Area Networks (CSMA, CSMA/CD, CSMA/CA): Dec 2020, Dec 2024, two in Jun 2025, Jun 2026, May 2023 (14 marks).
Collision Free Protocols: Basic Bit Map: definition and efficiency (unasked).
BRAP: definition and key points (unasked).
Binary Count Down: Dec 2024.
MLMA Limited Contention Protocols: Adaptive Tree Walk: May 2023.
Performance Measuring Metrics: Dec 2020.
IEEE Standards 802 series & their variant: Jun 2026, May 2023 (14 marks).

Account

Computer Networks (CS-602) - Unit 3 Short Notes

How unit 3 is examined

MAC Addressing

Binary Exponential Back-off (BEB) Algorithm

Distributed Random Access Schemes/Contention Schemes: for Data Services

ALOHA and Slotted-ALOHA

for Local-Area Networks (CSMA, CSMA/CD, CSMA/CA)

Collision Free Protocols: Basic Bit Map

BRAP

Binary Count Down

MLMA Limited Contention Protocols: Adaptive Tree Walk

Performance Measuring Metrics

IEEE Standards 802 series & their variant

Last-minute revision

Memory hooks

Coverage checklist