JB

COMP — Link Layer & LANs (Week 7)

Week 7 – Data/Link Layer & Local Area Networks (LANs)
Administrative Announcements

  • Trimester break has finished – welcome back.

  • Assignment 2 (second assignment) due Friday of this week

    • All info already given (see ~15 min of Week 6 lecture start).

    • Nothing from today is required to complete A2.

    • Contact lecturer early for misunderstandings / extension requests.

1 OSI Context & Why the Link Layer Matters

  • Layer ordering reminder:

    • Layer 1 Physical (cables, radio).

    • Layer 2 Data-Link (today’s focus).

    • Layer 3 Network/IP (last week).

    • Above: Transport, Application.

  • For network engineers, Layer 3 (routing) and Layer 2 (link/LAN) are where you spend most “logical” time; Layer 1 = pure hardware.

  • Link-layer = direct interface between host’s NIC and the medium.

    • It transfers a network-layer datagram one hop to a physically‐adjacent node (same “segment”).

  • Terminology mapping:

    • L3 packet → Frame at L2.

    • “Subnet” (IP) ≈ “segment/collision domain” (Ethernet).

    • Two hosts in same subnet normally talk using MAC, not IP, on the wire.

2 Frames & Encapsulation

  • Encapsulation: IP datagram ⟶ encapsulated in a link-layer frame with:

    • Link Header (MAC src/dst, type, etc.)

    • Payload (IP datagram)

    • Link Trailer (CRC, End-of-Frame delimiters).

  • At this layer we always add both header and trailer (unlike TCP/UDP that add only a header).

  • Trailer roles:

    • Mark end of frame for sniffers.

    • Store error-checking codes (e.g.xC FCS) for reliability.

3 Network Interface Cards (NICs)

  • Still called NIC for historical reasons (once plug-in cards).

    • Today integrated in laptop motherboard / SoC / Wi-Fi chip.

    • Implements Link & Physical layers: hardware + firmware + driver software.

  • Connects to host via various buses (PCIe, USB, SPI/I²C on embedded).

    • Signal modulation different for Cat5 vs Antenna, emphasising hardware nature.

4 Link-Layer Services

• Framing

• Medium Access Control (MAC) – rules for placing bits on medium.

– Crucial in Wi-Fi (different modulations).

• Optional Reliable Delivery

– Often enabled on error-prone links (wireless, dial-up, LoRaWAN).

– Not needed for very low-error fibre.

• Flow Control (protect limited switch/host buffers).

• Error Detection (always) & Error Correction (sometimes).

– Link-layer algorithms more powerful than higher-layer Internet Checksum because can be hardware-accelerated.

5 Error‐Handling Algorithms
5.1 Parity

  • Even parity bit: choose p such that Sum from i=1 to n of b_i + p is congruent to 0 (mod 2).

    • Detects one-bit errors but cannot locate which bit.

  • 2-D Array parity: rows + columns parity bits.

    • Detects & corrects single-bit flips (intersection reveals error).

    • Fails on multi-bit patterns.

5.2 CRC (Cyclic Redundancy Check)

  • Sender treats bits as polynomial G(x), appends R(x) s.t. divisible by generator.

  • Receiver divides; non-zero remainder ⇒ error.

  • Ethernet uses 32-bit CRC, Wi-Fi varies, classical modems used 16-bit.

  • Detects:

    • all 1- & 2-bit errors,

    • all odd-bit errors,

    • burst ≤16 bits with 16-bit CRC (larger for 32-bit).

  • Cheap hardware: XOR gates + shift register.

6 Duplex & Physical Characteristics

  • Simplex: one-way only (rare in networks).

  • Half-duplex: both can send, but not simultaneously (Wi-Fi, walkie-talkie).

  • Full-duplex: simultaneous send/receive (modern Ethernet pairs, most fibre pairs).

7 Link Types
7.1 Point-to-Point (P2P)

  • Exactly two nodes share link (router ↔ router serial, dial-up, microwave link).

  • Media access trivial; historically common in Cisco serial labs.

7.2 Multiple Access / Broadcast

  • Shared medium: collisions possible.

  • Need MAC protocols to coordinate.

Channel partitioning techniques

  1. TDMA (Time-division): predictable slots, wasted if node idle.

  2. FDMA (Freq-division): distinct sub-carriers; used in WDM fibre.

  3. Token (taking turns): e.g.xC IBM Token Ring; deterministic but non-scalable.

  4. Random access (dominant): allow collisions, recover (Ethernet CSMA/CD); or try to avoid (Wi-Fi CSMA/CA).

8 Evolution of Ethernet
8.1 Coax Bus Ethernet (802.3 original)

  • Thick/Yellow coax; 500 m segment, min 3 m tap spacing.

  • 10 Mb/s; shared collision domain.

8.2 Hubs (Layer-1 Repeaters)

  • Multi-port device that electrically replicates incoming bits to all ports.

    • Still one collision domain.

    • Cheap ($≈5), dumb; uncommon today.

8.3 Repeaters & Length limits

  • Amplify/regen signals, extend segment but still part of same collision domain.

  • Limited count—propagation delay must stay < “slot time” so collisions detectable.

8.4 Bridges (2-port Layer-2)

  • Listen promiscuously on each attached segment.

  • Forward only complete, error-free frames whose destination lies on other segment.

  • Effect: break up collision domains; localise traffic.

8.5 Switches (Multi-port Bridges)

  • Every port = its own segment ⇒ virtually zero collisions.

  • Internally a matrix of bridges; store-and-forward, optional buffering.

  • Can still attach hubs or cascaded switches → hierarchical LAN.

Collision Domain Recap

  • Segment(s) where simultaneous transmissions may collide.

  • Bridges/switches create separate collision domains even inside one IP subnet.

9 CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

  1. Host wishing to send listens; if idle send immediately.

  2. If busy, waits random back-off (exponential after each collision).

  3. While sending, keeps sensing: collision detected ⇒ transmit 32-bit Jam signal, abort.

  4. Everybody hearing jam backs off random slot.

  • Efficiency acceptable; disappears entirely if every host on its own switch port.

10 MAC (LAN) Addressing

  • 48-bit globally unique address burned into NIC (can be overridden).

    Format: six octets hex, e.g. 64:6E:0C:12:A4:FE.

  • First 3 octets = OUI (Organisationally Unique ID) purchased from IEEE.

    • Lookup tools map OUI → vendor.

  • Bits of first octet:

    • b0 (I/G) 0 = unicast, 1 = group.

    • b1 (U/L) 0 = globally administered, 1 = locally (randomised) address.

  • MAC addresses are portable (unlike IP) and only significant on local segment.

11 Address Resolution Protocol (ARP)

  • Needed because L3 IP ↔ L2 MAC mapping not implicit.

  • Steps (IPv4 only):

  1. If destination IP within same subnet, sender broadcasts ARP Request: "Who has 192.168.0.5?".

  2. Owner replies ARP Reply unicast with its MAC.

  3. Sender caches entry in ARP table (timeout ageing).

  4. If dest IP remote, host ARPs only for default-gateway’s MAC.

  • IPv6 replaces ARP with Neighbour Discovery / ICMPv6.

  • Commands: arp -a (Windows) shows table.

12 Ethernet II Frame Format
| Preamble 7B | SFD 1B | Dest MAC 6B | Src MAC 6B | Type 2B |
| Payload 46–1500B ( + optional 4B 802.1Q VLAN tag ) |
| CRC/FCS 4B |

  • Preamble: 101010… (7 bytes) + Start-of-Frame‐Delimiter 10101011 (1 byte) for clock sync.

  • Type/Length field: e.g. 0x0800 = IPv4, 0x86DD = IPv6, 0x0806 = ARP.

  • Min payload 46 bytes (pads if shorter).

  • Max payload 1500 B (standard MTU); jumbo ≈ 9000 possible in DC.

  • CRC-32 trailer checks integrity + marks end.

  • Optional 802.1Q VLAN tag (4 bytes) inserted after Src MAC (see next lecture).

13 Switch Operation in Detail

  1. Learning: on every received frame the source MAC is stored in CAM/TCAM table with ingress port, timer reset.

  2. Forwarding/Filtering:

    • If Dest MAC known → send out associated port only.

    • If unknown / multicast / broadcast → Flood out all ports except incoming.

  3. Aging: entries expire (default 5 min Cisco) so moved hosts are relearned.

  4. Store-and-forward: entire frame buffered, CRC checked, then forwarded; allows speed mismatches (1 Gb port ↔ 100 Mb printer).

14 Switches vs Routers

Aspect

Switch (Layer 2)

Router (Layer 3)

Operates on

MAC addresses & Frame header

IP addresses & Datagram header

Table built by

Automated learning + flooding

Routing protocols, static routes

Collision handling

Eliminated per-port

Not applicable (next-hop decision only)

Typical speed

Very high (ASIC)

Slightly lower; more processing

Scope

Single broadcast domain (subnet)

Interconnects multiple subnets

Terminology

CAM/TCAM, FDB

RIB/FIB, routing table

  • Multilayer (L3) switches incorporate both behaviours: ASIC switching within VLAN, routing between VLANs.

15 Full Summary / Key Takeaways

  • Link-layer frames move one hop; routers forward datagrams hop-by-hop.

  • Ethernet dominates wired LANs; evolved from coax → hubs → switches.

  • CSMA/CD allows efficient random access on shared media; modern switched Ethernet removes collisions entirely.

  • NICs integrate hardware error detection (CRC), signalling, buffering.

  • MAC addresses are global, 48-bit, vendor-assigned; ARP binds them to IPv4 addresses.

  • Switches learn MAC→port mappings, flood unknowns, and age tables to adapt.

  • Error detection parity/CRCs give high reliability before upper layers even see data.

  • Conceptual mapping:

    • Segment/collision-domain ≈ VLAN broadcast domain.

    • Subnet (IP) commonly maps 1 : 1 to broadcast domain, but virtualisation (VLANs) can break this (next lecture).

Practical CLI Nuggets (for lab)

  • Show NICs: ipconfig /all (Win) / ip a (Linux).

  • View ARP cache: arp -a (Win) / ip neigh (Linux).

  • Change duplex on NIC (Windows Advanced tab / ethtool Linux).

  • OUI lookup online reveals vendor (e.g.xD Dell, Intel, Cisco).

  • Packet capture requires promiscuous mode (e.g.xD Wireshark) to see non-destined frames.

Next lecture/lab: VLAN tagging, trunk links, switch-router interoperation, hands-on flooding/learning demos.