ACtE0502 Data link layer¶
Services and framing¶
- Data link layer provides node-to-node delivery over a single link.
- It converts the raw bit stream into frames.
- Major services: framing, physical addressing, flow control, error control, and medium access control.
- LLC and MAC are the two classic IEEE 802 sublayers.
Error detection and correction¶
| Method | Purpose | Cue |
|---|---|---|
| Parity | Detect simple odd/even bit errors | Very low overhead, weak protection |
| Checksum | Detect many common errors | Used more widely at higher layers too |
| CRC | Strong burst-error detection | Standard Ethernet frame check sequence |
| Hamming code | Detect and correct limited bit errors | Forward error correction idea |
Facts:
- Detection means the receiver knows corruption occurred.
- Correction means the receiver can reconstruct the original bits, or the system retransmits.
- ARQ combines detection with retransmission.
Flow control and data-link protocols¶
| Concept | Core idea |
|---|---|
| Stop-and-wait | Send one frame, wait for ACK |
| Sliding window | Allow multiple outstanding frames |
| Piggybacking | Carry ACK on a reverse-direction data frame |
Data-link protocols often combine framing, sequence numbering, ACKs, timers, and retransmission.
On a path with a large bandwidth-delay product, a sliding window keeps multiple frames in flight and can fill the path; stop-and-wait often leaves capacity idle. Go-Back-N is itself a sliding-window ARQ protocol, so an item offering both “sliding window” and “Go-Back-N” does not provide mutually exclusive choices.
Multiple access protocols¶
| Method | Principle | Typical cue |
|---|---|---|
| ALOHA / slotted ALOHA | Random access | Historical collision-based access |
| CSMA | Listen before transmit | Carrier sense |
| CSMA/CD | Detect collision during transmission | Classic shared Ethernet |
| CSMA/CA | Try to avoid collisions | Wireless LAN |
| Token passing | Explicit permission token | Token bus, token ring |
Important trap:
- Ethernet with hubs used CSMA/CD.
- Modern switched full-duplex Ethernet does not have collisions, so CSMA/CD is effectively irrelevant there.
- Wireless LAN does not use collision detection like classic Ethernet; it uses collision avoidance.
LAN addressing and ARP¶
- A MAC address is a data-link physical address used for local delivery on a LAN.
- ARP maps an IPv4 address to a MAC address on the local network.
- ARP is often described as between Layer 2 and Layer 3 or as a link-layer support protocol for the network layer.
- For MCQ purposes, do not place ARP in the transport or application layer.
ARP flow:
- Sender knows target IP but not target MAC.
- Sender broadcasts ARP request on the local LAN.
- Owner of the IP replies with its MAC address.
- Sender caches the mapping.
Related distinction:
- ARP: IPv4 address to MAC address.
- RARP: MAC address to IPv4 address; historically important, now mostly replaced by BOOTP or DHCP workflows.
Ethernet and IEEE LAN standards¶
| Standard | Topic | Exam cue |
|---|---|---|
| IEEE 802.3 | Ethernet | CSMA/CD heritage |
| IEEE 802.4 | Token Bus | Token on bus topology |
| IEEE 802.5 | Token Ring | Token on ring topology |
| IEEE 802.11 | Wireless LAN | Wi-Fi family |
Ethernet facts:
- Uses frame-based LAN communication.
- Original Ethernet used a shared medium; switched Ethernet dominates today.
- Ethernet addressing uses MAC addresses.
- In classic half-duplex Ethernet, the PHY/MAC detects a collision and sends a jam sequence so all stations recognize it.
- Minimum frame size and slot time keep the sender transmitting long enough to detect a late collision.
- The frame check sequence (FCS) detects frame corruption; it does not detect a collision.
Wireless LANs¶
- Wireless LANs typically refer to IEEE 802.11 networks.
- Uses radio channels and contention methods such as CSMA/CA.
- Access point mode is infrastructure WLAN; direct station-to-station mode is ad hoc.
- Hidden terminal problem is more relevant in wireless than wired Ethernet.
PPP and WAN protocols¶
- PPP stands for Point-to-Point Protocol.
- It is a data-link protocol for direct point-to-point links.
- PPP supports framing, link configuration, optional authentication, and network-layer protocol encapsulation.
- It is not a multiple-access LAN protocol.
Wide-area protocol cues:
- WAN protocols connect sites over long distances.
- PPP, HDLC-family links, and leased-line framing ideas are classical WAN examples.
Data-link examples¶
- If a host must find the MAC for local IPv4 delivery, it uses ARP, not DNS.
- If the question mentions token circulation on a ring, it points to IEEE 802.5.
- If the link is point-to-point and framing plus authentication are highlighted, PPP is likely.
ACtE0502 revision box¶
- Data link = framing, MAC, local delivery, error control, flow control.
- ARP maps IPv4 to MAC on a local LAN.
- 802.3 Ethernet, 802.4 Token Bus, 802.5 Token Ring, 802.11 WLAN.
- CSMA/CD belongs to classic shared Ethernet; CSMA/CA belongs to Wi-Fi.
- PPP is for point-to-point links, not broadcast LAN access.