Role of the data link layer: hop-to-hop delivery and MAC addresses

1. Circuit switching total time formula?
   Stuck? Peek at 1.3.4 Circuit vs packet switching: delay numericals

   • Circuit: setup + M/R + h·T_p.
   • Message switching: h·(M + H)/B + h·T_p.
   • Packet switching: (⌈M/p⌉ + h − 1)·(p + H)/B + h·T_p.
   • Optimal p ≈ √(M·H/(h − 1)); in MCQs evaluate each option.

   Read lesson 1.3.4

2. N packets over h equal links: total time?
   Stuck? Peek at 1.3.2 Propagation, queuing and processing delay; multi-hop pipelining

   • T_p = d / v, v = 2 × 10⁸ m/s default; independent of size and rate.
   • Nodal delay = d_proc + d_queue + T_t + T_p.
   • One packet, h equal links: h·T_t + h·T_p.
   • N packets, h equal links: (N + h − 1)·T_t + h·T_p (+ (h − 1)·d_proc).
   • Unequal links: first packet ΣT_t + ΣT_p, then + (N − 1) × max T_t.
   • Traffic intensity L·a/R → 1: queues explode.

   Read lesson 1.3.2

3. Which address changes at every hop — link-layer or network-layer?
   Stuck? Peek at 1.1.3 Encapsulation, headers, PDU names, addresses and devices per layer

   • Encapsulation: payload + header (+ trailer at data link); decapsulation in reverse.
   • PDUs: message (app) → segment (transport) → packet/datagram (network) → frame (data link) → bits.
   • Addresses: link (per hop, changes) · network (end to end, fixed) · which-program id · names.
   • Devices: hub/repeater 1 · bridge/switch 2 · router 3 · gateway/host up to 7.
   • Efficiency = data / (data + all headers + trailer).

   Read lesson 1.1.3

A long-distance bus trip is planned city to city, but each leg is driven by a different local driver who only knows the next stop. The network layer plans the whole trip; the data link layer is that local driver. Our message, now a packet, must be carried across one link at a time — and on a shared link, the right neighbour must pick it up.

A packet travels laptop → switch → router → … → server. At the first link, whose address must be on the outside of the wrapper: the server's, or the next box's?

The next box's. The frame on each link carries the link-layer address of the next hop (and of the sender on that link). The server's address lives inside, in the network header, and stays unchanged. Each hop builds a fresh frame.

What the data link layer does

The data link layer moves a frame from one node to the next directly connected node — hop to hop. Its jobs:

• Framing: marking where each frame starts and ends in the bit stream.
• Link addressing: naming the sender and receiver on this link.
• Error detection (Ch 2.2): noticing damaged bits.
• Medium access control (Ch 2.3): deciding who may send on a shared link.
• Optionally error control and flow control on the link (names only for now).

Hop-to-hop delivery

Across a path of h links there are h separate data-link conversations. At every router, the incoming frame is opened (decapsulated), the packet goes up to the network layer, and a new frame with new link addresses is built for the next link.

MAC addresses

• 2⁴⁸ ≈ 2.8 × 10¹⁴ possible addresses.
• The first 24 bits identify the manufacturer (vendor prefix); the last 24 bits are numbered by that vendor — 2²⁴ = 16,777,216 per prefix.
• A MAC address is flat: it says which card, not where it is. That is why a different, location-based address is needed to cross networks (Phase 3).

Framing (Foundation)

Putting it together

• The data link layer carries frames one hop at a time.
• MAC addresses are flat 48-bit card names; they change with every hop.
• Framing marks frame boundaries (bit stuffing keeps the flag out of the data).

• The MAC destination in a frame is the next hop, not the final server (unless they are on the same link).
• MAC addresses are flat identifiers, not locations — they cannot be aggregated for routing.
• 48 bits, not 32: 12 hex digits.

"MAC = name tag on the card; it says who, not where."

We can now carry a frame to the next box. Next (Ch 2.2): a scratch on the cable or a burst of radio noise flips some bits on the way — how can the receiver notice that the frame is damaged?

1. NAT

   How many bits are in a MAC address?

   Write your answer before opening the key.

2. MCQ

   In the frame on the link from router R1 to router R2, the destination MAC address is that of:

   1. the original source host
   2. the final destination host
   3. R2's interface
   4. R1's interface
3. MSQ

   Which are data link layer jobs?

   1. Framing
   2. Choosing a route across many networks
   3. Detecting damaged bits on a link
   4. Deciding who may send on a shared link
4. MCQ

   Bit stuffing the data 11111111 (flag 01111110) gives:

   1. 111110111
   2. 11111111
   3. 111111011
   4. 011111111

• Data link layer = hop-to-hop delivery of frames to the next directly connected node.
• Jobs: framing, link addressing, error detection, medium access (+ error/flow control).
• MAC address: 48 bits, 6 bytes, 12 hex digits; 24-bit vendor prefix; flat.
• A new frame (new MAC addresses) is built on every link; the packet inside is unchanged.
• Foundation: bit stuffing inserts a 0 after five consecutive 1s.

• data link layer role
• hop-to-hop (node-to-node) delivery
• MAC address (48-bit, hex)
• vendor prefix
• framing (Foundation): flags, bit stuffing
• error control / flow control (names)

1. MAC address size?
   Reveal answer

   48 bits (6 bytes, 12 hex digits).
2. Destination MAC on an intermediate link?
   Reveal answer

   The next hop's interface.
3. Bit stuffing rule?
   Reveal answer

   Insert a 0 after five consecutive 1s.

• Frames per path = number of links h; MAC addresses change on every link. — routers in between (a switch inside a LAN keeps the frame's MAC addresses unchanged).