Chapter Six

1. Chapter Six The Data Link Layer

2. Objectives • Learn about physical addressing on the network. • Explore the MAC and LLC sublayers of Data Link. • Get introduced to some of the hardware protocols in greater detail. • Learn about end-to-end error correction. • Examine flow control methods in detail.

3. The Two Layers of Data Link • Media access control • Dictates how data will access the medium • Defines physical addressing mechanisms • Logical link control • Defines the hardware protocols

4. Addressing in Data Link • Physical addressing • The MAC address is burned onto the interface at the factory. • Physical addressing uses a flat addressing model. • Logical addressing • Service access points (SAP) are generated by Data Link.

5. The MAC Address • A 48-bit address • 24 bits is the organizational unique identifier • Assigned by IEEE and identifies the make and model of the device • 24 bits is a manufacturer assigned interface serial number. • No two devices can have the same MAC address. 00 - F1 - DB - 01 - 85 - A1

6. Hardware Protocols • CSMA/CD (Ethernet) • Demand Priority (now defunct) • Token Ring • FDDI

7. CSMA/CD • Carrier Sense Multiple Access/Collision Detection • It listens to the wire for a quiet moment, then transmits. • If two devices transmit at once, a collision occurs. • After a collision, a backoff algorithm is issued. • Each device waits a predetermined interval before trying to transmit again. • It is known as probabilistic networking.

8. Ethernet over Copper

9. Ethernet over Fiber

10. Demand Priority • It was a good idea that never made it. • Now defunct, but enough existing networks remain to make it worth discussing • An “intelligent” hub controls access to the network by the devices attached. • When a device needs to transmit, it sends a signal to the hub “demanding access.” • The hub determines when and if the device can transmit.

11. CSMA/CA • Carrier Sense Multiple Access/Collision Avoidance • It is used by Apple’s LocalTalk. • A device broadcasts its intention to transmit. • Upon receiving this broadcast, all other devices are induced to wait a predetermined amount of time.

12. Token Passing • A small packet of data called the token runs round and round the network. • For a device to transmit, it must take possession of the token. • It is used by token ring and FDDI.

13. Token Ring • It uses a virtual ring network topology. • A multi-station access unit (MAU) interconnects devices in the manner of a hub. • The first device to power up on the network becomes the active monitor (and generates tokens as needed).

14. Beaconing • When the token does not arrive at a workstation, that workstation broadcasts messages in both directions across the ring. • The point at which the two broadcasts each reach a dead end is dropped from the MAU. • The active monitor generates a new token and life goes on.

15. FDDI • FDDI is another ring-based token passing network. • It uses two concentric rings (although a network can be built on a single ring). • Rings are virtual structures and do not even need to be geographically adjacent. • Devices can be dual-attached (dual ring) or single-attached (single ring).

16. Error Detection in Data Link • Data Link is responsible for finding and fixing: • Lost frames • Incorrect checksum or CRC • Offending frame sizes • Buffer overflow • Frame corrupted through interference

17. Lost Frames • When a device is transmitting data, it waits for ACK packets for each data packet sent. • If an ACK does not arrive within the time to live specified in the header, the transmitting workstation will retransmit the frame. • If duplicate frames arrive, the receiving workstation will discard the second to arrive.

18. Incorrect Checksum or CRC • The same calculations that are performed by the transmitting workstation are done by the receiver. • If the value achieved by the receiving workstation does not match the value in the trailer, a NACK is issued. • The transmitting workstation retransmits the data.

19. Offending Frame Size • Protocols are picky about the size of frames they create. • Frames that are too small (runts) or frames that are too large (giants) are discarded. • Eventually the TTL will expire and the transmitting workstation will retransmit.

20. Buffer Overflow • If data is coming into the receiving workstation too quickly, the memory buffer will fill up. • Depending on the flow control method used, the session will either be aborted or transmission speeds will be renegotiated.

21. Interference • EMI or other interference can result in corrupted frames. • These are dropped and treated as lost frames.

22. Flow Control • Most flow control is under the umbrella of the Transport layer. • One Data Link protocol, HDLC, uses flow control. • X-on/X-off • Stop and wait • Sliding window

23. X-on/X-off • Data flows until the buffer fills. • The receiving device issues a STOP command. • Receiving device processes data and then issues a stop command. • Data flows until the buffer fills.

24. Stop and Wait • Transmitting device sends one packet and waits. • Receiving device receives packet and sends an ACK; receives a corrupted packet and sends a NACK; or doesn’t receive it and the TTL expires. • Transmitting device receives ACK and sends next packet; receives a NACK and retransmits previous packet; or TTL expires and it retransmits previous packet.

25. Sliding Window • Transmitting packet sends a series of packets and sits back to wait for the ACKs. • If all ACKs return, another blast of packets goes out. • If only some of the packets return ACKs, the number of packets is reduced for the next burst.

26. Data Link Protocols • HDLC • SLIP • PPP