Chapter Six

The Data Link Layer

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.

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

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.

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.

Hardware Protocols

• CSMA/CD (Ethernet)

• Demand Priority (now defunct)

• Token Ring

• FDDI

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.

Ethernet over Copper10BaseT 10Mb/s CAT3

10Base2 10Mb/s RG-58

10Base5 10Mb/s RG-8

100BaseT 100Mb/s CAT5

100BaseT4 100Mb/s CAT5 using all four pairs

1000BaseT 1000Mb/s CAT5e or CAT6

1000BaseCX 1000Mb/s Balanced copper over coax

Ethernet over Fiber

100BaseFL 100Mb/s

100BaseFB 100Mb/s fiber backbone

1000BaseLX 1000Mb/s

1000BaseSX 1000Mb/s

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.

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.

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.

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).

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.

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).

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

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.

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.

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.

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.

Interference

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

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

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.

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.

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.

Data Link Protocols

• HDLC

• SLIP

• PPP