1. Example DLL Protocols 1. High-Level Data Link Control (HDLC).
2. Point-to-Point Protocol (PPP).

2. HDLC
Flag
Identifies
host.
ACK,
seq. #
CRC

3. PPP Internet’s DLL. Router-to-router. Home user-to-ISP. RFC 1661, etc.
PPP is a multi-protocol framing mechanism that can be used over multiple PHYs (dial-up, dedicated point-to-point connections).

4. Why MAC? Point-to-point versus shared-medium networks.
Shared-medium networks use “broadcast” channels.
multi-access or random access channels.
MAC layer protocols regulate access to medium in shared-medium networks.

5. MAC and LANs LANs typically use shared-medium. Examples?
MAC layer critical!
BTW, in wireless networks also!
WANs typically use point-to-point connections

6. Channel Allocation Problem
How to allocate single shared, broadcast channel among several stations/users.
If no arbitration, several stations/users may transmit at the same time: COLLISIONS!

7. MAC Protocols Contention-based Round-robin : token-based protocols.
ALOHA and Slotted ALOHA.
CSMA.
CSMA/CD.
Round-robin : token-based protocols.
Token bus.
Token ring.

8. Pure ALOHA
In pure ALOHA, frames are transmitted at completely arbitrary times.

9. Carrier Sense Multiple Access
The capacity of ALOHA or slotted ALOHA is limited by the large vulnerability period of a packet.
By listening before transmitting, stations try to reduce the vulnerability period to one propagation delay.
This is the basis of CSMA.

10. CSMA
Station that wants to transmit first listens to check if another transmission is in progress (carrier sense).
If medium is in use, station waits; else, it transmits.
Collisions can still occur.
Transmitter waits for ACK; if no ACKs, retransmits.

11. CSMA Protocol CSMA Packet ready Channel Busy? no transmit delay packet
wait for a
round-trip time
positive
ack?
yes
compute random
backoff integer k
delay packet
transmission
k times
Packet
ready
Channel
Busy?

12. CSMA (cont’d)
Collisions can occur only when 2 or more stations begin transmitting within short time.
If station transmits and no collisions during the time leading edge of frame propagates to farthest station, then NO collisions.

13. CSMA Flavors
After detecting carrier, a station can persist trying to transmit after the channel is idle again.
1-persistent CSMA (IEEE 802.3)
If medium idle, transmit; if medium busy, wait until idle; then transmit.
If collision, waits random period and starts again.
Non-persistent CSMA: if medium idle, transmit; otherwise wait a random time before re-trying.
Thus, station does not continuously sense channel when it is in use.
P-persistent: when channel idle detected, transmits packet in the first slot.

14. CSMA versus Aloha
Comparison of the channel utilization versus load for various random access protocols.

15. CSMA/CD CSMA with collision detection.
Problem: when frames collide, medium is unusable for duration of both (damaged) frames.
For long frames (when compared to propagation time), considerable waste.
What if station listens while transmitting?

16. CSMA/CD Protocol 1. If medium idle, transmit; otherwise 2.
2. If medium busy, wait until idle, then transmit with p=1.
3. If collision detected, transmit brief jamming signal and abort transmission.
4. After aborting, wait random time, try again.

17. CSMA/CD Performance
Wasted capacity restricted to time to detect collision.
Time to detect collision < 2*maximum propagation delay.
Rule in CSMA/CD protocols: frames long enough to allow collision detection prior to end of transmission.

18. CSMA with Collision Detection
CSMA/CD can be in one of three states: contention, transmission, or idle.

19. Ethernet
IEEE 802. family.
Standards for LANs and MANs.
Ethernet defined in the IEEE standard.

20. Ethernet MAC
CSMA/CD.
Binary exponential back-off.
CRC

21. Ethernet Frame
Frame formats. (a) DIX Ethernet, (b) IEEE

22. Ethernet MAC (Cont’d)

23. Ethernet Frame Length At 10Mbps with 2,500 m maximum distance:
RTT ~ 50 microsec.
Thus, at least 500-bit frames.
It is actually 512 bits.
If fewer bits than that, add “padding”.

24. Ethernet Cabling
The most common kinds of Ethernet cabling.

25. Switched Ethernet
A simple example of switched Ethernet.

26. Fast Ethernet
The original fast Ethernet cabling.

27. Gigabit Ethernet
Gigabit Ethernet cabling.

28. Intro to 802.5 and ATM 802.5 Token ring networks
ATM – Asynchronous Transfer Mode