Two types of links / channels / transmission media: shared broadcast and point-to-point. A natural shared broadcast medium is wireless. For example, a WiFi channel. Ethernet is also a shared broadcast medium. That is, multiple transmitters connected to the channel can all hear each other. The other type of links are point-to-point. Only the two end points connected to the link can communicate with each other. Link layer design varies for these two types of links.
Medium access: when multiple nodes share a shared broadcast medium, the link layer decides who transmits when using a medium access control (MAC) protocol. One technique is to partition the channel using techniques such as TDMA or FDMA. However, this is inefficient if all users do no always use their allocated partition. Another class of MAC protocols called random access protocol, which is more suited for bursty traffic patterns found in packet switched networks. The first random access protocol was ALOHA. In “slotted” ALOHA, time was divided into slots. At the beginning of the slot, the N nodes which have packets to send try to independently access the medium with a probability “p”. The transmission succeeds if exactly one node accesses the medium. If two or more nodes transmit together, the transmissions fail due to “collisions”. The utilization of the medium, defined as the probability that the channel does useful work, is defined as the probability that only one of the N nodes transmits. Therefore, U = N p (1-p)^{N-1}. This utilization is maximum when p = 1 / N, and the asymptotic value of the optimum utilization is 1/e = 0.37 for large N. That is, ALOHA works well when the value of the access probability p is set carefully, and the best it can work is still only 37% of the capacity of the link.
Random access protocols perform better when they are based on “carrier sense” - that is, nodes sense the energy on the medium to determine whether someone else is sending or not, and transmit only if the medium is free. CSMA (carrier sense multiple access) protocols work this way. Are collisions completely eliminated with carrier sense? No, because there is a finite detection delay between a node starting transmission and other nodes identifying the medium to be busy. Therefore, if two nodes start transmissions within the detection delay, then collisions can still happen. What causes non-zero detection delay? Two factors: the propagation delay of the signal (at speed of light), and hardware processing delay to detect medium as busy. The longer this “blind spot”, the greater the chance of collisions.
Ethernet uses the CSMA/CD MAC protocol (CSMA with collision detection). Whenever a node has a packet to send, it senses the medium a small duration, and start transmitting if medium is idle. If medium is busy, it waits till it gets idle for a certain period of time. Once transmission starts, the transmitter monitors for collision. If a transmitter detects a collision has occurred after sending its preamble, (say, by finding that energy on channel is more than what can be caused by one transmission), then it sends a special jamming sequence and aborts the transmission. The node then waits for some time before retransmitting again. The wait time between successive retransmissions of the same frame increases exponentially (“exponential backoff”) to avoid collisions.
Other details of Ethernet. Ethernet frame consists of a payload (46 to 1500 bytes), source and destination MAC addresses, a CRC for error detection, and a preamble at start of frame. Note that Ethernet does not do any error correction - no extra bits to recover from errors. No retransmissions by sender if checksum fails at receiver. Ethernet uses Manchester encoding: 0 is transmitted as a low to high transition, and 1 is encoded as a high to low transition.
WiFi is another shared broadcast medium. It also uses CSMA. However, the main difference is that detecting collisions is hard in wireless channels (for reasons that are hard to explain in this course). Therefore, WiFi has link layer ACKs. When a sender transmits a frame, it waits for an ACK from receiver. If no ACK, assumes collision and retries. This is called CSMA/CA (CSMA with collision avoidance). Because wireless channel introduces a lot of errors, WiFi link layer also uses error correcting codes (not just error detection).
As the number of transmitters increases in a shared broadcast channel, the probability of collisions increases. Therefore such link layers are not scalable for a large number of users.