Outline

• Quiz
• Ethernet
  • Physical Properties
  • Access Protocol
  • In the read world
• Rings (802.5, FDDI, RPR)

Quiz

Suppose for a moment that a frame could consists of all zero bits. We now compute a CRC-32 checksum for this frame, tack it on to the end, and transmit it. Suppose the lower order bit 5 and bit 6 of the message part of the frame are corrupted. What can the receiver say about the message?

1. These errors would go undetected.
2. Not enough information is given to know for sure.
3. The errors would be detected.

Physical Properties of Ethernet

• Originally, an Ethernet segment was implemented on a 50 ohm coaxial cable of up to 500m.
• Hosts connected to such a segment by tapping into it; taps had to be at least 2.5m apart.
• A transceiver, which was directly attached to the tap, detected when the line is idle and drove the signal when the host is transmitting.
• The transceiver also received incoming signals.
• It is connected to the Ethernet Adaptor of the given host.
• Multiple segments could be joined by repeaters up to a maximum total reach of 2500m.
• Bits on the Ethernet use Manchester encoding.
• Signals on Ethernet are heard by all hosts attached to it.
• Up 1,024 host may be attached to an Ethernet.

Variations on Physical Properties of Ethernet

• More recent Ethernets replace the 50 ohm cable (also called 10Base 5 or thicknet cable) with a thinner cable known as 10Base2. Here 10 means 10Mbps, and 2 refers to 200m rather than 500m.
• There is also 10BaseT Ethernet which uses twisted-pair wire and is restricted to 100m. This is what's used in Fast (100Mbps) Ethernet and in Gigabit Ethernet.
• The typical configuration for 10Base2 and 10BaseT is to have several point-to-point segments coming out of a multiway repeater rather than use taps. This kind of repeater is called a hub.
• Regardless of the set-up, all hosts can hear whatever anyone transmits on an Ethernet. So everyone on an Ethernet is competing for the shared resource. i.e., everyone is in the same collision domain.

Access Protocol

• We will look in a moment at the algorithm used by Ethernet to share the link. This algorithm is known as the media access control (MAC) algorithm.
• Before we do this, let us briefly refresh our knowledge about what an Ethernet frame looks like and what an Ethernet address looks like...

Ethernet Frame Format

• Ethernet is a bit-oriented framing protocol.
• The Preamble allows the receiver to synchronize with the signal; it is a sequence of alternating 0s and 1s.
• Both the Source and Destination hosts are identified by a 6 byte number. (We'll talk more about this on the next slide).
• The high-order bit of the destination address in IEEE 802.3 can be used to say single or group addresses (multicast).
• The Type in Digital-Intel-Xerox Ethernet is used to indicated to which of the many possible higher level protocols this frame should be delivered. In 802.3 Ethernet it is replaced with a 16-bit length field, and the first 16 bits of the Body contain the higher level protocol info. Since all type values are greater than 1500, it is possible using ANDing to get the two frame formats to coexist.
• The Body of the frame is at least 46 bytes long (so the whole frame is at least 64 bytes long). A whole frame can be at most 1500 bytes long.
• The CRC, of course, is used to detect errors.

Ethernet Addresses

• The 6 byte Ethernet address also called (MAC) is unique to each Network Adaptor (burned into its ROM).
• The IEEE manages a registry of the addresses (assigning different manufacturers different prefixes) which have been used so far. For example, the prefix 8:0:20 is for AMD.
• Addresses are usually written in hexadecimal with colons used to separate each byte and leading 0's dropped. For example: 8:0:2b:e4:b1:2 .
• Recall each digit in hex corresponds to a 4 bit nibble of the byte. So 2b is 0010 1011.
• FF:FF:FF:FF:FF:FF is used to indicate a broadcast frame for all hosts on the Ethernet.
• Each device on an Ethernet sees all frames that are sent on the Ethernet, but should only accept: (a) those addressed to it, (b) those broadcast to all hosts, or (c) those multi-cast to it. In the case where it is in promiscuous mode it should accept all frames.

MAC Algorithm

The smarts of Ethernet resides in how the sender sends frames it needs to send. It uses the following transmitter algorithm:

• If a network adaptor has a frame to send, and the line is idle, then the adaptor sends the frame immediately.
• If a network adaptor has a frame to send, and the line is busy, the adaptor waits for the line to become idle and then immediately send the frame. This is called 1-persistence; p-persistence would be to transmit with probability p and wait for the next frame "time slot" to consider transmitting with probability 1-p.
• If two transmitters try to transmit at the same time, then the frames are said to collide. The time it takes for a signal to transmit the whole Ethernet line length determines how many bits someone might have transmitted before noticing a collision. Once a collision is noticed, the transmitter is supposed to transmit a special 32-bit jamming sequence. This jammed frame is called a runt frame and contains at least the preamble and the jamming sequence (96 bits), and at most 512 bytes.
• Once the adaptor has detected a collision and stopped its transmission, it chooses to wait either 0s or 51.2μs at random; after which it starts transmitting again. If it fails again, the adaptor flips a four-sided coin and depending on its value tries again in either 0s, 51.2μs, 102.4μs, 153.6μs. If the kth try fails it flips a 2^k sided coin to get a number r between 0 and 2^k-1 and tries to transmit at time 51.2*r. An adaptor tries this up to k=16 before failing with an error. This is called exponential backoff.

The worst case for detecting a collision is when the hosts are on opposite ends of the Ethernet. If d is the time for a signal to go the complete length of the Ethernet. Then Host A might transmit for d seconds and Host B might just start transmitting and immediately send a jamming sequence, this takes d second to get back to A. So A might transmit for 2*d before detecting a collision. On a 10Mbps Ethernet limited to 2500m, the delay would be at most 51.2μs and this would correspond to 512 bits = 96 bytes, which is why this is used as the shortest frame length.

Real world experience with Ethernet

• In the real-world, it has been observed that Ethernets tend to work best when they are lightly loaded.
• Utilization over 30% is considered heavy on Ethernet -- too much traffic is wasted on collisions.
• Since typically Ethernet have less than 200 hosts, and are far shorter than 2500m, both the round-trip delay and the likelihood of collision are much smaller.
• Ethernet is successful because it is reasonably easy to implement and administer. There is no hardware to maintain such as switches, cable is cheap, and the only cost to adding an additional node is the cost of a network adaptor.

Rings

• Ring networks, like Ethernets are shared-media networks.
• The grand-daddy of ring networks is IBM's Token Ring which was standardized as 802.5.
• Another ring network is Fibre Distributed Data Interface (FDDI) token ring.
• Neither of these is in common use; however, more recently Resilient Packet Ring (RPR), standardized as 802.17, has been proposed for MAN (metropolitan area networks) settings.
• In a ring network each host is connected to two other machines such that the graph of all the machines looks like a circle (that's why its called a ring).
• In a token ring, a special bit sequence circulates around the ring in a fixed direction.
• Each host receives and then forwards this token.
• When a node that a has a frame to transmit sees the token, it takes the token off the ring (does not propagate it). It instead then sends the frame it wants to send.
• When this frame makes it way around the loop and comes back to the sender, the sender takes it off the ring, and then reinserts the token.