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

64 bits	48 bits	48 bits	16 bits		32 bits
Preamble	Destination Address	Source Address	Type	Body	CRC

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 indicate 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.

In fast ethernet pseudo-random number generation is done using Linear feedback shift registers.

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.

Keeping Rings connected

So far we described the basics of how to transmit frames in a token ring.
Of course, this relied on machines being connected in a ring. What happens if one machine fails?
- If we do nothing, then it is no longer a ring and the token will fly off one end to oblivion, and no one will be able to talk.
- One solution is to connect each station to a electromechanical relay. If the station is healthy the relay stays open and the current goes through the given host. If the host dies and stops providing power to the relay, the relay closes, and the ring automatically bypasses the station. This only works for electrical cable, not fiber.
- Typically, several relays are packed into a single box called a multistation access unit (MSAU).
- A pair of connections (probably one cable but with an incoming/outgoing line) is then run from each host to this box.
- It is also possible to chain boxes.

Multistation Access Unit Image

Token Ring Media Access Control

Let's look at how a token ring works in more detail...
The network adaptor for a token ring contains a receiver and a transmitter.
Most of the time an adaptor is merely retransmitting the data that it receives.
When a station has the token (so is not retransmitting it), it is allowed to send one or more packets of data.
Each packet contains the destination address of the intended receiver; it may also contain a multi-cast address.
As the packet flows around the ring each node looks to see if it is the recipient. If so, it copies the packet to its buffer, but does not remove it from the ring.
The sending station is responsible for removing the packet from the ring.
There are two choices for how to do this: (a) send the token immediately after the last frame to send (early release); or (b) send the token after waiting for the last frame to go around the ring (delayed release). Initially, 802.5 used method (b), but now supports both.

Token Holding Time

The time each station can hold the token (and so send packets) before giving it to the next station is called the token holding time (THT).
We would like this time to be less when many people need to send.
On the other hand, to ensure good ring utilization, we want this time to be longer when only one station has stuff to send.
To address the issue of fair utilization, the token contains a 3-bit priority field.
Each device that wants to send a packet assigns a priority to that packet, and the device can only seize the token to transmit a packet if the packet's priority is at least as great as the token's.
To change the priority of the token, a station can set a 3-bit reservation field in a data packet to the priority of the packet it wants to send, provided this reservation field does not have a higher value already.
When a sender is finished sending, it is supposed to set the priority of the token to the value of reservation bits and set the reservation bits to the lowest priority.

Reliable Delivery

802.5 (Token Ring) uses a 2 bit field in the trailer of packets to determine if the packet got to its intended recipient.
The bits are called A and C and when the packet is sent these are both 0.
When the recipient sees the packet it sets A to 1. If it copies the frame to the network adaptor buffer, it sets the C bit to 1.
If, when the packet comes back to the sender, the sender sees A set to 0, it knows the recipient is absent. If A is set but C is not, then the sender will probably assume the recipient's buffer is full and try again later.

Token Ring Maintenance

Each token ring has one station designated as a monitor.
The monitor tries to keep the ring healthy. It does this by doing things like making sure the token is not lost.
A healthy monitor periodically announces its presence with a special control message.
If a station fails to see this message for some period of time, it assumes the monitor has failed, and will try to become the monitor.
A "claim token" is transmitted by the station trying to become the monitor.
If the station see its token go around the ring and come back to it, it then assumes it can be the monitor.
If someone else's, claim token arrives first, then it compares its address with the token's sender. If it is higher, it still becomes monitor; otherwise, it lets the other station be monitor.
To detect a missing token, the monitor maintains a timer, initially set to
```
NumStations×THT + RingLatency
```
If the timer expires without seeing a token a new token is inserted by the monitor.

Corrupt and Orphaned Frames and Dead Stations

The monitor checks for corrupt frames by looking for incorrect checksums or invalid formats to frames. This is important because, a sender address corruption might mean no one knows to take this packet off the ring.
The monitor takes corrupt frames off the ring and re-inserts the token.
An orphaned frame is a correct frame but whose sender has died (so no one will take it off the ring).
These are detected by the monitor by setting a monitor bit to 1 when the frame goes by the monitor. If the monitor sees a frame with a monitor bit of 1, it knows it's gone around the loop at least once and should be removed.
It is possible for a station to die in some way that the MSAU doesn't know the station is dead. i.e., some kind of software death.
Any station can send out beacon frames to try to figure out how far down the ring the error is and then tell the MSAU to bypass this station.

802.5 Frame Format

8	8	8	48	48		32	8	8
Start Delimeter	Access Control	Frame Control	Destination Address	Source Address	Body	Checksum	End Delimiter	Frame Status

The frame is sent using Manchester encoding, except start and end delimiters.

Access Control - contains the frame priority, and reservation bits.

Frame Control - says the higher level protocol.

Frame Status - has the A and C bits.

Ethernet and Token Rings

Outline

Quiz