LAN Technology

Point-to-point Communication

The first computer communication systems had each communication channel, e.g. a leased circuit, connecting exactly two computers. This is known as point-to-point communication and has three useful properties.

• Each connection is independent of the others and can use appropriate hardware.
• The two end points have exclusive access and can decide how to send data across the connection.
• Since only two computers have access to the channel, it is easy to enforce security and privacy.

However, point-to-point communications also have disadvantages. The main disadvantage is the proliferation of connections, as illustrated in Figure 1.

Figure 1: Point-to-point connections

In (a), two computers require one connection, while in (b), three computers require three connections. In (c), four computers requires six connections. The number of connections for N computers is proportional to the square of N, namely N(N-1)/2.
In practice, the cost is high because many connections follow the same physical path. Figure 2 illustrates the situation with five computers located in two locations.

Figure 2: Different locations

There are six connections between the two locations, more than the total number of computers being connected. Another computer added to location 1 increases the number of connections between the two locations to nine.
Apart from the fact that point-to-point communication does not scale, as the physical distance between computers increases, the cost of installing the point-to-point connections becomes prohibitively expensive.

LAN Topologies

In the late 1960s and the early 1970s researchers developed a form of computer communication known as Local Area Networks (LANs). These are different from long-distance communications because they rely on sharing the network. Each LAN consists of a single shared medium, usually a cable, to which many computers are attached. The computers co-ordinate and take turns using the medium to send packets.
Unfortunately, this mechanism does not scale. Co-ordination requires communication, and the time to communicate depends on distance - large geographic separation between computers introduces longer delays. Therefore, shared networks with long delays are inefficient. In addition, providing high bandwidth communication channels over long distances is very expensive.
There are a number of different LAN technologies. Each technology is classified into a category according to its topology, or general shape. The first of these is a star topology, as illustrated in Figure 3.

Figure 3: Star topology

The hub accepts data from a sender and delivers it to the receiver. In practice, a star network seldom has a symmetric shape; the hub often resides in a separate location from the computers attached to it.
A network using a ring topology arranges the computers in a circle - the first computer is cabled to the second. Another cable connects the second computer to the third, and so on, until a cable connects the final computer back to the first. This is illustrated in Figure 4.

Figure 4: Ring topology

Once again, the ring, like the star topology, refers to logical connections, not physical orientation.
A network that uses a bus topology consists of a number of computers all connected to a single, long cable. Any computer attached to the bus can send a signal down the cable, and all computers receive the signal. This is illustrated in Figure 5.

Figure 5: Bus topology

The computers attached to a bus network must co-ordinate to ensure that only one computer sends a signal at any time. In addition, the ends of a bus network must be terminated to prevent electrical signals from reflecting back along the bus.

Ethernet

Ethernet is a widely used technology employing a bus topology. The original standard was published by Digital Equipment Corporation, Intel Corporation, and Xerox Corporation in 1982. IEEE currently controls Ethernet standards, e.g. IEEE 802.3 was published in 1985.
In its original form, an Ethernet LAN consists of a single coaxial cable called the ether, but often referred to as a segment. A segment is limited to 500 m in length, with a minimum separation of 3 m between each pair of connections. It operates at 10 Mbps; a later version, Fast Ethernet, operates at 100 Mbps; the latest version, Gigabit Ethernet, operates at 1,000 Mbps or 1 Gbps.

Manchester Encoding

The standard specifies that Ethernet frames are transmitted using Manchester Encoding, which uses the fact that hardware can detect a change in voltage more easily than a fixed value, e.g. RS-232. Technically, the hardware is edge triggered, with the changes known as rising or falling edges. The sender transmits a falling edge to encode a 0 and a rising edge to encode a 1, as illustrated in Figure 6.

Figure 6: Manchester encoding

The voltage change that encodes a bit occurs exactly half-way through the time slot. Exactly half-way through the first time slot, the voltage becomes positive (+0.85 v) to encode a 1. Similarly, exactly half-way through the second time slot, the voltage becomes negative (-0.85 v) to encode a 0. If two contiguous bits have the same value, an additional change in voltage occurs at the edge of the time slot.
Manchester Encoding uses a preamble to allow for synchronisation. The preamble consists of 64 alternating 1s and 0s sent before the frame. These produce a square wave with transitions exactly in the middle of each slot. Receiving hardware uses the preamble to synchronise with the time slots. The last two bits of the preamble are both 1s to signal the end of the preamble.

Sharing Ethernet

Ethernet requires multiple computers to share access to the ether. A sender transmits a signal, which propagates towards both ends of the cable. This is illustrated in Figure 7.

Figure 7: Ethernet sharing

The sending computer has exclusive use of the cable during the transmission of the frame. Multiple frames cannot be sent at the same time, all other computers must wait.

Carrier Sense Multiple Access/Collision Detection (CSMA/CD)

All computers attached to the Ethernet use CSMA/CD to co-ordinate their activities. A computer wishing to transmit checks for electrical activity on the cable, informally called a carrier. If there is no carrier, the computer can transmit. If a carrier is present, the computer waits for the sender to finish before proceeding.
However, it is possible for two or more computers to detect the lack of carrier and start transmission simultaneously. The signals travel at approximately 70% of the speed of light and interfere with one another. This interference is called a collision. A sending computer monitors the signal on the cable and if it differs from the signal it is sending, then a collision has occurred and the computer stops transmitting.
Following a collision, a computer waits for the cable to become idle before retransmitting. However, if the computers start transmitting as soon as the cable becomes free, another collision will occur. Ethernet requires each computer to delay after a collision. The standard specifies a maximum delay, d, and requires each computer to choose a random delay less than d. In this case, the computer choosing the shortest delay will transmit first.
If subsequent collisions still occur, the computers double the maximum delay (2d, 4d, ...) until the range is large enough for one computer to choose a short delay and transmit without a collision. This technique is called binary exponential backoff.

Wireless LANs

Instead of transmitting signals across a cable, wireless LANs use antennas to broadcast RF signals through the air. All the computers using a wireless LAN share the same radio frequency, so they must take turns to transmit. Wireless LAN transmitters use low power, so the signal only propagates a short distance. In addition, metallic obstructions can block the signal.
This lack of full communication means that wireless LANs cannot use the CSMA/CD mechanism used by Ethernet. Consider the situation illustrated in Figure 8.

Figure 8: Wireless topology

Given a maximum tranmission distance d, computer 2 will receive all transmissions, but computers 1 and 3 will not receive transmissions from each other. Wireless LANs use Carrier Sense Multiple Access/Collision Advoidance (CSMA/CA) to share the air waves. Suppose that computer 1 wants to send a frame to computer 2. It first sends a short Request To Send (RTS) control message. Computer 2 receives the RTS and responds with a short Clear To Send (CTS) control message. Now computer 1 can transmit its frame.
In Figure 8, computer 3 will not receive the RTS from computer 1, but it will receive the CTS from computer 2 - so it knows that it must wait. In the event of computers 1 and 3 both sending a control message to computer 2 at the same time, they apply random backoff before resending. Since control messages are much shorter than data frames, the probability of a second collision is much lower than with conventional Ethernet.

IBM Token Ring

Most LANs that employ ring technology use an access mechanism known as token passing. A token ring operates as a single, shared medium. When a computer wants to send data, it must wait until it obtains the token, when it is in control of the ring as illustrated in Figure 9.

Figure 9: Token ring network

All stations except the sender forward bits around the ring. The sender compares the data being received with the data being sent to ensure that no transmission errors have occurred. The receiver makes a copy of the message as it passes the bits round the ring.
Co-ordination is achieved by using a special message called a token. A computer wanting to transmit must wait for the token to arrive. It removes the token from the ring and uses the ring to transmit data. After sending one frame it retransmits the token; this token passing guarantees that stations wanting to transmit will take turns. Ensuring there is one, and only one, token on the ring is handled by the ring hardware.

Fibre Distributed Data Interconnect (FDDI) Topology

A major disadvantage of token ring networks is that the failure of a single computer or connection will disable the complete network. FDDI transmits data at 100 Mbps, uses optical fibres to interconnect computers, and uses redundancy to overcome failures. This is illustrated in Figure 10.

Figure 10: FDDI network

In (a), the outer ring is used to transfer data. In (b), a station has failed and the adjacent stations detect the disconnection and reconfigure to use the reverse path to form a closed ring. This process of reconfiguring is called self-healing and FDDI is known as a self-healing network.