Basic Network Design

The most common form of network is Ethernet. This is a bus-like network that uses Carrier-Sense Multiple Access with Collision Detection (CMSA-CD). Interpreting this we have a network where stations apply a voltage to the bus when they wish to send data, by sensng the bus for this voltage we can determine if the bus is in use; multiple access implies many hosts may be on this bus; collision detect is used to detect multiple hosts sending data at the same time. Initially, it would seem unnecessary to need collision detection, after all, a station on sends data on the bus when there is no one else sending. Due to the propagation delay of electrical signals we can have to stations decide to send data at the same time, when each station looks at the bus it is clear, however before the data they send reaches it's destination they will collide and the result will be garbage. The collision detection circuitry monitors the line to verify there were no collisions and the data does not need to be resent. Understanding the CMSA-CD concept is fundamental to understanding how ethernet works. All limitations found on the design of ethernet networks are there do to issues surrounding CMSA-CD. The biggest design limitation is that reading data on an ethernet is a passive operation, the sending stations has no way to "sense" when this has happened. However, the sending station must perform collision detection until it knows the receiving station has gotten the packet! To do, lenght restrictions must be developed so that a sending station knows that within a finite time the receiving stations must have gotten the packet. This time limit controls most aspects of network design. A basic way of calculating this time limit is to look at how long a machine must monitor the network is to look at the underlying physics. By it's definition ethernet operates at a speed of 10 Mhz (10 million bits/sec). The maximum packet size is 1500 bytes (12,000 bits). Presently ethernet has a maximum lenght of 500 meters. The time required to transmit 1500 bytes over 500 meters is: Time to transmit a packet is 12000 bits/10,000,000 bits/sec is .0012 seconds Time to transmit a bit 500 meters is defined by the speed that electrical signals travel, which is the speed of light. This figure turns out to be : 500 meters / 60000 meters/sec which equaks .0008333 seconds Other characteristics that define ethernet deal with the waveform that a ethernet signal assumes. The waveform on a thick ethernet segment is 2.5 meters in lenght, that is why stations are seperated by 2.5 meters. Ethernet HardwareEthernet has evolved over time. Ethernet version 2 released in 1982 was originally developed by Xerox-Intel-Dec. In 1985 the IEEE released a new standard for ethernet. This standard is named IEEE 802.2. In general, these two versions of ethernet can inter-operate, however there are a few minor differences. The first difference is that in the ethernet packet header Version 2 defined a two byte Type field while IEEE created a 2 byte length field in that location. Luckily, values for type cannot conflict with valid length values and most systems can determine the Ethernet Frame type by examining this field. A second difference was that the Ethernet version 2 spec required that a transciever send a heartbeat signal each second. The IEEE 802.2 spec removed this. This has resulted in most vendors offerring transcievers that have a switch to enable or disable hearbeat. It should be off unless connected to a piece of equipment using the ethernet version 2 spec. Luckily, all new devices are built to conformto the 802.2 spec; however there are occasionally devices found that were installed years ago that still need this. In either specification, ethernet uses a 48 bit identifier to uniquely identify each source and destination device.A range of addresses is assigned to each manufactuer of ethernet equipment. There are basically two categories of ethernet components, one type that passes the signal onto other devices, generally these are known as repeaters. A secondtype of device which takes the signal and regenerates the signal onto a new network, these types of devices are generally known as bridges or routers. Repeaters are useful for propagating a network signal, a signal comes in on an input port is often output to many ports.However since they add some delay to the transmittal of packets they reduce the maximum size a segment can be. However, repeaters can simplify the design of a network. Devices such as bridges and routers, which regenerate the signal, allow you to build larger networks. Since the signal is regenerated, it becomes the responsibility of the bridge or router to gaurentee the packets arrival at the destination (or the next router or bridge). Bridges and routers work at different levels of the network. Bridges work at the ethernet frame level while routers work at the protocol level. In both cases, the bridge or router, has the property of filtering traffic and only transmitting the signal onto networks where it makes sense. Thus, in each case they have the effect of reducing unnecessary traffic.