Whether a system uses a System V or a BSD based kernel all versions of Unix now ship with 4.3 BSD networking software. Since this software was developed at Berkeley, under a US government grant, it has been available to any vendor or university at minimal cost. The TCP/IP code developed at Berkely has been ported to other operating systems, such as the DEC VMS , Macintsh, DOS, Windows, IBM CMS, IBM MVS, andmany other systems.Due to the ubiquity in the platforms where TCP/IP is available it has become the primary means for interconnecting systems in a heterogeneous computing environment.
Unix has been the platform for TCP/IP development. While Berkeley has been the main contributor countless others have contributed to the effort. This work has produced a system for networking that has proven itself over the years. Presently, there are estimated to be over 5 million systems running the TCP/IP software suite, the overwhelming majority are microcomputers. Unix has evolved as the platform to use for integrating these many different systems into something useful. As a system administrator on a Unix system it is very likely you will be involved in networking issues and need to have a basic understanding of things work.
Many vendors have provided other network on Unix systems other than (or in addition too) TCP/IP. DEC has offered a version of itŐs DECNET software for systems running itŐs version of Unix, named Ultrix. IBM also offers a version of their propreitary SNA network software on IBM AIX machines. However, the emphasis in this course will be on the TCP/IP
All modern networks are now designed using a layered approach. Each layer presents a predefined interface to the layer above it. By doing so, a modular design can be developed so as to minimize problems in the development of new applications or in adding new interfaces.
The ISO/OSI protocol with seven layers is the usual reference model. SInce TCP/IP was designed before the ISO model was developed it has four layers; however the differences between the two are mostly minor. Below, is a comparison of the TCP/IP and OSI protocol stacks:
OSI Protocol Stack 7. Application -- End user services such as email. 6. Presentation -- Data problems and data compression 5. Session -- Authenication and authorization 4. Transport -- Gaurentee end-to-end delivery of packets 3. Network -- Packet routing 2. Data Link -- Transmit and receive packets 1. Physical -- The cable or physical connection itself. TCP/IP Protocol Stack. 5. Application -- Authenication, compression, and end user services. 4. Transport -- Handles the flow of data between systems and provides access to the network for applications via the (BSD socket library) 3. Network -- Packet routing 2. Link -- Kernel OS/device driver interface to the network interface on the computer.Below are the major difference between the OSI and TCP/IP:
Over the next few months we will be examining these components as we work our way up from the bottom. First, we need to get a basic upderstanding of how networks are designed and how the basic hardware used to interconnect them.
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
Ethernet 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
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.
Each media type has different signal properties and limits. For example, (10BASE-T) only supports one machine per segment and is limited in distance to 100 meters. Thinwire (10BASE-2) can support up to 29 stations and is limited to a maximum distance of 185 meters. Fiber optic cabling can support 1024 devices and can operate at distances up to 2 Kilometers. Thickcoaxial cable (10BASE-5) can operate up to 500 meters and support up to 1024 stations.
Trancievers often allow you to attach dis-similar devices togethor. Many machines have a 15 pin Ethernet AUI interface. Tranceivers exist which allow you to adapt the AUI interface to whatever media you have running to the desktop.