Info about LAN Ethernet Transmission Media

Early implementations of Ethernet LANs employed thick coaxial cable. In fact, it was a thick yellow coaxial cable – original recipe Ethernet cable. The cable was defined by the 10Base-5 standard. This implementation was called Thicknet.

It could deliver a BER of 10-8. It supported a data rate of 10 MBPS. The maximum LAN cable segment length was 500 meters. The segment length is the maximum distance between data terminal equipment. These are attractive features.

The unfortunately, the thick coaxial cable is difficult to work with. As a result, the second wave of the implementation of the Ethernet LAN using thin coaxial cable. The cable was RG58 A/U coaxial cable - sometimes called Cheapernet. This cable is made from 10Base-2 standard. The implementation was called Thinnet. It supported a data rate of 10 MBPS. But, it had a BER somewhat degraded relative to Thicknet. The LAN cable segment length was reduced to the order of 185 meters.

Thinnet ultimately gave way to the replacement of coaxial cable with unshielded twisted pair (UTP) cable. This is done through an interesting Ethernet LAN architecture combined with another local area network (LAN) flavor called StarLAN, the AT &T.

10M/100M/1000M Gigabit Media Converter

StarLAN was based on a telecommunications, Telephone Company; usually do for the enterprise to provide voice communications. The Transmission Medium a Telecom used in a facility for voice communications are shielded twisted pair (STP) cable. It provides voice communication within a facility and the external world connect all telephone, mobile phone, closet, or by telephone wiring closet. The distance from handset to telephone closet is relatively limited, maybe 250 meters.

The StarLAN idea is the basic method of voice that uses it to connect a local area network (LAN). The LAN stations would be connected through a closet. The existing UTP cable present in a facility for voice would be used for the LAN data traffic. There would be no need to install a new and separate transmission medium. Installation costs would be contained. Unfortunately, StarLAN only supported 1 MBPS. It has never left the ground.

However, in 1990 aspects of StarLAN were taken and merged with the Ethernet LAN architecture. This leads to a new Ethernet LAN based on UTP and 10Based-T standard definition. Based on this method, Ethernet UTP really starts the market place.

Ethernet under the 10Base-T standard has a hub and spoke architecture. This is illustrated in Figure 1. The various data equipment units, radio, are connected to a central point called multi-point repeater or Hub. The connections are by UTP cable. This architecture does support the Broadcast Channel – Ethernet Bus. This is because all of the data equipment unit can be broadcast to all the other data units through the Hub. Likewise, all data equipment units can listen to the transmissions from all other data equipment units as they are received via the UTP cable connection to the Hub. The Hub takes the place of the telephone closet. The Hub may be strictly passive or it may perform signal restoration functions.

Figure 1: 10Base-T hub-and-spoke architecture

The illustration Figure 2 indicates how the 10Base-T topology may actually look in an office set-up at some facility. The data units are computer equipment here. One serves as the file server. The illustration shows what is usually referred to as a 10Base-T Work Group. It may serve one specific department in a company. By connecting together these work groups Ethernet LANs may be extended. This can be achieved by using local area network (LAN) connection Hub network elements called bridges, routers and switches. Description of their operations is beyond the focus of the present discussion.

Figure 2:  Ethernet operating as a 10Base-T work group

But, let us get back to 10Base-T. It supports a data rate of 10 MBPS. It has a BER comparable to Thinnet. However, the LAN segment length is reduced even further. With 10Base-T LAN segment length is only 100m – a short distance, but distance, it is permissible to many data equipment standing in a typical business. However, it may be too short to others.

This is a place which fiber optic cable can come to the rescue. For the LAN market place 10Base-T was far from the last word. It led to the development of 100Base-T – Fast Ethernet. It is also based on using UTP cable for transmission medium. However, it supports a data rate of 100 MBPS over cable segments of 100 meters. Fast Ethernet, itself, is not the end of the road. Suppliers are starting to promote Giga Bit Ethernet which is capable of supporting 1 GBPS.

However, we will stop at Fast Ethernet and the problem that both it and 10Base-T have the short cable segment of 100 meters. It will be worth defined two terms before continuing in discussing the characteristics of the Ethernet. These are 1) the network diameter, and 2) slot time. The Network Diameter is simply the maximum end-to-end distance between data equipment users, stations, in an Ethernet network.

It is really what has been referred to above as the cable segment. The Network Diameter is the same for both 10Base-T and 100Base-T, 100 meters. After a BIU has begun the transmission of a packet the Slot Time is the time interval that a BIU listens for the presence of a collision with an interfering packet. The Slot Time cannot be infinite. It is set for both the 10Base-T and 100Base-T Ethernet architectures. It is defined for both standards as the time duration of 512 bits. With a 10Base-T Ethernet network operating at 10 MBPS the Slot Time translates to 51.2msec. With a 100Base-T Ethernet network operating at 100 MBPS the Slot Time translates to 5.12msec.

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