A network can be understood to be the capability of two or more entities to communicate over a given medium. The development of any network relies on this same principle for establishing communication. Commonly the entities within a network that are responsible for the transmission and reception of communication are known as end stations, while the means by which communication is enabled is understood to be the medium. Within an enterprise network, the medium exists in a variety of forms from a physical cable to radio waves.
The coaxial cable represents a more historic form of transmission medium that may today be limited in usage within the enterprise network. As a transmission medium, the coaxial cable comprises generally of two standards, the 10Base2 and 10Base5 forms, that are known as Thinnet or Thinwire, and Thicknet or Thickwire respectively.
The standards both support a transmission capacity of 10Mbps transmitted as baseband signals for respective distances of 185 and 500 meters. In today’s enterprise networks, the transmission capacity is extremely limited to be of any significant application. The Bayonet Neill-Concelman (BNC) connector is the common form of connector used for thin 10Base2 coaxial cables, while a type N connector was applied to the thicker 10Base5 transmission medium.
Ethernet cabling has become the standard for many enterprise networks providing a transmission medium that supports a much higher transmission capacity. The medium supports a four copper wire pair contained within a sheath which may or may not be shielded against external electrical interference. The transmission capacity is determined mainly based on the category of cable with category 5 (CAT5) supporting Fast Ethernet transmission capacity of up to 100Mbps, while a higher Gigabit Ethernet transmission capacity is supported from Category 5 extended (CAT5e) standards and higher.
The transmission over Ethernet as a physical medium is also susceptible to attenuation, causing the transmission range to be limited to 100 meters. The RJ-45 connector is used to provide connectivity with wire pair cabling requiring specific pin ordering within the RJ-45 connector, to ensure correct transmission and reception by end stations over the transmission medium.
Optical media uses light as a means of signal transmission as opposed to electrical signals found within both Ethernet and coaxial media types. The optical fiber medium supports a range of standards of 10Mbps, 100Mbps, 1Gbps and also 10Gbps (10GBASE) transmission. Single or multi-mode fiber defines the use of an optical transmission medium for propagating of light, where single mode refers to a single mode of optical transmission being propagated, and is used commonly for high speed transmission over long distances.
Multi mode supports propagation of multiple modes of optical transmission that are susceptible to attenuation as a result of dispersion of light along the optical medium, and therefore is not capable of supporting transmission over longer distances. This mode is often applied to local area networks which encompass a much smaller transmission range. There are an extensive number of fiber connector standards with some of the more common forms being recognized as the ST connector, LC connector and SC, or snap connector.
Serial represents a standard initially developed over 50 years ago to support reliable transmission between devices, during which time many evolutions of the standard have taken place. The serial connection is designed to support the transmission of data as a serial stream of bits. The common standard implemented is referred to as (Recommended Standard) RS-232 but it is limited somewhat by both distance and speed. Original RS-232 standards define that communication speeds supported be no greater that 20Kbps, based on a cable length of 50ft (15 meters), however transmission speeds for serial is unlikely to be lower than 115 Kbps. The general behavior for serial means that as the length of the cable increases, the supported bit rate will decrease, with an approximation that a cable of around 150 meters, or 10 times the original standards, the supported bit rate will be halved.
Other serial standards have the capability to achieve much greater transmission ranges, such as is the case with the RS-422 and RS-485 standards that span distances of up to 4900ft (1200 meters) and are often supported by V.35 connectors that were made obsolete during the late 1980’s but are still often found and maintained today in support of technologies such as Frame Relay and ATM, where implemented. RS-232 itself does not define connector standards, however two common forms of connector that support the RS-232 standard include the DB-9 and DB-25 connectors. Newer serial standards have been developed to replace much of the existing RS-232 serial technology, including both FireWire and the universal serial bus (USB) standards, that latter of which is becoming common place in many newer products and devices.
In order to enable communication over physical links, signals must be transmitted between the transmitting and receiving stations. This signal will vary depending on the medium that is being used, as in the case of optical and wireless transmission. The main purpose of the signal is to ensure that synchronization (or clocking) between the sender and receiver over a physical medium is maintained, as well as support transmission of the data signal in a form that can be interpreted by both the sender and receiver.
A waveform is commonly recognized as a property of line encoding where the voltage is translated into a binary representation of 0 and 1 values that can be translated by the receiving station. Various line coding standards exist, with 10Base Ethernet standards supporting a line encoding standard known as Manchester encoding. Fast Ethernet with a frequency range of 100MHz invokes a higher frequency than can be supported when using Manchester encoding.
An alternative form of line encoding is therefore used known as NZRI, which in itself contains variations dependant on the physical media, thus supporting MLT-3 for 100Base-TX and 100Base-FX together with extended line encoding known as 4B/5B encoding to deal with potential clocking issues. 100Base-T4 for example uses another form known as 8B/6T extended line encoding. Gigabit Ethernet supports 8B/10B line encoding with the exception of 1000Base-T which relies on a complex block encoding referred to as 4D-PAM5
Ethernet represents what is understood to be a multi-access network, in which two or more end stations share a common transmission medium for the forwarding of data. The shared network is however susceptible to transmission collisions where data is forwarded by end stations simultaneously over a common medium. A segment where such occurrences are possible is referred to as a shared collision domain.
End stations within such a collision domain rely on contention for the transmission of data to an intended destination. This contentious behavior requires each end station monitor for incoming data on the segment before making any attempt to transmit, in a process referred to as Carrier Sense Multiple-Access Collision Detection (CSMA/CD). However, even after taking such precautions the potential for the occurrence of collisions as a result of simultaneous transmission by two end stations remains highly probable.
Transmission modes are defined in the form of half and full duplex, to determine the behavior involved with the transmission of data over the physical medium.
Half duplex refers to the communication of two or more devices over a shared physical medium in which a collision domain exists, and with it CSMA/CD is required to detect for such collisions. This begins with the station listening for reception of traffic on its own interface, and where it is quiet for a given period, will proceed to transmit its data. If a collision were to occur, transmission would cease, followed by initiation of a backoff algorithm to prevent further transmissions until a random value timer expires, following which retransmission can be reattempted.
Full duplex defines the simultaneous bidirectional communication over dedicated point-to-point wire pairs, ensuring that there is no potential for collisions to occur, and thus there is no requirement for CSMA/CD.
Summary
Gigabit Ethernet transmission is supported by CAT 5e cabling and higher, and also any form of 1000Base Fiber Optic cabling or greater.
A collision domain is a network segment for which the same physical medium is used for bi-directional communication. Data simultaneously transmitted between hosts on the same shared network medium is susceptible to a collision of signals before those signals reach the intended destination. This generally results in malformed signals either larger or smaller than the acceptable size for transmission (64 bytes – 1500 bytes), also know as runts and giants, being received by the recipient.
CSMA/CD is a mechanism for detecting and minimizing the possibility of collision events that are likely to occur in a shared network. CSMA requires that the transmitting host first listen for signals on the shared medium prior to transmission. In the event that no transmissions are detected, transmission can proceed. In the unfortunate circumstance that signals are transmitted simultaneously and a collision occurs, collision detection processes are applied to cease transmission for a locally generated period of time, to allow collision events to clear and to avoid further collisions from occurring between transmitting hosts.
Ref : [1]