A collision domain is also referred to as a network segment. Hubs and repeaters therefore have the effect of increasing the size of the collision domain. As shown in the figure, the interconnection of hubs form a physical topology called an extended star.
The extended star can create a greatly expanded collision domain. An increased number of collisions reduces the network's efficiency and effectiveness until the collisions become a nuisance to the user. Therefore, other mechanisms are required when large numbers of users require access and when more active network access is needed. We will see that using switches in place of hubs can begin to alleviate this problem. Ethernet Timing Faster Physical layer implementations of Ethernet introduce complexities to the management of collisions.
As discussed, each device that wants to transmit must first "listen" to the media to check for traffic. If no traffic exists, the station will begin to transmit immediately. The electrical signal that is transmitted takes a certain amount of time latency to propagate travel down the cable. Each hub or repeater in the signal's path adds latency as it forwards the bits from one port to the next.
This accumulated delay increases the likelihood that collisions will occur because a listening node may transition into transmitting signals while the hub or repeater is processing the message. Because the signal had not reached this node while it was listening, it thought that the media was available. This condition often results in collisions. In half-duplex mode, if a collision has not occurred, the sending device will transmit 64 bits of timing synchronization information, which is known as the Preamble.
The sending device will then transmit the complete frame. Ethernet with throughput speeds of 10 Mbps and slower are asynchronous. An asynchronous communication in this context means that each receiving device will use the 8 bytes of timing information to synchronize the receive circuit to the incoming data and then discard the 8 bytes. Ethernet implementations with throughput of Mbps and higher are synchronous. Synchronous communication in this context means that the timing information is not required.
For each different media speed, a period of time is required for a bit to be placed and sensed on the media. This period of time is referred to as the bit time. At Mbps, that same bit requires 10 nS to transmit. And at Mbps, it only takes 1 nS to transmit a bit. As a rough estimate, At Mbps, the device timing is barely able to accommodate meter cables. At Mbps, special adjustments are required because nearly an entire minimum-sized frame would be transmitted before the first bit reached the end of the first meters of UTP cable.
For this reason, half-duplex mode is not permitted in Gigabit Ethernet. These timing considerations have to be applied to the interframe spacing and backoff times both of which are discussed in the next section to ensure that when a device transmits its next frame, the risk of a collision is minimized.
In half-duplex Ethernet, where data can only travel in one direction at once, slot time becomes an important parameter in determining how many devices can share a network. For all speeds of Ethernet transmission at or below Mbps, the standard describes how an individual transmission may be no smaller than the slot time. Determining slot time is a trade-off between the need to reduce the impact of collision recovery backoff and retransmission times and the need for network distances to be large enough to accommodate reasonable network sizes.
The compromise was to choose a maximum network diameter about meters and then to set the minimum frame length long enough to ensure detection of all worst-case collisions. Slot time for and Mbps Ethernet is bit times, or 64 octets. Slot time for Mbps Ethernet is bit times, or octets. The slot time ensures that if a collision is going to occur, it will be detected within the first bits for Gigabit Ethernet of the frame transmission.
This simplifies the handling of frame retransmissions following a collision. Slot time is an important parameter for the following reasons:. The bit slot time establishes the minimum size of an Ethernet frame as 64 bytes. Any frame less than 64 bytes in length is considered a "collision fragment" or "runt frame" and is automatically discarded by receiving stations. Slot time is calculated assuming maximum cable lengths on the largest legal network architecture.
All hardware propagation delay times are at the legal maximum and the bit jam signal is used when collisions are detected. The actual calculated slot time is just longer than the theoretical amount of time required to travel between the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected.
See the figure. For the system to work properly, the first device must learn about the collision before it finishes sending the smallest legal frame size.
To allow Mbps Ethernet to operate in half-duplex mode, the extension field was added to the frame when sending small frames purely to keep the transmitter busy long enough for a collision fragment to return.
This field is present only on Mbps, half-duplex links and allows minimum-sized frames to be long enough to meet slot time requirements. Extension bits are discarded by the receiving device. The Ethernet standards require a minimum spacing between two non-colliding frames. This gives the media time to stabilize after the transmission of the previous frame and time for the devices to process the frame. Referred to as the interframe spacing , this time is measured from the last bit of the FCS field of one frame to the first bit of the Preamble of the next frame.
After a frame has been sent, all devices on a 10 Mbps Ethernet network are required to wait a minimum of 96 bit times 9. On faster versions of Ethernet, the spacing remains the same - 96 bit times - but the interframe spacing time period grows correspondingly shorter. Synchronization delays between devices may result in the loss of some of frame preamble bits. This in turn may cause minor reduction of the interframe spacing when hubs and repeaters regenerate the full 64 bits of timing information the Preamble and SFD at the start of every frame forwarded.
On higher speed Ethernet some time sensitive devices could potentially fail to recognize individual frames resulting in communication failure. As you will recall, Ethernet allows all devices to compete for transmitting time.
But remember, when a larger number of devices are added to the network, it is possible for the collisions to become increasingly difficult to resolve. As soon as a collision is detected, the sending devices transmit a bit "jam" signal that will enforce the collision. This ensures all devices in the LAN to detect the collision. It is important that the jam signal not be detected as a valid frame; otherwise the collision would not be identified. The most commonly observed data pattern for a jam signal is simply a repeating 1, 0, 1, 0 pattern, the same as the Preamble.
The corrupted, partially transmitted messages are often referred to as collision fragments or runts. Normal collisions are less than 64 octets in length and therefore fail both the minimum length and the FCS tests, making them easy to identify. After a collision occurs and all devices allow the cable to become idle each waits the full interframe spacing , the devices whose transmissions collided must wait an additional - and potentially progressively longer - period of time before attempting to retransmit the collided frame.
The waiting period is intentionally designed to be random so that two stations do not delay for the same amount of time before retransmitting, which would result in more collisions. This is accomplished in part by expanding the interval from which the random retransmission time is selected on each retransmission attempt. The waiting period is measured in increments of the parameter slot time.
If media congestion results in the MAC layer unable to send the frame after 16 attempts, it gives up and generates an error to the Network layer. Such an occurrence is rare in a properly operating network and would happen only under extremely heavy network loads or when a physical problem exists on the network. The methods described in this section allowed Ethernet to provide greater service in a shared media topology based on the use of hubs.
Ethernet is covered by the IEEE Four data rates are currently defined for operation over optical fiber and twisted-pair cables:. While there are many different implementations of Ethernet at these various data rates, only the more common ones will be presented here. The figure shows some of the Ethernet PHY characteristics.
The portion of Ethernet that operates on the Physical layer will be discussed in this section, beginning with 10Base-T and continuing to 10 Gbps varieties. These implementations are no longer used and are not supported by the newer However, Cat5 or later cabling is typically used today.
The pair connected to pins 1 and 2 are used for transmitting and the pair connected to pins 3 and 6 are used for receiving. The replacement of hubs with switches in 10BASE-T networks has greatly increased the throughput available to these networks and has given Legacy Ethernet greater longevity.
In the mid to late s, several new These standards used different encoding requirements for achieving these higher data rates. The most popular implementations of Mbps Ethernet are:.
Because the higher frequency signals used in Fast Ethernet are more susceptible to noise, two separate encoding steps are used by Mbps Ethernet to enhance signal integrity. The figure shows an example of a physical star topology. Although the encoding, decoding, and clock recovery procedures are the same for both media, the signal transmission is different - electrical pulses in copper and light pulses in optical fiber.
Fiber implementations are point-to-point connections, that is, they are used to interconnect two devices. These connections may be between two computers, between a computer and a switch, or between two switches.
The development of Gigabit Ethernet standards resulted in specifications for UTP copper, single-mode fiber, and multimode fiber. On Gigabit Ethernet networks, bits occur in a fraction of the time that they take on Mbps networks and 10 Mbps networks.
With signals occurring in less time, the bits become more susceptible to noise, and therefore timing is critical. The question of performance is based on how fast the network adapter or interface can change voltage levels and how well that voltage change can be detected reliably meters away, at the receiving NIC or interface. At these higher speeds, encoding and decoding data is more complex. Gigabit Ethernet uses two separate encoding steps. Data transmission is more efficient when codes are used to represent the binary bit stream.
Encoding the data enables synchronization, efficient usage of bandwidth, and improved signal-to-noise ratio characteristics. Gigabit Ethernet over copper wire enables an increase from Mbps per wire pair to Mbps per wire pair, or Mbps for the four pairs. Each wire pair signals in full duplex, doubling the Mbps to Mbps. This encoding scheme enables the transmission signals over four wire pairs simultaneously. It translates an 8-bit byte of data into a simultaneous transmission of four code symbols 4D , which are sent over the media, one on each pair, as 5-level Pulse Amplitude Modulated PAM5 signals.
This means that every symbol corresponds to two bits of data. Because the information travels simultaneously across the four paths, the circuitry has to divide frames at the transmitter and reassemble them at the receiver.
This traffic flow creates permanent collisions on the wire pairs. These collisions result in complex voltage patterns. The hybrid circuits detecting the signals use sophisticated techniques such as echo cancellation, Layer 1 Forward Error Correction FEC , and prudent selection of voltage levels.
Using these techniques, the system achieves the 1-Gigabit throughput. To help with synchronization, the Physical layer encapsulates each frame with start-of-stream and end-of-stream delimiters. Loop timing is maintained by continuous streams of IDLE symbols sent on each wire pair during the interframe spacing. Unlike most digital signals where there are usually a couple of discrete voltage levels, BASE-T uses many voltage levels.
In idle periods, nine voltage levels are found on the cable. During data transmission periods, up to 17 voltage levels are found on the cable.
With this large number of states, combined with the effects of noise, the signal on the wire looks more analog than digital. Like analog, the system is more susceptible to noise due to cable and termination problems. Because of the overhead of this encoding, the data transfer rate is still Mbps. Each data frame is encapsulated at the Physical layer before transmission, and link synchronization is maintained by sending a continuous stream of IDLE code groups during the interframe spacing.
These differences are shown in the figure. The Because the frame format and other Ethernet Layer 2 specifications are compatible with previous standards, 10GbE can provide increased bandwidth to individual networks that is interoperable with the existing network infrastructure.
Frame format is the same, allowing interoperability between all varieties of legacy, fast, gigabit, and 10 gigabit Ethernet, with no reframing or protocol conversions necessary. With 10Gbps Ethernet, flexible, efficient, reliable, relatively low cost end-to-end Ethernet networks become possible. Although 1-Gigabit Ethernet is now widely available and Gigabit products are becoming more available, the IEEE and the Gigabit Ethernet Alliance are working on , , or even Gbps standards.
The technologies that are adopted will depend on a number of factors, including the rate of maturation of the technologies and standards, the rate of adoption in the market, and the cost of emerging products.
Legacy Ethernet - Using Hubs In previous sections, we have seen how classic Ethernet uses shared media and contention-based media access control. Classic Ethernet uses hubs to interconnect nodes on the LAN segment. Hubs do not perform any type of traffic filtering. Instead, the hub forwards all the bits to every device connected to the hub. This forces all the devices in the LAN to share the bandwidth of the media.
Additionally, this classic Ethernet implementation often results in high levels of collisions on the LAN. Because of these performance issues, this type of Ethernet LAN has limited use in today's networks. Ethernet implementations using hubs are now typically used only in small LANs or in LANs with low bandwidth requirements.
Sharing media among devices creates significant issues as the network grows. The figure illustrates some of the issues presented here. In a hub network, there is a limit to the amount of bandwidth that devices can share. With each device added to the shared media, the average bandwidth available to each device decreases.
With each increase in the number of devices on the media, performance is degraded. Network latency is the amount of time it takes a signal to reach all destinations on the media. Each node in a hub-based network has to wait for an opportunity to transmit in order to avoid collisions. Latency can increase significantly as the distance between nodes is extended. Latency is also affected by a delay of the signal across the media as well as the delay added by the processing of the signals through hubs and repeaters.
Increasing the length of media or the number of hubs and repeaters connected to a segment results in increased latency. With greater latency, it is more likely that nodes will not receive initial signals, thereby increasing the collisions present in the network. Because classic Ethernet shares the media, any device in the network could potentially cause problems for other devices. If any device connected to the hub generates detrimental traffic, the communication for all devices on the media could be impeded.
This harmful traffic could be due to incorrect speed or full-duplex settings on a NIC. If two nodes send packets at the same time, a collision occurs and the packets are lost. Then both nodes send a jam signal, wait for a random amount of time, and retransmit their packets. Any part of the network where packets from two or more nodes can interfere with each other is considered a collision domain. A network with a larger number of nodes on the same segment has a larger collision domain and typically has more traffic.
As the amount of traffic in the network increases, the likelihood of collisions increases. Switches provide an alternative to the contention-based environment of classic Ethernet. Ethernet - Using Switches In the last few years, switches have quickly become a fundamental part of most networks. Switches allow the segmentation of the LAN into separate collision domains.
Each port of the switch represents a separate collision domain and provides the full media bandwidth to the node or nodes connected on that port. With fewer nodes in each collision domain, there is an increase in the average bandwidth available to each node, and collisions are reduced. A LAN may have a centralized switch connecting to hubs that still provide the connectivity to nodes. Or, a LAN may have all nodes connected directly to a switch.
Theses topologies are shown in the figure. In a LAN where a hub is connected to a switch port, there is still shared bandwidth, which may result in collisions within the shared environment of the hub.
However, the switch will isolate the segment and limit collisions to traffic between the hub's ports. Nodes are Connected Directly. In a LAN where all nodes are connected directly to the switch, the throughput of the network increases dramatically. The three primary reasons for this increase are:. These physical star topologies are essentially point to point links. Each node has the full media bandwidth available in the connection between the node and the switch.
Because a hub replicates the signals it receives and sends them to all other ports, classic Ethernet hubs form a logical bus. This means that all the nodes have to share the same bandwidth of this bus. With switches, each device effectively has a dedicated point-to-point connection between the device and the switch, without media contention.
As an example, compare two Mbps LANs, each with 10 nodes. In network segment A, the 10 nodes are connected to a hub. Each node shares the available Mbps bandwidth. This provides an average of 10 Mbps to each node. In network segment B, the 10 nodes are connected to a switch. In this segment, all 10 nodes have the full Mbps bandwidth available to them.
Even in this small network example, the increase in bandwidth is significant. As the number of nodes increases, the discrepancy between the available bandwidth in the two implementations increases significantly. Collision-Free Environment. A dedicated point-to-point connection to a switch also removes any media contention between devices, allowing a node to operate with few or no collisions.
In a switched Ethernet network - where there are virtually no collisions - the overhead devoted to collision recovery is virtually eliminated. This provides the switched network with significantly better throughput rates.
Switching also allows a network to operate as a full-duplex Ethernet environment. Before switching existed, Ethernet was half-duplex only. This meant that at any given time, a node could either transmit or receive. With full-duplex enabled in a switched Ethernet network, the devices connected directly to the switch ports can transmit and receive simultaneously, at the full media bandwidth. The connection between the device and the switch is collision-free.
This arrangement effectively doubles the transmission rate when compared to half-duplex. For example, if the speed of the network is Mbps, each node can transmit a frame at Mbps and, at the same time, receive a frame at Mbps.
Using Switches Instead of Hubs. Most modern Ethernet use switches to the end devices and operate full duplex. Because switches provide so much greater throughput than hubs and increase performance so dramatically, it is fair to ask: why not use switches in every Ethernet LAN?
There are three reasons why hubs are still being used:. Availability - LAN switches were not developed until the early s and were not readily available until the mid s. Early Ethernet networks used UTP hubs and many of them remain in operation to this day. The next section explores the basic operation of switches and how a switch achieves the enhanced performance upon which our networks now depend.
A later course will present more details and additional technologies related to switching. The following image describes the data flow from one application to the other.
According to the above image, the application data is being passed downwards through all layers of the protocol stack, i. Each layer embeds the data it receives into its specific frames, a process called encapsulation. After the data has been transmitted through the physical layer, it will be passed upwards through all layers up to the application process. Each layer will remove its specific frames to comply with the requirements of the upper layer, and then pass the remainder of the data to that layer.
Naturally, this process requires logical interactions within each layer to complete the network connection. The disadvantage of this design is, however, a disproportionately high overhead for communication with devices that have to exchange small quantities of data frequently.
Note: Similar to other networking technologies e. Fully updated for the newest innovations, it demonstrates each protocol in action through realistic examples from modern Linux, Windows, and Mac OS environments.
Building on the late W. More Information All prices are in USD. Sitemap Powered by BigCommerce. Site Information. When the data packets are sent over a network, they may or may not take the same route -- it doesn't matter. At the receiving end, the data packets are re-assembled into the proper order. After all packets are received, a message goes back to the originating network. If a packet does not arrive, a message to "re-send" is sent back to the originating network.
TCP, paired with IP, is by far the most popular protocol at the transport level. Several protocols overlap the session, presentation, and application layers of networks. There protocols listed below are a few of the more well-known:. Florida Center for Instructional Technology.
College of Education ,. University of South Florida ,. This publication was produced under a grant from the Florida Department of Education. The information contained in this document is based on information available at the time of publication and is subject to change.
Although every reasonable effort has been made to include accurate information, the Florida Center for Instructional Technology makes no warranty of claims as to the accuracy, completeness, or fitness for any particular purpose of the information provided herein.
Nothing herein shall be construed as a recommendation to use any product or service in violation of existing patents or rights of third parties. What is a Protocol? OSI model related to common network protocols Figure 1 illustrates how some of the major protocols would correlate to the OSI model in order to communicate via the Internet. Ethernet The original Ethernet standard was developed in and had a maximum speed of 10 Mbps phenomenal at the time over coaxial cable.
Gigabit Ethernet Gigabit Ethernet standard is a protocol that has a transmission speed of 1 Gbps Mbps. TCP and SPX Transport Layer The transport layer is concerned with efficient and reliable transportation of the data packets from one network to another. Fowler Ave.
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