Network+ 2 Flashcards
Time-division multiplexing (TDM): TDM supports different communication sessions (for example, different telephone conversations in a telephony network) on the same physical medium by causing the sessions to take turns. For a brief period, defined as a time slot, data from the first session is sent, followed by data from the second session. This continues until all sessions have had a turn, and the process repeats itself.
Statistical time-division multiplexing (StatTDM): A downside to TDM is that each communication session receives its own time slot, even if one of the sessions does not have any data to send at the moment. To make a more efficient use of available bandwidth, StatTDM dynamically assigns time slots to communications sessions on an as-needed basis.
Frequency-division multiplexing (FDM): FDM divides a medium’s frequency range into channels, and different communication sessions send their data over different channels.
TDM
Data Link : Packaging data into frames and transmitting those frames on the network, Performing error detection/correction, Uniquely finding network devices with an address, Handling flow control.
Data Link > MAC > physical addressing, logical topology, method of transmitting on media
Data Link > LLC > connection services, synchronizing transmissions
1st 24 bits of 48-bit MAC address is vendor code assigned by IEEE, last 24 bits assigned by manufacturer and as serial number for device.
data link
LLC : Connection services: When a device on a network receives a message from another device on the network, that recipient device can give feedback to the sender in the form of an acknowledgment message.
Flow control: Limits the amount of data a sender can send at one time; this prevents the sender from overwhelming the receiver with too much information.
Error control: Allows the recipient of data to let the sender know whether the expected data frame was not received or whether it was received but is corrupted. The recipient figures out whether the data frame is corrupt by mathematically calculating a checksum of the data received. If the calculated checksum does not match the checksum received with the data frame, the recipient of the data draws the conclusion that the data frame is corrupted and can then notify the sender via an acknowledgment message.
LLC
Isochronous: With isochronous transmission, network devices look to a common device in the network as a clock source, which creates fixed-length time slots. Network devices can determine how much free space, if any, is available within a time slot and then insert data into an available time slot. A time slot can accommodate more than one data frame. Isochronous transmission does not need to provide clocking at the beginning of a data string (as does synchronous transmission) or for every data frame (as does asynchronous transmission). As a result, isochronous transmission uses little overhead when compared to asynchronous or synchronous transmission methods.
Isochronous transmission
Asynchronous: With asynchronous transmission, network devices reference their own internal clocks, and network devices do not need to synchronize their clocks. Instead, the sender places a start bit at the beginning of each data frame and a stop bit at the end of each data frame. These start and stop bits tell the receiver when to monitor the medium for the presence of bits. An additional bit, called the parity bit, might also be added to the end of each byte in a frame to detect an error in the frame. For example, if even parity error detection (as opposed to odd parity error detection) is used, the parity bit (with a value of either 0 or 1) would be added to the end of a byte, causing the total number of 1s in the data frame to be an even number. If the receiver of a byte is configured for even parity error detection and receives a byte where the total number of bits (including the parity bit) is even, the receiver can conclude that the byte was not corrupted during transmission.
asynchronous transmission
Network : logical addressing, switching, route discovery and selection, connection services, bandwidth usage, multiplexing strategy.
Logical addressing: Whereas the data link layer uses physical addresses to make forwarding decisions, the network layer uses logical addressing to make forwarding decisions. A variety of routed protocols (for example, AppleTalk and IPX) have their own logical addressing schemes, but by far, the most widely deployed routed protocol is Internet Protocol (IP),
Switching, at its essence, is making decisions about how data should be forwarded.
Network
Packet switching: With packet switching, a data stream is divided into packets. Each packet has a Layer 3 header that includes a source and destination Layer 3 address. Another term for packet switching is routing.
Circuit switching dynamically brings up a dedicated communication link between two parties for those parties to communicate.
message switching, a data stream is divided into messages. Each message is tagged with a destination address, and the messages travel from one network device to another network device on the way to their destination. Because these devices might briefly store the messages before forwarding them, a network using message switching is sometimes called a store-and-forward network.
packet switching
Connection Services at Network Layer : Flow control (also known as congestion control): Helps prevent a sender from sending data more rapidly than the receiver is capable of receiving it. Packet reordering: Allows packets to be placed in the proper sequence as they are sent to the receiver. This might be necessary because some networks support load balancing, where multiple links are used to send packets between two devices. Because multiple links exist, packets might arrive out of order.
*** With buffering, a device (for example, a router) uses a chunk of memory (sometimes called a buffer or a queue) to store segments if bandwidth is not available to send those segments.
Network Layer Connection Services
Transport Layer > TCP/UDP, Windowing, Buffering (messages are taken from upper layers (Layers 5–7) and are encapsulated into segments for transmission to the lower layers (Layers 1–3). Similarly, data streams coming from lower layers are de-encapsulated and sent to Layer 5 (the session layer), or some other upper layer, depending on the protocol).
Transport Layer
TCP communication uses windowing, in that one or more segments are sent at one time, and a receiver can attest to the receipt of all the segments in a window with a single acknowledgment. TCP uses a sliding window, where the window size begins with one segment. If there is a successful acknowledgment of that one segment (that is, the receiver sends an acknowledgment asking for the next segment), the window size doubles to two segments. Upon successful receipt of those two segments, the next window holds four segments. This exponential increase in window size continues until the receiver does not acknowledge successful receipt of all segments within a certain amount of time—known as the round-trip time (RTT), which is sometimes called real transfer time—or until a configured maximum window size is reached.
TCP
