EECS340: Link Layer
carrier sensing, it detects no transmissions for a short amount of time, collision detection, stops transmitting and waits a random amount of time, carrier sense multiple access (CSMA), CSMA with collision detection (CSMA/CD)
2 important rules for polite human conversation: 1) Listen before speaking: Wait until someone finishes speaking before starting. In networking, this is called _____: a node listens to the channel before transmitting. If a frame from another node is currently being transmitted into the channel, a node then waits until _____ and then begins transmission. 2) If someone else starts talking at the same time, stop talking. In networking, this is called ______: a transmitting node listens to the channel while it is transmitting. If it detects that another node is transmitting an interfering frame, it ________ before repeating the sense-and-transmit-when-idle cycle. These two rules are embodied in the protocols: ____ and ____
parity value, error, correct, forward error correction (FEC), audio CDs, by themselves, link-layer ARQ, decrease the number of sender retransmissions required, immediate correction, round-trip propagation delay
2D PARITY SCHEME Suppose that a single bit error occurs in the original d bits of information. The d bits in D are divided into i rows and j columns. A ____ is computed for each row and for each column. The resulting i + j + 1 parity bits comprise the link-layer frame's error-detection bits. The parity of both the column and the row containing the flipped bit will have an ____. RECEIVER: Can now not only detect an error, but also to identify the corrupt bit and sometimes ___ the error at the receiver The ability of the receiver to both detect and correct errors is known as ______. These techniques are commonly used in audio storage and playback devices such as ____. In a network setting, FEC techniques can be used ____, or in conjunction with _____ techniques FEC techniques are valuable because they can ____. Perhaps more important, they allow for _____ of errors at the receiver. This avoids having to wait for the _____ needed for the sender to receive a NAK packet and for the retransmitted packet to propagate back to the receiver—a potentially important advantage for real-time network applications or links with long propagation delays
link-layer, LAN, physical, 6 bytes, hexadecimal, byte, permanent, no two adapters have the same address, flat, adapter, network, host, moves, MAC, LAN, broadcasts, isn't addressed, destination, own, extracts the enclosed datagram, passes the datagram up the protocol stack, discards the frame, a string of 48 consecutive 1s (that is, FF-FF-FF-FF-FF- FF in hexadecimal notation).
A MAC address is also called a ___ address, a ____ address, or a ____ address. For most LANs, the MAC address is ____ long, giving 248 possible MAC addresses, and are expressed in ____ notation, with each____ of the address expressed as a pair of hexadecimal numbers. Although MAC addresses were designed to be_____, it is now possible to change an adapter's MAC address via software. One interesting property of MAC addresses is that ______, and this is made possible because IEEE manages the MAC address space. An adapter's MAC address has a_____ structure (as opposed to a hierarchical structure) and doesn't change no matter where the____ goes. In contrast, IP addresses have a hierarchical structure (that is, a____ part and a ____ part), and a host's IP addresses needs to be changed when the host_____, i.e, changes the network to which it is attached. When an adapter wants to send a frame to some destination adapter, the sending adapter inserts the destination adapter's ____ address into the frame and then sends the frame into the ___. A switch occasionally ____ an incoming frame onto all of its interfaces. Thus, an adapter may receive a frame that ____ to it. Thus, when an adapter receives a frame, it will check to see whether the _____ MAC address in the frame matches its_____ MAC address. If there is a match, the adapter _____ and ____. If there isn't a match, the adapter ______, without passing the network-layer datagram up. Thus, the destination only will be interrupted when the frame is received. However, sometimes a sending adapter does want all the other adapters on the LAN to receive and process the frame it is about to send. In this case, the sending adapter inserts A SPECIAL MAC BROADCAST ADDRESS into the destination address field of the frame. For LANs that use 6-byte addresses (such as Ethernet and 802.11), the broadcast address is ______
point-to-point link, point-to-point pro- tocol (PPP), high-level data link control (HDLC)
A _______ consists of a single sender at one end of the link and a single receiver at the other end of the link. Many link-layer protocols have been designed for this type of link, such as: 1) ____ 2) _____
self-learning, MAC address, interface from which the frame arrived, current time, aging, plug-and-play,
A switch is ___, which means its switch table is built automatically, dynamically, and autonomously—without any intervention from a network administrator or from a configuration protocol. 1. The switch table is initially empty. 2. For each incoming frame received on an interface, the switch stores in its switch table (1) the ____ in the frame's source address field, (2) the _____, and (3) the ___. 3. The switch deletes an address in the table if no frames are received with that address as the source address after some period of time (the ____ time). Switches are ______ devices because they require no intervention from a network administrator or user. The administrator need not configure the switch tables at the time of installation or when a host is removed from one of the LAN segments. Switches are also full-duplex, meaning any switch interface can send and receive at the same time.
No collisions, buffer, sum of all the switch interface rates, Heterogeneous links, Management,
ADVANTAGES OF SWITCHES: • ____: In a LAN built from switches (and without hubs), there is no wasted bandwidth due to collisions! The switches____ frames and never transmit more than one frame on a segment at any one time. As with a router, the maximum aggregate throughput of a switch is the ____. Thus, switches provide a significant performance improvement over LANs with broadcast links. • _____: Because a switch isolates one link from another, the different links in the LAN can operate at different speeds and can run over different media. For example, the uppermost switch in Figure 5.22 might have three 1 Gbps 1000BASE-T copper links, two 100 Mbps 100BASE-FX fiber links, and one 100BASE-T copper link. Thus, a switch is ideal for mixing legacy equipment with new equipment. • _____: In addition to providing enhanced security, a switch also eases network management. For example, if an adapter malfunctions and continually sends Ethernet frames (called a jabbering adapter), a switch can detect the problem and internally disconnect the malfunctioning adapter. Similarly, a cable cut disconnects only that host that was using the cut cable to connect to the switch. Switches also gather statistics on bandwidth usage, collision rates, and traffic types, and make this information available to the network manager. This information can be used to debug and correct problems, and to plan how the LAN should evolve in the future.
the multiple access problem
AN IMPORTANT PROBLEM FOR THE LINK LAYER: ______: How to coordinate the access of multiple sending and receiving nodes to a shared broadcast channel
plug-and-play, automatically, configured by a system administrator, deleted from the other ARP tables in the subnet
ARP is ______; that is, an ARP table gets built ____—it doesn't have to be ____. And if a host becomes disconnected from the subnet, its entry is eventually ____.
exponential backoff
After aborting, NIC enters : after mth collision, NIC chooses K at random from {0,1,2,...,2m-1}. NIC waits K·512 bit times, returns to Step 2
move a datagram, adjacent, a single communication link, vary, framing, datagram, link-layer frame, link access, idle, coordinate, reliable delivery, move each network-layer datagram across the link without error, TCP, acknowledgments, retransmissions, error detection and correction, bit error, include error-detection bits in the frame, perform an error check, checksum, hardware, correction, determines exactly where in the frame the errors have occurred, corrects
Although the basic service of any link layer is to _____ from one node to an_____ node over _____, the details of the provided service can___ from one link-layer protocol to the next. Services offered by a link-layer protocol: 1)_____: Almost all link-layer protocols encapsulate each network-layer _____ within a ____ before transmission over the link. 2) ____: A MAC protocol specifies the rules by which a frame is transmitted onto the link. For point-to-point links with 1 sender sender and 1 receiver, the MAC protocol is: the sender can send a frame whenever the link is______. For the multiple access problem, the MAC protocol serves to_____ the frame transmissions of the many nodes. 3) _____:When a link-layer protocol provides reliable delivery service, it guarantees to ______. Like the transport-layer ____ which also provides a reliable delivery service, a link-layer reliable delivery service can be achieved with_____ and_____ 4) ______: Because there is no need to forward a datagram that has a ____, many link-layer protocols provide a mechanism to detect them by having the transmitting node ______, and having the receiving node _____. The Internet's transport layer and network layer also provide a limited form of error detection—the Internet_____. Error detection in the link layer is usually more sophisticated and is implemented in______. Error_____ is similar to error detection, except that a receiver not only detects when bit errors have occurred in the frame but also _______ (and then____ these errors).
cyclic redundancy check (CRC), polynomial, generator, 1, R, append, D, d + r, exactly divisible by G, divides the d + r received bits by G, nonzero, correct, remainder(D left shifted by r / G), XOR, r + 1, all consecutive bit errors of r bits or fewer will be detected, odd number
An error-detection technique used widely in today's computer networks based on cyclic redundancy check (CRC) codes. aka ______ codes, since it is possible to view the bit string to be sent as a polynomial whose coefficients are the 0 and 1 values in the bit string, with operations on the bit string interpreted as polynomial arithmetic. Consider the d-bit piece of data, D, that the sending node wants to send to the receiving node. The sender and receiver must first agree on an r + 1 bit pattern, known as a______, which we will denote as G. We will require that the most significant (leftmost) bit of G be a ___ The sender will choose r additional bits, ____, and ____ them to ___ such that the resulting _____ bit pattern (interpreted as a binary number) is _____ (i.e., has no remainder) using modulo-2 arithmetic. The process of error checking with CRCs is thus simple: 1) The receiver ______. 2) If the remainder is____, the receiver knows that an error has occurred 3) otherwise the data is accepted as being____. HOW THE SENDER COMPUTES R: R = ? KEY POINTS: - multiplying D by 2^(r) leftshifts D by r places - r = G bits - 1 - when (D leftshifted by r) / G, instead of subtracting, use ____ Each of the CRC standards can detect burst errors of < ____ bits, meaning ______. Each of the CRC standards can also detect any _____ of bit errors.
ceases transmission, datagram, link-layer frame, idle, waits, other adapters using the broadcast channel, aborts the transmission,
DIFFERENCE BETWEEN CSMA AND CSMA/CD: When a node performs collision detection, it _____ as soon as it detects a collision. The two nodes will each abort their transmission a short time after detecting a collision. Adding collision detection to a multiple access protocol will help protocol performance by not transmitting a useless, damaged frame in its entirety. PROCESS of CSMA/CD protocol from the perspective of an adapter (in a node) attached to a broadcast channel: 1) The adapter obtains a_____ from the network layer, prepares a ____, and puts the frame adapter buffer. 2) If the adapter senses that the channel is____ (that is, there is no signal energy entering the adapter from the channel), it starts to transmit the frame. If, on the other hand, the adapter senses that the channel is busy, it_____ until it senses no signal energy and then starts to transmit the frame. 3) While transmitting, the adapter monitors for the presence of signal energy coming from _____. 4) If the adapter transmits the entire frame without detecting signal energy from other adapters, the adapter is finished with the frame. If, on the other hand, the adapter detects signal energy from other adapters while transmitting, it ______ (that is, it stops transmitting its frame). 5) After aborting, the adapter waits a random amount of time and then returns to step 2.
connectionless, handshaking, unreliable, CRC check, acknowledgment, negative acknowledgment, discards the frame, gaps, yes, will not, retransmit
Ethernet provides_____ service to the network layer. - when adapter A wants to send a datagram to adapter B, adapter A encapsulates the datagram in an Ethernet frame and sends the frame into the LAN, without first____ with adapter B. - This layer-2 connectionless service is analogous to IP's layer-3 datagram service and UDP's layer-4 connectionless service. - Ethernet technologies provide an____ service to the network layer. - Specifically, when adapter B receives a frame from adapter A, it runs the frame through a ____, but neither sends an____ when a frame passes the CRC check nor sends a ____ when a frame fails the CRC check. - When a frame fails the CRC check, adapter B simply ____. Thus, adapter A has NO IDEA whether its transmitted frame reached adapter B and passed the CRC check. - This lack of reliable transport (at the link layer) helps to make Ethernet simple and cheap. But it also means that the stream of datagrams passed to the network layer can have____. If there are gaps due to discarded Ethernet frames, does the application at Host B see gaps as well? - depends on whether the application is using UDP or TCP: - If using UDP, then ____ - If using TCP, then TCP in Host B _____ acknowledge the data contained in discarded frames, causing TCP in Host A to____. Note that when TCP retransmits data, the data will eventually return to the Ethernet adapter at which it was discarded. Thus, in this sense, Ethernet does retransmit data, although Ethernet is unaware of whether it is transmitting a brand-new datagram with brand-new data, or a datagram that contains data that has already been transmitted at least once.
when there are 3 you can detect but can't correct, but at 2 or 1 errors you can detect AND correct
For a 2D parity scheme, if there are 3 errors, can you detect / correct it? 2 errors? 1 errors?
link-layer, link-layer addresses, link-layer addresses, IP addresses, carry datagrams between hosts and routers,
Hosts and routers have network-layer addresses AND their adapters (network interfaces) have ___ addresses A host or router with multiple network interfaces will thus have multiple _____ associated with it, just as it would also have multiple _____ associated with it. link-layer switches DO NOT have ______ associated with their interfaces that connect to hosts and routers. This is because the job of the link-layer switch is to ______; a switch does this job without the host or router having to explicitly address the frame to the intervening switch.
adapter, ARP, MAC address, router interface, first-hop router, frame, MAC address, forwarding table,
How does ARP operate when a host wants to send a datagram to another host on a DIFFERENT subnet? Consider a simple network consisting of 2 subnets interconnected by a router. Each host has exactly 1 IP address and 1 adapter. The router has an IP address for each of its interfaces, and for each router interface there is also an ARP module (in the router) and an adapter. Because the router in our example has two interfaces, it has two IP addresses, two ARP modules, and two adapters. And, each adapter in the network has its own MAC address. Now let's answer how a host on Subnet 1 would send a datagram to a host on Subnet 2. 1) The sending host passes the datagram to its____, as usual. 2) The sending host uses____ to get the ____ of the ___, which is the IP address of the ____ on the path to the final destination 3) The sender creates a____ containing the datagram addressed to the ____ and sends the frame to the router 4) The router now has to determine the correct interface on which the datagram is to be forwarded. It uses its ____ in the router to get the next interface and uses ARP to get that next interface's MAC address. It puts this info in a frame and sends it to its final destination into Subnet 2.
sending controller, sets, frame header, receiving controller
If the link layer performs error detection, then it is the _____ that___ the error-detection bits in the ____ and it is the _____ that performs error detection.
k-bit integers, sum, use the resulting sum as the error-detection bits, Internet checksum, bytes, 16-bit integers, summed, one's complement, segment header, 1, 0, all, header, data, 16 bits, IP header, overhead, cyclic redundancy check, link,
In checksumming techniques, the d bits of data are treated as a sequence of ____. One simple checksumming method is to simply____ these k-bit integers and _____. The ____ is based on this approach:______ of data are treated as ____ and____. The _____ of this sum then forms the Internet checksum that is carried in the ____. the receiver checks the checksum by taking the 1s complement of the sum of the received data (including the checksum) and checking whether the result is all _____ bits. If any of the bits are ___, an error is detected. In the TCP and UDP protocols, the Internet checksum is computed over _____ fields (____and ___ fields included) and the checksums use only ____ In IP the checksum is computed over the ______ (since the UDP or TCP segment has its own checksum). Checksumming methods require relatively little packet_____, but they are weak compared with ______, which is often used in the ____ layer.
end-to-end channel propagation delay of a broadcast channel, propagation delay, sense a transmission that has already begun at another node
MOST IMPORTANT CHARACTERISTICS OF CSMA AND CSMA/CD: If all nodes perform carrier sensing in CSMA, why do collisions occur in the first place? Consider a space-time diagram of four nodes (A, B, C, D) attached to a linear broadcast bus. The horizontal axis shows the position of each node in space; the vertical axis represents time. At time t0, node B senses the channel is idle, as no other nodes are currently transmitting. Node B thus begins transmitting, with its bits propagating in both directions along the broadcast medium. The downward propagation of B's bits in Figure 5.12 with increasing time indicates that a nonzero amount of time is needed for B's bits actually to propagate (albeit at near the speed of light) along the broadcast medium. At time t1 (t1 > t0), node D has a frame to send. Although node B is currently transmitting at time t1, the bits being transmitted by B have yet to reach D, and thus D senses the channel idle at t1. In accordance with the CSMA protocol, D thus begins transmitting its frame. A short time later, B's transmission begins to interfere with D's transmission at D. From Figure 5.12, it is evident that the _______—the time it takes for a signal to propagate from one of the nodes to another—will play a crucial role in determining its performance. The longer this ______, the larger the chance that a carrier-sensing node is not yet able to ______ in the network.
communication service, datagrams, packet switches, switches, routers,
NETWORKING LAYER: the network layer provides a ___ between any two network hosts. Between the two hosts, ___travel over a series of communication links, some wired and some wireless, starting at the source host, passing through a series of _____ (____and ____) and ending at the destination host.
Slotted Aloha, transmitting, stops transmitting, all the nodes, the slot ends, the beginning of the next slot, the entire frame in the slot, retransmits its frame in each subsequent slot with probability p, biased, toss the coin again in the next slot, (1 - p), independently, continuously at the full rate, R, it has frames to send, decentralized, collisions, probabilistic, exactly one node transmits, successful,
One of the simplest random access protocols is the ______ protocol. a node neither pays attention to whether another node happens to be_____ when it begins to transmit, nor_____ if another node begins to interfere with its transmission. In our description of this protocol, we assume the following: • All frames consist of exactly L bits. • Time is divided into slots of size L/R seconds (1 slot = the time to transmit 1 frame). • Nodes start to transmit frames only at the beginnings of slots. • The nodes are synchronized so that each node knows when the slots begin. • If 2 or more frames collide in a slot, then ____ detect the collision event before _____. Let p be a probability, that is, a number between 0 and 1. The operation of this protocol in each node is simple: • When the node has a fresh frame to send, it waits until _____ and transmits_____ HOW IT HANDLES COLLISIONS: • If there isn't a collision, the node has successfully transmitted its frame and thus need not consider retransmitting the frame. (The node can prepare a new frame for transmission, if it has one.) • If there is a collision, the node detects the collision before the end of the slot. The node ______ until the frame is transmitted without a collision. By retransmitting with probability p, we mean that the node effectively tosses a ____ coin; the event heads corresponds to "retransmit," which occurs with probability p. The event tails corresponds to "skip the slot and ____"; this occurs with probability _____. All nodes involved in the collision toss their coins_____. ADVANTAGES: - allows a node to transmit ______ when that node is the only active node. (A node is said to be active if ____.) -highly______ bc each node detects collisions + independently decides when to retransmit. (it does, however, require the slots to be synchronized in the nodes, meaning each node knows when the slots begin) - extremely simple protocol. 2 POSSIBLE EFFICIENCY CONCERNS WHEN THERE'S MULTIPLE ACTIVE NODES: 1) fraction of the slots are "wasted" when there are___ 2) fraction of the slots will be empty because all active nodes refrain from transmitting as a result of the_____ transmission policy. The only "unwasted" slots will be those in which _______, which is said to be a ______ slot.
count the number of 1s, undetected, bursts, 50%
Simplest form of error detection is the use of a *SINGLE PARITY BIT* Suppose D has d bits. SENDER: For even parity schemes, the sender chooses the parity bit so the total number of 1s in the d + 1 bits (the original information + a parity bit) = even For even parity schemes, the sender chooses the parity bit so the total number of 1s in the d + 1 bits (the original information + a parity bit) = odd The single parity bit is stored in a separate field. RECEIVER: The receiver need only _______ in the received d + 1 bits. If it's an even parity scheme with an odd number of 1s, then the receiver knows that an odd number of >= 1 bit error has occurred. BUT, if an even number of bit errors occur? The error would be___. for single parity bit errors, they are often clustered together in "____", and other these conditions, the chance of undetected errors in a frame protected by single-bit parity can approach ____ So, a more robust error-detection scheme is needed and used in practice
frequency-division multiplexing (FDM), frequencies, R/N, frequency, N smaller channels, packets to send, wait for its turn
Suppose the channel supports N nodes and that the transmission rate of the channel is R bps. _____ divides the R bps channel into different____ (each with a bandwidth of ____) and assigns each ___ to one of the N nodes. - creates _____ of R/N bps out of the single, larger R bps channel. SHARES THE SAME POSITIVES AND NEGATIVES AS THE OTHER PARTITIONING PROTOCOL POSITIVES: 1) eliminates collisions 2) is perfectly fair: each node gets a dedicated transmission rate of ___ bps during each frame time. NEGATIVES: 1) a node is limited to an average rate of R/N bps even if it is the only node with ____. 2) a node must always _____ in the transmission sequence even if it is the only node with a frame to send.
time-division multiplexing (TDM), assigned time slot, revolving TDM frame, single packet, R/N, packets to send, wait for its turn
Suppose the channel supports N nodes and that the transmission rate of the channel is R bps. ______ divides time into time frames and further divides each time frame into N time slots. Each time slot is then assigned to one of the N nodes. Whenever a node has a packet to send, it transmits the packet's bits during its _____ in the _____. Typically, slot sizes are chosen so that a _____ can be transmitted during a slot time. In this case, the cocktail party would allow 1 person to speak for a fixed amount of time, then allow another person to speak for the same amount of time, etc. Once everyone had a chance to speak, the pattern repeats. SHARES THE SAME POSITIVES AND NEGATIVES AS THE OTHER PARTITIONING PROTOCOL POSITIVES: 1) eliminates collisions 2) is perfectly fair: each node gets a dedicated transmission rate of ___ bps during each frame time. NEGATIVES: 1) a node is limited to an average rate of R/N bps even if it is the only node with ____. 2) a node must always _____ in the transmission sequence even if it is the only node with a frame to send.
store-and-forward packet switch, store-and-forward packet switch, MAC, filtering, forwarding, ARP tables, broadcast storms, spanning tree, firewall,
Switches Versus Routers - a router is a ___ that forwards packets using network-layer addresses. - Although a switch is also a ____, it is fundamentally different from a router in that it forwards packets using ___ addresses. Whereas a router is a layer-3 packet switch, a switch is a layer-2 packet switch. - Even though switches and routers are fundamentally different, network administrators must often choose between them when installing an interconnection device. PROS AND CONS OF SWITCHES: PROS: - plug-and-play - Switches can also have relatively high_____ and____ rates since switches have to process frames only up through layer 2, whereas routers have to process datagrams up through layer 3. CONS: On the other hand, to prevent the cycling of broadcast frames, the active topology of a switched network is restricted to a spanning tree. Also, a large switched network would require large ______ in the hosts and routers and would generate substantial ARP traffic and processing. Furthermore, switches are susceptible to ______—if one host goes haywire and transmits an endless stream of Ethernet broadcast frames, the switches will forward all of these frames, causing the entire network to collapse. PROS AND CONS OF ROUTERS: PROS: - Because routers do not have the ___ restriction, they have allowed the Internet to be built with a rich topology that includes, for example, multiple active links between Europe and North America. - they provide_____ protection against layer-2 broadcast storms. - not plug-and-play - have a larger per-packet processing time than switches, because they have to process up through the layer-3 fields. TO PICK: - choose SWITCHES for SMALL networks because they localize traffic and increase aggregate throughput without requiring any configuration of IP addresses. - choose ROUTERS for LARGE networks (in addition to switches) because they provide a more robust isolation of traffic, control broadcast storms, and use more "intelligent" routes among the hosts in the network
link layer switches, another host/router, receiving, forwarding, temporarily exceed, buffers
The role of the_____ is to receive incoming link-layer frames and forward them onto outgoing links - it is transparent to the hosts and routers in the subnet, meaning a host/router addresses a frame to _____ (rather than addressing the frame to the switch) and happily sends the frame into the LAN, unaware that a switch will be____ the frame and____ it. - The rate at which frames arrive to any one of the switch's output interfaces may _____ the link capacity of that interface. To accommodate this problem, switch output interfaces have____, in much the same way that router output interfaces have buffers for datagrams.
time-division multiplexing (TDM), frequency-division multiplexing (FDM),
What are 2 techniques that can be used to partition a broadcast channel's bandwidth among all nodes sharing that channel? THEY BOTH HAVE THE SAME ADVANTAGES AND DISADVANTAGES
parity checks, checksumming, cyclic redundancy checks
What are the 3 techniques for detecting errors in the transmitted data? 1) ______ : simplest form of error detection 2)______ : more typically used in the transport layer 3) _____: more typically used in the link layer in an adapter
Data field, fragment, stuffed, Destination address, Source address, Type field, Cyclic redundancy check (CRC), Preamble,
What are the 6 fields of the Ethernet frame? 1) _____(46 to 1,500 bytes): carries the IP datagram. - The maximum transmission unit (MTU) of Ethernet is 1,500 bytes. This means that if the IP datagram exceeds 1,500 bytes, then the host has to____ the datagram. - The minimum size of the data field is 46 bytes. This means that if the IP datagram is less than 46 bytes, the data field has to be "____" to fill it out to 46 bytes. When stuffing is used, the data passed to the network layer contains the stuffing as well as an IP datagram. The network layer uses the length field in the IP datagram header to remove the stuffing. 2) ____ (6 bytes). contains the MAC address of the destination adapter - If the adapter is the destination, it passes the contents of the frame's data field to the network layer, else it discards the frame 3) _____ (6 bytes): contains the MAC address of the adapter that transmits the frame onto the LAN 4) _____ (2 bytes): permits Ethernet to multiplex network-layer protocols. - hosts can use other network-layer protocols besides IP and may support multiple network-layer protocols using different protocols for different applications. - when the Ethernet frame arrives at adapter B, adapter B needs to know to which network-layer protocol it should pass (that is, demultiplex) the contents of the data field. - IP and other network-layer protocols (for example, Novell IPX or AppleTalk) each have their own, standardized type number. - Furthermore, the ARP protocol has its own type number, and if the arriving frame contains an ARP packet (i.e., has a type field of 0806 hexadecimal), the ARP packet will be demultiplexed up to the ARP protocol. - the type field is analogous to the protocol field in the network-layer datagram and the port-number fields in the transport-layer segment; all of these fields serve to glue a protocol at one layer to a protocol at the layer above. 5) ______ (4 bytes): allows the receiving adapter, adapter B, to detect bit errors in the frame. 6) ____ (8 bytes): Each of the first 7 bytes of the preamble has a value of 10101010; the last byte is 10101011. The first 7 bytes of the preamble serve to "wake up" the receiving adapters and to synchronize their clocks to that of the sender's clock. - Why should the clocks be out of synchronization? Keep in mind that adapter A aims to transmit the frame at 10 Mbps, 100 Mbps, or 1 Gbps, depending on the type of Ethernet LAN. However, because nothing is absolutely perfect, adapter A will not transmit the frame at exactly the target rate; there will always be some drift from the target rate, a drift which is not known a priori by the other adapters on the LAN. A receiving adapter can lock onto adapter A's clock simply by locking onto the bits in the first 7 bytes of the preamble. The last 2 bits of the eighth byte of the preamble (the first two consecutive 1s) alert adapter B that the "important stuff" is about to come.
reliable delivery service, wireless, on the link, end-to-end retransmission of the data, link-layer reliable delivery, wired,
When a link-layer protocol provides ______, it guarantees to move each network-layer datagram across the link without error often used for links that are prone to high error rates, such as a ____ link, with the goal of correcting an error locally—____ where the error occurs—rather than forcing an ____ by a transport- or application-layer protocol. However, ______ can be considered an unnecessary overhead for low bit-error links such as fiber, coax, and many twisted-pair copper links, so many_____ link-layer protocols do not provide this service.
software, host's CPU, link-layer addressing information, controller hardware, controller interrupts, error conditions, datagram, network
While most of the link layer is implemented in hardware, part of the link layer is implemented in____ that runs on the ____. The software components of the link layer implement higher-level link- layer functionality such as 1) assembling _____ and 2) activating the ____. On the receiving side, link-layer software: 1) responds to ____ (e.g., due to the receipt of one or more frames), 2) handling _____ and 3) passing a____ up to the____ layer. Thus, the link layer is a combination of hardware and software—the place in the protocol stack where software meets hardware.
software, simple, fast, dedicated hardware in adapters, complex
Why is checksumming used at the transport layer and cyclic redundancy check used at the link layer? Recall that the transport layer is typically implemented in software in a host as part of the host's operating system. Because transport-layer error detection is implemented in_____, it is important to have a____ and ____ error-detection scheme such as checksumming. On the other hand, error detection at the link layer is implemented in _____, which can rapidly perform the more____ CRC operations.
Filtering, Forwarding, switch table, MAC, switch interface, time, broadcasts, filtering, discarding, forwarding, output buffer,
_____: the switch function that determines whether a frame should be forwarded to some interface or should just be dropped. _____: the switch function that determines the interfaces to which a frame should be directed, and then moves the frame to those interfaces. These 2 processes are done with a ____, which contains entries for some, but not necessarily all, of the hosts and routers on a LAN. An entry in the switch table contains: (1) a ___ address, (2) the ___ that leads toward that MAC address, and (3) the____ at which the entry was placed in the table. Suppose a frame with destination address DD-DD-DD-DD-DD-DD arrives at the switch on interface x. The switch indexes its table with the MAC address DD-DD-DD-DD-DD-DD. There are three possible cases: 1) If there is no entry in the table for DD-DD-DD-DD-DD-DD, then the switch ___ the frame. 2) If there is an entry in the table and the frame is coming from a LAN segment that contains adapter DD-DD-DD-DD-DD-DD, the switch performs the____ function by____ the frame. 3) If there is an entry in the table and the frame needs to be forwarded to the LAN segment attached to interface y. The switch performs its_____ function by putting the frame in an _____ that precedes interface y.
multiple access protocols, wired, wireless, satellite, adapter, directly, transmitting frames, multiple frames, collide, all of the receivers, collision, frames, lost, wasted, channel partitioning, random access, taking-turns, R, R/M, decentralized,
______ protocols used by nodes to regulate their transmission into the shared broadcast channel and coordinate the transmissions of the active nodes - needed in a wide variety of network settings, including both_____ and____ access networks, and___ networks. - each node accesses the broadcast channel through its____ - hundreds or thousands of nodes can____ communicate over a broadcast channel. - Because all nodes are capable of _____, more than two nodes can transmit frames at the same time. - When this happens, ALL OF THE NODES receive ______ at the same time; that is, the transmitted frames_____ at ______. - Typically, when there is a_____, none of the receiving nodes can make any sense of any of the_____ that were transmitted and all the frames involved in the collision are____, and the broadcast channel is_____ during the collision interval. - This is a PROBLEM because if many nodes want to transmit frames frequently, many transmissions will result in collisions, and much of the bandwidth of the broadcast channel will be wasted. - What are the 3 categories of multiple access protocols? 1) _____ protocols, 2) _____ protocols, and 3) _____ protocols. Ideally, a multiple access protocol for a broadcast channel of rate R bps should: 1. When only 1 node has data to send, that node has a throughput of ____ bps. 2. When M nodes have data to send, each of these nodes has a throughput and average transmission rate of ____ bps. 3. The protocol is_____ so there is no master node that is a single point of failure 4. The protocol is simple and inexpensive to implement.
node, links, links, 6, frame, frame, link
a ____ is any device that runs a link-layer (i.e., layer 2) protocol include hosts, routers, switches, and WiFi access points ___ are the communication channels that connect adjacent nodes along the communication path - In order for a datagram to be transferred from source host to destination host, it must be moved over each of the individual___ in the end-to-end path. - a datagram will pass through ____links/hops to send a datagram from one of the wireless hosts to one of the servers: - a Wifi link between sending host and WiFi access point, an Ethernet link between the access point and a link-layer switch; a link between the link-layer switch and the router, a link between the two routers; an Ethernet link between the router and a link-layer switch; and finally an Ethernet link between the switch and the server. - Over a given link, a transmitting node encapsulates the datagram in a link-layer_____ and transmits the_____ into the ____. EXAMPLE: Consider a travel agent who is planning a trip for a tourist traveling from Princeton, New Jersey, to Lausanne, Switzerland. The travel agent decides that it is most convenient for the tourist to take a limousine from Princeton to JFK airport, then a plane from JFK airport to Geneva's airport, and finally a train from Geneva's airport to Lausanne's train station. Once the travel agent makes the three reservations, it is the responsibility of the Princeton limousine company to get the tourist from Princeton to JFK; it is the responsibility of the airline company to get the tourist from JFK to Geneva; and it is the responsibility of the Swiss train service to get the tourist from Geneva to Lausanne. Each of the three segments of the trip is "direct" between two "adjacent" locations. Note that the three transportation segments are managed by different companies and use entirely different transportation modes (limousine, plane, and train). Although the transportation modes are different, they each provide the basic service of moving passengers from one location to an adjacent location. In this transportation analogy, the tourist is a datagram, each transportation segment is a link, the transportation mode is a link-layer protocol, and the travel agent is a routing protocol.
hub, broadcast LAN
a physical-layer device that acts on individual bits rather than frames. When a bit, representing a zero or a one, arrives from one interface, it simply re-creates the bit, boosts its energy strength, and transmits the bit onto all the other interfaces. Thus, it's also a ____
32, xxx-xxx-xxx-xxx where each group is a decimal number that represents a 4-bit number
an IP address can be at most ___ bits, and is formatted like?
ARP packet, query, response, broadcast, standard, ARP query packet
an ____ has several fields that include the sending and receiving IP and MAC addresses. Both ARP____ and ____ packets have the same format. The query ARP message is sent within a___ frame, whereas the response ARP message is sent within a____ frame. The purpose of the ____ is to query all the other hosts and routers on the subnet to determine the MAC address corresponding to the IP address that is being resolved.
broadcast link, the channel broadcasts the frame, each of the other nodes receives a copy, send, receive, talk, when
can have multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. The term broadcast is used here because when any one node transmits a frame, _____ and ____. Examples: Ethernet and wireless LANs Nodes on a computer network broadcast channel can both____ and ___. ANALOGIES: 1) a cocktail party, where people gather to talk/listen in a large room and the air is the broadcast medium 2) a classroom where teacher and students share the same, single, broadcast medium. A central problem in both scenarios is determining who gets to____ (that is, transmit into the channel), and ____.
transport, link, networking
checksumming is used at the ___ layer and cyclic redundancy check is used at the ___ layer and FEC is used at the ___ layer
bit-level error detection and correction
detecting and correcting the corruption of bits in a link-layer frame sent from one node to another physically connected neighboring node - these are 2 services often provided by the link layer.
Ethernet, broadcast LAN, switch-based star topology, active switch, nodes do not collide with each other, separate Ethernet protocol,
dominant" wired LAN technology, cheap, first widely used LAN technology, kept up with speed race: 10 Mbps - 100 Gbps Originally used bus topology, but then moved to hub-based star topology (which is also a ___), but finally now it's ____ In this current topology, an ____ is in the center and each arm of the star runs a ____ so that ___
random access protocols, full rate, repeatedly retransmits its frame, retransmit, random delay, independent random delays, ALOHA, carrier sense multiple access (CSMA), Ethernet,
one of the two broad class of multiple access protocols in this class of protocols, a transmitting node always transmits at the ____ of the channel, namely, R bps. When there is a collision, EACH node involved in the collision _________ (that is, packet) until its frame gets through without a collision. But when a node experiences a collision, it doesn't necessarily_____ the frame right away. Instead it waits a ______ before retransmitting the frame. Each node involved in a collision chooses _____ to avoid collision again Two most commonly used types of random access protocols are: 1) the ____ protocols 2) the _____ protocols (example:_____)
Address Resolution Protocol (ARP), MAC address, link-layer frame, destination's MAC address, send the frame into the LAN, An ARP module in the sending host takes any IP address on the same LAN as input and returns the corresponding MAC address, hosts, router, subnet, mappings of IP addresses to MAC addresses, time-to-live (TTL) value, ARP packet, adapter, MAC broadcast address, link-layer frame, broadcast address, subnet, adapters, ARP module, its IP address matches the destination IP address in the ARP packet, response ARP packet with the desired mapping, updates its ARP table
provides a mechanism to translate IP addresses to link-layer (MAC) addresses. Suppose a host wants to send another host in the same subnet an IP datagram. To send a datagram, the source must give its adapter not only the IP datagram but also the ____ . The sending adapter will then construct a _____ containing the _____ and ____ How does the sending host determine the MAC address for the destination host if it knows the destination's IP address if they are in the same LAN / subnet? This protocol resolves IP addresses only for ____ and____ interfaces on the same_____. HOW IT WORKS: Each host and router has an ARP table in its memory, which contains _______ and a ____ which indicates when each mapping will be deleted from the table (typically 20 mins from when an entry is placed in an ARP table) What happens if a host wants to send a datagram that is IP-addressed to another host or router on that subnet and the sender's ARP table DOES NOT have an entry for the destination node? 1) the sender constructs an ____ which has several fields including the sending and receiving IP and MAC addresses 2) the sender passes the ARP query packet to the _____ along with an indication that the adapter should send the packet to the _____, namely, FF-FF-FF-FF-FF-FF. 3) The adapter encapsulates the ARP packet in a _____, uses the _____ for the frame's destination address, and transmits the frame into the ____. 4) The frame containing the ARP query is received by all the other_____ on the subnet, and (because of the broadcast address) each adapter passes the ARP packet within the frame up to its _____. 5) Each of these ARP modules checks to see if _____. 6) The one with a match sends back to the querying host a _____. 7) the sender then ____ and sends its IP datagram, encapsulated in a link-layer frame whose destination MAC is that of the host or router responding to the earlier ARP query.
networking, network-layer datagrams, link, link-layer frames
routers operate at the __ layer and route ___ switches operate at the __ layer and switch ___
medium access control (MAC) protocol, the link is idle, coordinate the frame transmissions
specifies the rules by which a frame is transmitted onto the link For point-to-point links that have a single sender at one end of the link and a single receiver at the other end of the link, the protocol is simple (or nonexistent): the sender can send a frame whenever _______. When multiple nodes share a single broadcast link (multiple access problem) the protocol serves to ______ of the many nodes.
link, frames, datagrams, RIP, OSPF, link-layer addresses, link-layer frames,
switched local area networks: Consider a switched local network connecting three departments, two servers and a router with four switches. Because switches operate at the___ layer, they switch link-layer ____ (rather than network-layer____), don't recognize network-layer addresses, and don't use routing algorithms like ____ or ____ to determine paths through the network of layer-2 switches. Instead of using IP addresses, switches use _____ to forward _____ through the network of switches.
router's line card (hardware), network adapter, network interface card (NIC), network adapter, controller, chip, hardware, controller, link-layer frame, communication link, link-access, controller, frame, network-layer datagram, error detection, sets the error-detection bits in the frame header, error detection
the link layer is implemented in a ____, and for the most part, the link layer is implemented in a ____, also sometimes known as a _____. At the heart of the _____ is the link-layer_____, usually a single, special-purpose____ that implements many of the link-layer services (framing, link access, error detection, etc). Thus, much of a link-layer controller's functionality is implemented in_____. ON THE SENDING SIDE: the ____ takes a datagram that has been created and stored in host memory by the higher layers of the protocol stack, encapsulates the datagram in a ______ (filling in the frame's various fields), and then transmits the frame into the _____, following the ____ protocol. ON THE RECEIVING SIDE: a_____ receives the entire____, and extracts the ____. If the link layer performs _____, then it is the sending controller that ______ and it is the receiving controller that performs _____.
end-to-end channel propagation delay of a broadcast channel
the time it takes for a signal to propagate from one of the nodes to another play a crucial role in determining performance of CSMA and CSMA/CD protocols
broadcast channels, medium access protocol, central controller, hosts, point-to-point communication link
what are the 2 different types of link-layer channels? 1) ____: connect multiple hosts in wireless LANs, satellite networks, and hybrid fiber-coaxial cable (HFC) access networks. Since many hosts are connected to the same broadcast communication channel, a _____ is needed to coordinate frame transmission. In some cases, a _____ may be used to coordinate transmissions; in other cases, the___ themselves coordinate transmissions. 2) ___: like the type found between two routers connected by a long-distance link, or between a user's office computer and the nearby Ethernet switch to which it is connected.
bit-level error detection, bit-level error correction, error-detection and error-correction bits (EDC), datagram, link-level addressing information, sequence numbers, link frame header, link-level frame, sequence of bits, D', EDC', D' is the same as the original D, D', EDC', undetected bit errors, corrupted datagram, a field in the frame's header, parity checks, checksumming, cyclic redundancy checks
what are the 2 error services provided by the link layer? At the sending node, the data D is protected against bit errors by being augmented with ___ Typically, the data to be protected includes not only the _____ passed down from the network layer for transmission across the link, but also ____, _____, and other fields in the ____. Both D and EDC are sent to the receiving node in a ____. At the receiving node, a ____, ___, and___ is received. NOTE: D' and EDC' may differ from the original D and EDC as a result of in-transit bit flips. The receiver's challenge is to determine whether or not ____, given that it has only received ___ and ___. Error-detection and error-correction techniques allow the receiver to SOMETIMES, but NOT ALWAYS, detect that bit errors have occurred. Even with the use of error-detection bits there still may be _____; that is, the receiver may be unaware that the received information contains bit errors. As a consequence, the receiver might deliver a ______ to the network layer, or be unaware that the contents of ______ has been corrupted. What are the 3 techniques for detecting errors in the transmitted data? 1) ______ : simplest form of error detection 2)______ : more typically used in the transport layer 3) _____: more typically used in the link layer in an adapter
framing, link access, reliable delivery, error detection and correction
what are the 4 services offered by a link layer protocol?
A data field (in which the network-layer datagram is inserted) and a number of header fields, link-layer protocol
what does a link-layer frame consist of? The structure of the frame is specified by the ____
the receiver checks the checksum by taking the 1s complement of the sum of the received data (including the checksum) and checking whether the result is all 1 bits. If any of the bits are 0, an error is indicated.
what is the process of checksumming?
bit errors, hardware, correction, determines exactly where in the frame the errors have occurred, corrects
when link-layer hardware in a receiving node incorrectly decides that a bit in a frame is zero when it was transmitted as a one and vice versa introduced by signal attenuation and electro-magnetic noise Error detection in the link layer is usually more sophisticated and is implemented in______. Error_____ is similar to error detection, except that a receiver not only detects when bit errors have occurred in the frame but also _______ (and then____ these errors).
