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The Data Link Layer • Highlights of this chapter – Data Link Layer Design Issues. – Error Detection and Correction. – Medium access control. – The Ethernet The Data Link Layer (Cont’d) • Data Link Layer Design Issues – Providing a well-defined service interface to the network layer. – Dealing with transmission errors. – Regulating the flow of data so that slow receivers are not swamped by fast senders. The Data Link Layer (Cont’d) • Provide interface to network layer Relationship between packets and frames. (a) Virtual communication. (b) Actual communication. The Data Link Layer (Cont’d) • Three possible services – Unacknowledged connectionless service. • Send independent frames to the destination machine. • Appropriate when the error rate is very low or real-time traffic. • Most LANs use this kind of service. – Acknowledged connectionless service. • Over unreliable channels, such as wireless systems. • Providing acknowledgements in the data link layer is just an optimization, never a requirement! – Acknowledged connection-oriented service. • E.g., connections established between modems. The Data Link Layer (Cont’d) • Error Detection and Correction – Where, why, and how? • In local loops, wireless communication, errors are common. • Thermal, electric noises. • tend to come in bursts rather than singly! – Two basic strategies for dealing with errors • error-correcting codes (forward error correction): include enough redundant information along with each block of data sent, to enable the receiver to deduce what the transmitted data must have been. (wireless comm) • error-detecting codes: include only enough redundancy to allow the receiver to deduce that an error occurred, but not which error, and have it request a retransmission. The Data Link Layer (Cont’d) • A simple error detection algorithm (a code in which a single parity bit) – The parity bit is chosen so that the number of 1 bits in the codeword is even (or odd). – When 1011010 is sent in even parity, a bit is added to the end to make it 10110100. With odd parity 1011010 becomes 10110101. • More sophisticate error detection and correction algorithms – CRC (Cyclic Redundancy Check) —— rather popular. – We stop here for they are rather sophisticated. The Data Link Layer (Cont’d) • The Medium Access Control – networks can be divided into two categories: those using point-to-point connections and those using broadcast channels. – Due to the cost considerations, many modern network architecture employs broadcast channels. (e.g., Ethernet, Wireless Ad hoc network, etc.) Seldom of them are fully connected. – Challenge: Who can access the network? When and how? The Data Link Layer (Cont’d) • The Medium Access Control – In the literature, broadcast channels are sometimes referred to as multiaccess channels or random access channels. – The protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer. The Data Link Layer (Cont’d) • Frequency Division Multiplexing (FDM) – If there are N users, the bandwidth is divided into N equal-sized portions, each user being assigned one portion. each user has a private frequency band, there is no interference between users. – Does FDM fit for the real use? (number of senders is large and continuously varying; traffic is bursty) The Data Link Layer (Cont’d) • Problems of FDM – Waste of the bandwidth: If the spectrum is cut up into N regions and fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted. If more than N users want to communicate, some of them will be denied permission for lack of bandwidth, even if some of the users who have been assigned a frequency band hardly ever transmit or receive anything. – Inefficiency: assuming that the number of users could somehow be held constant at N, when some users are quiescent, their bandwidth is simply lost. They are not using it, and no one else is allowed to use it either. As data traffic is extremely bursty, most of the channels will be idle most of the time. The Data Link Layer (Cont’d) • Problems of FDM – for a channel of capacity C bps, with an arrival rate of λ frames/sec, each frame having a length drawn from an exponential probability density function with mean 1/µ bits/frame. With these parameters the arrival rate is λframes/sec and the service rate is µC frames/sec. From queueing theory, we have the mean time delay T: – Now let us divide the single channel into N independent subchannels, each with capacity C/N bps. The mean input rate on each of the subchannels will now be λ/N. Recomputing T we get – The mean delay using FDM is N times worse than if all the frames were somehow magically arranged orderly in a big central queue. The Data Link Layer (Cont’d) • Pure ALOHA – Problem: Multiple users share a single ground-based radio broadcasting channel. They transmit whenever they have data to be sent. There will be collisions, of course, and the colliding frames will be damaged. However, due to the feedback property of broadcasting, a sender can always find out whether its frame was destroyed by listening to the channel, the same way other users do. If the frame was destroyed, the sender just waits a random amount of time and sends it again. The waiting time must be random or the same frames will collide over and over, in lockstep. – Systems in which multiple users share a common channel in a way that can lead to conflicts are widely known as contention systems. The Data Link Layer (Cont’d) • Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times. – Whenever two frames try to occupy the channel at the same time, there will be a collision and both will be garbled. If the first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted later. The checksum cannot (and should not) distinguish between a total loss and a near miss. Bad is bad. – An interesting question is: What is the efficiency of an ALOHA channel? The Data Link Layer (Cont’d) • Pure ALOHA – We assume infinite users share the same pure ALOHA channel, and the generate new frames according to a Poisson distribution with mean N frames per frame time. – If N > 1, the user community is generating frames at a higher rate than the channel can handle, and nearly every frame will suffer a collision. For reasonable throughput we would expect 0 < N < 1. – In addition to new frames, the stations also generate retransmissions that previously suffered collisions. We further assume that the probability of k transmission attempts per frame time, old and new combined, is also Poisson, with mean G per frame time. Clearly, G ≥ N. The Data Link Layer (Cont’d) • Pure ALOHA – At low load (i.e., N ≈ 0), there will be few collisions, hence few retransmissions, so G ≈ N. At high load there will be many collisions, so G > N. – If P0 is the probability that a frame does not suffer a collision, S, the offered load can be calculated: S = GP0 – Let t be the time needed to transmit a single frame (frame time). The probability that k frames are generated during a given frame time is given by the Poisson distribution: – so the probability of zero frames is just e-G. The Data Link Layer (Cont’d) • Pure ALOHA – In an interval two frame times long, the mean number of frames generated is 2G. The probability of no other traffic being initiated during the entire vulnerable period is thus given by P0 = e-2G. Using S = GP0, we get: S = Ge-2G – The maximum throughput occurs at G = 0.5, with S = 1/2e, which is about 0.184 (18%). The Data Link Layer (Cont’d) • Slotted ALOHA – Divide time into discrete intervals, each interval corresponding to one frame. This approach requires the users to agree on slot boundaries. One way to achieve synchronization would be to have one special station emit a pip at the start of each interval, like a clock. (Roberts, 1972) – Since the vulnerable period is now halved, the probability of no other traffic during the same slot as our test frame is e-G which leads to: S = Ge-G – See the above figure, slotted ALOHA peaks at G = 1, with a throughput of S =1/e or about 0.368, twice that of pure ALOHA. The Data Link Layer (Cont’d) • Carrier Sense Multiple Access Protocols (CSMA) – In local area networks, however, it is possible for stations to detect what other stations are doing, and adapt their behavior accordingly. – Protocols in which stations listen for a carrier (i.e., a transmission) and act accordingly are called carrier sense protocols. – A number of them have been proposed. (1persistent CSMA、non-persistent CSMA、ppersistent CSMA.) The Data Link Layer (Cont’d) • 1-persistent CSMA – When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. If the channel is busy, the station waits until it becomes idle. When the station detects an idle channel, it transmits a frame. If a collision occurs, the station waits a random amount of time and starts all over again. – The protocol is called 1-persistent because the station transmits with a probability of 1 when it finds the channel idle. The Data Link Layer (Cont’d) • Problems of 1-persistent CSMA – The propagation delay has an important effect on the performance of the protocol. – Two stations want to send their respective frames. If the first station's signal has not yet reached the second one, the latter will sense an idle channel and will also begin sending, resulting in a collision. – The longer the propagation delay, the more important this effect becomes, and the worse the performance of the protocol. – Even if the propagation delay is zero, there will still be collisions. – higher performance than pure ALOHA and slotted ALOHA. The Data Link Layer (Cont’d) • Non-persistent CSMA – In this protocol, a conscious attempt is made to be less greedy than in the 1-persistent CSMA. – If the channel is already in use, the station does not continually sense it for the purpose of seizing it immediately upon detecting the end of the previous transmission. Instead, it waits a random period of time and then repeats the algorithm. – This algorithm leads to better channel utilization but longer delays than 1-persistent CSMA. The Data Link Layer (Cont’d) • p-persistent CSMA – It applies only to slotted channels. – When a station becomes ready to send, it senses the channel. If it is idle, it transmits with a probability p. With a probability q = 1 - p, it defers until the next slot. If that slot is also idle, it either transmits or defers again, with probabilities p and q. The Data Link Layer (Cont’d) • Comparison of the channel utilization versus load for various random access protocols. The Data Link Layer (Cont’d) • CSMA with Collision Detection (CSMA/CD) – If two stations sense the channel to be idle and begin transmitting simultaneously, they will both detect the collision almost immediately. Rather than finish transmitting their frames, which are irretrievably garbled anyway, they should abruptly stop transmitting as soon as the collision is detected. – Quickly terminating damaged frames saves time and bandwidth. – This protocol, is widely used on LANs in the MAC sublayer. In particular, it is the basis of the popular Ethernet LAN. The Data Link Layer (Cont’d) • An analysis of CSMA/CD – Suppose that two stations both begin transmitting at exactly time t0. The minimum time to detect the collision is then just the time it takes the signal to propagate from one station to the other. We consider the following worst-case scenario. – Let the time for a signal to propagate between the two farthest stations be τ. At t0, one station begins transmitting. At τ-ε, an instant before the signal arrives at the most distant station, that station also begins transmitting. Of course, it detects the collision almost instantly and stops, but the little noise burst caused by the collision does not get back to the original station until time 2τ-ε. The Data Link Layer (Cont’d) • An analysis of CSMA/CD (Cont’d) – This means that in the worst case a station cannot be sure that it has seized the channel until it has transmitted for 2τ without hearing a collision. – we will model the contention interval as a slotted ALOHA system with slot width 2τ, and the offered bandwidth can be denoted as: S = Ge-τ – As τ is small (in most conditions), the bandwidth will be large. The Data Link Layer (Cont’d) • Notes on CSMA/CD – a sending station must continually monitor the channel, listening for noise bursts that might indicate a collision. For this reason, CSMA/CD with a single channel is inherently a half-duplex system. – It is impossible for a station to transmit and receive frames at the same time because the receiving logic is in use, looking for collisions during every transmission. The Data Link Layer (Cont’d) • Ethernet – Ethernet Cabling – The Ethernet MAC Sublayer Protocol – Switched Ethernet – Fast Ethernet & Gigabit Ethernet The Data Link Layer (Cont’d) • Ethernet Cabling – The most common kinds of Ethernet cabling (a) 10Base5. (b) 10Base2. (c) 10Base-T. The Data Link Layer (Cont’d) • Ethernet Cabling – 10Base2 (thin Ethernet) • Thin Ethernet is much cheaper and easier to install, but it can run for only 185 meters per segment, each of which can handle only 30 machines. • Detecting cable breaks, excessive length, bad taps, or loose connectors can be a major problem. – 10Base-T • Hubs are used to connect the machines. • Maximum rang is 100 meters, allowing 1024 hosts connection. • Hubs do not buffer incoming traffic. They can be regarded as bus system, which connect parts of the network. The Data Link Layer (Cont’d) • The Ethernet MAC Sublayer Protocol – Frame structure – The high-order bit of the destination address is a 0 for ordinary addresses and 1 for group addresses. – When a frame is sent to a group address, all the stations in the group receive it. Sending to a group of stations is called multicast. The address consisting of all 1 bits is reserved for broadcast. The Data Link Layer (Cont’d) • Switched Ethernet – As more and more stations are added to an Ethernet, the traffic will go up. Eventually, the LAN will saturate. – there is an additional way to deal with increased load: switched Ethernet. The Data Link Layer (Cont’d) • Switched Ethernet – When a station wants to transmit an Ethernet frame, it outputs a standard frame to the switch. The plug-in card getting the frame may check to see if it is destined for one of the other stations connected to the same card. If so, the frame is copied there. If not, the frame is sent over the high-speed backplane to the destination station's card. – From non-store to store-and-forward. The Data Link Layer (Cont’d) • Fast Ethernet & Gigabit Ethernet – FDDI (Fiber Distributed Data Interface) • ring-based optical LAN – Gigabit Ethernet • All configurations of gigabit Ethernet are point-to-point rather than multidrop as in the original 10 Mbps standard. Each individual Ethernet cable has exactly two devices on it, no more and no fewer. The Data Link Layer (Cont’d) – Gigabit Ethernet (Cont’d) • Gigabit Ethernet supports two different modes of operation: full-duplex mode and half-duplex mode. • Full-duplex mode allows traffic in both directions at the same time. This mode is used when there is a central switch connected to computers. In this mode, no contention is possible. • Half-duplex, is used when the computers are connected to a hub rather than a switch. Because a minimum (i.e., 64-byte) frame can now be transmitted 100 times faster than in classic Ethernet, the maximum distance is 100 times less, or 25 meters. The Data Link Layer (Cont’d) • Gigabit Ethernet cabling – 1000Base-SX is suitable for office network; – 1000Base-LX is suitable for backbone of campus network; – 1000Base-CX and 1000Base-T are always the poor man’s gigabit Ethernet. The Data Link Layer (Cont’d) • Retrospective on Ethernet – Ethernet has been around for over 20 years and has no serious competitors in sight, so it is likely to be around for many years to come. Why? • Probably the main reason for its longevity is that Ethernet is simple and flexible. In practice, simple translates into reliable, cheap, and easy to maintain. • Another point is that Ethernet interworks easily with TCP/IP, which has become dominant. IP is a connectionless protocol, so it fits perfectly with Ethernet, which is also connectionless. – How about the competitors? • FDDI、ATM、Fibre Channel and etc. • Although they are faster when they are introduced, they were incompatible with Ethernet, far more complex, and harder to manage. – Adoption without modifications on software is the key issue to survive! The Data Link Layer • Exercises – Please try to solve these problems: • 3.1, 3.4, 3.10, 3.12, 4.2, 4.3, 4.15, 4.24