Media Access Control (MAC) ch12 It is a sublayer of the Data_Link layer to define the coordination of multiple station to a common Link.

Transcription

1 Media Access Control (MAC) ch12 It is a sublayer of the Data_Link layer to define the coordination of multiple station to a common Link. MAC Protocols Random Control access Channelization 1)ALOHA 1)Reservation 1)FDMA 2)CSMA 2)Polling 2)TDMA 3)CSMA/CD 3) Token Passing 3)CDMA 4)CSMA/CA 5) No CS (wireless): MACA, MACAW Channelization: (Static/deterministic) Static (fixed)&simple, yet rigid (not very scalable), without prioritizing nodes/traffics, and varying reasons of wasting BW. Allows N users to share the channel spatially (FDMA each user gets a sub-band of the channel BW), temporally (TDMA each user gets an equal time slot in a round-robin fashion), or both (CDMA all users share the channel simultaneously via pre-assigned orthogonal chip-sequences to encode their data bits). Control access: (Dynamic/Deterministic) In a try to make better BW utilization and increase scalability, there will be more centralized control to control the channel access, yet more control overhead and less robustness (controller vulnerability). Still mostly no distinguishing of node and traffic types priorities, except in Token-Ring frame priority! Random: (Dynamic/Nondeterministic/Decentralized) No rigid assignment of time-slot/sub-bw which leads to wasting of BW, instead users should be polite in using the channel, with varying degrees. Meanwhile, random waiting time (exponential & function of access impairment) is used to separate/shake users temporally in channel access time, lowering users collisions. In addition, users might sense the channel politely for being idle/free. Most developed approaches have very polite access regimes where users might delay (fixed and/or random time) access to channel, even when it is idle.

2 I Random A station might transmit when having data, no scheduling Random. Stations compete for the link access with high potential for collisions. Sender re-tx the frame if no ACK after the longest round trip delay. A) ALOHA: ALOHA: University of Hawaii(1970) radio communication between islands. i)pure ALOHA: Vulnerable period or Possible collision period = 2 B A C bbbbbbbbbbbbbbbbbbbbbbbb aaaaaaaaaaaaaaaaaaaaaaaaa ccccccccccccccc Period One Period Two / Vulnerable Interval / Period One : A must wait for B to finish. Period Two: No one transmits while A is transmitting (e.g, C). Throughput (S): S = G e - 2G --Successful Tx. (Vol. period = 2) where G: Total number of frames (ret X +new) S max is when G = ½ ==> S max = 1/(2e) = (small!!)

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4 ii) Slotted ALOHA: To improve the pure ALOHA s throughput S by cutting the Vulnerable period by half.(how?) Time is slotted into slots each is the time to T X a frame (T f ), and a station (when has a frame) can not T X except at the start of a frame! Hence node A cannot catch up with B s frame above, yet still A and C can transfer at same time! Hence, we still have a VP but only one T f period. The V.P = one T f only now (when A & C T x at same time) S slotted = G e - 1*G (Vol. period = 2) S slotted (MAX/G=1) = 1/e = (doubling the S pure-aloha of 0.184)

5 B) Carrier Sense Multiple Access(CSMA): To reduce the chance of collision, a station will check if the channel is busy or idle before using it (i.e., it senses if the channel has a carrier signal (busy) or not (idle)) Do we still have collision possibility? Yes!(two station check at the same time an idle channel, or with time difference less than the cable delay between them) What is the Vol. Period? τ (one way end-to-end trip delay) Persistence Methods: (See Fig for protocols flow charts) The response of a station after sensing the channel: 1)busy and 2) idle. 1) Persistent: While channel is busy keep checking it. Highest probability of collision since more than one station can sense idle at same time. 2) Nonpersistent: If busy channel, wait random time then recheck it. There is lesser chance for collision because of the random wait by different stations. But, all stations might wait while channel is idle. How? If it becomes idle while all waiting, then no node will check it and it will remain idle until the first node wakes up to check. (i.e., too much politeness might be inefficient!)

6 3) P-Persistent: For slotted time channels with slot = τ (one way endto-end trip delay) combines lesser chance for collision, yet more efficient than nonpersistent. While channel is busy, keep checking. When channel becomes idle, the node waits for a slot, with probability P. If the node waits, then after waking up it will check it again, if idle repeat the idle random wait, otherwise waits until it becomes idle again and repeat the idle random wait. C) Carrier Sense Multiple Access with Collision Detection (CSMA/CD): While T x its frame, a station will monitor the channel detecting any collision.(i.e., CD while T x ing) See Fig Condition: It has to keep monitoring for 2 τ time! Why? Collision might be with the last station in the LAN (τ sec) and its echo must return back (τ sec), for a total of 2τ before the T x ing station can hear the collision. What about if the T f < 2 τ? Problem? Yes, a station can leave the Ether (cable) after finishing its frame, without feeling any collision, which does not mean that its frame did not collide since it might collide with the last station at a echo time distance 2τ > T f. The relation between data-rate (B b/s), network length (L), min. frame length (F min bit), and propagation speed (C m/s) is: (F min / B) = 2 τ = 2 (L/C) (= 51.2 µsec for 10Mb/s 2.5 Km Ethernet) Think varying the values of the involved parameters and see how we can still keep the protocol "/CD" (e.g., raising B [better data rate] will force to raise F [overhead of dummy bits] or C [expensive cables] and/or decreasing L [generally good, less $, but less Ethernet stations] to keep the equation balanced). (THINK of other scenarios.)

7 D) "CSMA/Collision" Avoidance (CSMA/CA): (wireless) Because of the nature of wireless media, collision cannot be detected (Why? Open medium low power, signal fading, need for more BW to T x and R x at same time, hidden terminal problem). Hence, we need to avoid the collision all together (How? By being more polite and wait much more than CSMA/CD, but can we? If not then at least to "alleviate" it by waiting more politely [IFS])!

8 CA is implemented with the aid of: 1) IFS: Even if station senses an idle channel it waits for an IFS --- (we can design it such that: short--> high priority and 2) Long--> low priority traffic). Then, it senses the channel again, if busy wait another IFS. Repeat until channel is found idle, then do not T x directly, instead wait on some contention period or random number of free slots (do not count busy slots!). 2) Contention Window: Time is slotted and a ready station (has frame to T x and channel is idle) will apply Bin. Expo. Back-off strategy for waiting for R random number of free slots to be selected form the range 0 -to- (2 k -1) where k is the number of failure to Tx a given frame by the station. Then after waiting for random R slots, a station T x its frame, then waits for ACK within a pre-specified timeout period. Max number of failure to Tx is K=15 trials. 3) ACK: Collision might still occur, destroying the T x ed frame, hence a station waits for ACK back from the receiver, otherwise it times out and increment k by one, then repeats all over again, if k allows (K <16), otherwise aborts and report to the upper layer!

9 There is also Virtual carrier sense, where sender/receiver process of handshaking is done, instead of the implicit carrier physical listening. Examples: Multiple Access Collision Avoidance (MACA) & MACA for wireless (MACAW) used in the IEEE MAC.

10 II Controlled Access Random access does not allow for the much needed upper bound on the frame delay! Hence, with a sort of deterministic control (DC), we can obtain such upper bound, while optimizing the channel BW utilization. With the aid of more control (DC overhead), the access of the channel is by mutual consent of all stations. 1) Reservation: (Centralized Control) Each station, when having a frame to sent reserves the channel to T x such frame.(how?) A reservation control frame (RCF), with mini slots one/station, precedes the data frames, in every data T x interval (variable size). A station with a frame to send marks its mini slot in the reservation frame, then T x in its turn. A station T x a frame only if it reserves its mini slot. A station missing the RCF will wait for an average of n/2 frames until being able to Tx it (or in worst case a max of n-1 frame Tx slots), yet there is an upper limit on the frame Tx delay. If all station have frames to Tx, i.e., heavy load, it will be a TDM mechanism, but with some extra overhead of the RF. In low traffic load, it resembles the statistical TDM, i.e., single Queue multi-servers, with minimal wasting of BW.

11 2) Polling: (Centralized Control) For better network scalability and maintenance, yet less robustness. One of the station (primary) arbitrates among the rest of stations (secondary) who should control the full access to the channel, one secondary station at a time (T x & R x is only via the primary!). Select phase Poll phase Poll Phase: The primary station polls all secondary nodes, if any has frame to send. NAK from a station is NO, Data frame is YES. NAK --> poll next station. Data frame --> get the data frame with receiver address. ACK --> send ACK (from primary to secondary). Problem-- Yes! The primary is lying, since it is not the intended receiver (IS)! (Solution: primary waits until communicating with the IS, and either send ACK or NAK to the sender. Select Phase: The Primary Station (PS) echoes the polled frame to its intended destination via the select function. SEL --> The PS waits for ACK from destination node (DN) indicating willingness to receive. When such ACK arrives, primary T x the data frame to the DN, and waits for DN s frame arrival ACK. Fast select: Primary sends select&data together and waits for ACK. Disadvantage: Station close to the primary hugs the T x starving the faraway stations.

12 3) Token Passing: (Centralized Control) Can we obtain the efficient statistical TDM without the burden of maintaining a global queue to control T x (especially, in lightly loaded network)? The answer is Yes-- it is the Token Ring MAC protocol! The right to access the channel is done via a special control frame called token, which circulates over the ring, giving the right for ring access. N stations are organized in a ring (physical/logical) where each station will have predecessor and successor (left/right) neighbors, it gets the token from its predecessor (hence the right to access the channel), uses it (only, if it has frames to T x ) for a limited predefined time slot, then passes the token to its successor (neighbor). Each station, when getting the token, a node T x frames for a Token Holding Time (THT) period only, then passes it to its successor, guaranteeing an upper bound on a frame total delay, over the ring N*THT. Centralized network monitoring is needed to monitor/control nodes access to the ring and protocol integrity (lost/destroyed token), plus other functions.

13 Types of Rings (LANS): 1) P_T_P physical Ring 2) Dual ring physical P_t_P 3) Logical ring 4) Hub central monitor to control and implement the ring (most used) The Token Ring (IBM)(IEEE 802.5) Point-to-point ring(physical) of twisted pair, Coaxial, or fiber optics. Digital, with known upper bound delay, about N*THT for N nodes. Number of bits in the ring at one time. Channel data rate = R Mbps ( one bit every 1/R μsec) Then each bit occupies 200/R meters ==> Ring physical length is important. 1-bit delay at each station interface to inspect (possibly modify). When a station needs to transmit, it seizes the special control frame token when it gets it. Each station interface is in one of the following modes at any time: 1) listen: read one bit from the ring into the local buffer and copy it back.( 1 bit delay) 2) transmit: break the ring and insert local data (after seizing the token). The sender removes the bits as they circulate back to it (why?). The sender will get some ACK about the state of its transmitted frame; then save it for reliability monitoring, or discard. After finishing the transmission (for a THT period), a station regenerates the token into the ring and switches back to the listen mode. In heavy load state, the round-robin fashion of rotating the token would make the ring efficiency about 100%, almost TDM. The is shielded twisted pairs running 4 or 6 Mbps using differential Manchester encoding. To make the ring more reliable, the wire centre technique (Hub) is used. Frame Format: 3 bytes token (first 3 bytes in the frame).

14 OCTs: SD AC ED TOKEN Format SD: Start Delimiter FC: Frame Control AC: Access Control ED: End Delimiter FS: Frame Status (no limit!!!!?) SD AC FC Destination Source Checksum ED FS Address Address (error check) Data (LLC fame) Data Frame Format No limit on the data length (except the token holding time, THT). The SD and ED contains invalid DME(differential Manchester Encoding).The ED has one bit set to one if the frames is in error, and one bit as end of file. The destination and source addresses are 6 bytes same format as in the and.4 (Ethernet & Token Bus). The FS byte: It has two bits: A and C that work as ACK bits. Auto ACK A=0 and C= 0 destination not present. A=1 and C =0 destination is present but frame is not accepted A=1 and C = 1 destination is present and frame is accepted(copied) The AC byte: Bits PPP T M RRR The PPP and RRR are used to handle multiple priority frames. A frame is transmitted if has higher priority than the token priority in the PPP field; otherwise not T X. But if not T x ed and its priority is higher than the token RRR field, then it can reserve the token priority for next time by replacing the token's RRR field with its own priority. To eliminate the low priority station starvation problem, the station raising the reservation priority (RRR) is also responsible for lowering it back when it is done by saving the overwritten RRR in its stack, and returning it back after finishing the use of the token (after getting it). The T bit is the token-bit = 0 (free token), = 1 (non-token) frame.

15 The FC byte: Bits 2 6 FF z z z z z z FF bits: 00 = control frame (MAC control PDU) 01 = Data frame (LLC PDU included) --> 6-bit z's will be r r r yyy Where r bit indicates reserved (unused), and YYY is the LLC frame priority (LLC) from source to destination. Monitor responsibilities: 1) Lost token: It keeps a timer with the longest tokenless interval. (sum of the token holding times of all stations), then it drains the ring and issues a new token. 2) Cleaning garbled frames from the network.(invalid format or error) 3) Detecting orphan frames through the M bit in the FC byte of the frame. The Monitor looks at the M bit: If M = 0 the frame is OK, and it will set it to 1; otherwise the Monitor decides that frame's sender ("source") node did not drain it (for some reason), since this is the second time the Monitor sees it, hence the frame is orphan and the Monitor drains it from the ring. 4) Inserting extra delay in the ring, when needed, to be able to hold the token, in case of low number of stations. Problems? Yes, tough to handle control in case of a sick monitor! We can easily detect/handle a dead monitor, rather than to deal with a sick one where we do not know really when it is sick! Based on the traffic load volume (heavy/light), compare the TR protocol with the Reservation and Polling protocols? Do we still have an upper bound on the frame delay over a prioritized TR??

16 III Channelization An approach to allow multi_access to single channel by number of stations utilizing Freq. Division (FDMA), Time division (TDMA), and code division (CDMA) Muxing. Frequency Division Multiple Access (FDMA) Spatial division of the channel BW among users, each access a sub-band of the total BW, all the time (forcing broadband Tx).

17 Time Division Multiple Access (TDMA) Code Division Multiple Access(CDMA) Digits (d) are encoded using chip sequence (c)

18 The entire bandwidth of the channel is used all the time by all stations. Each station will have its orthogonal to the others chip sequence for encoding. Bit to be encoded chip multiplier final encoding for T x > > 1 * chip > > 1 * chip silence > > 0 * chip = [00 0] Notice: How to get N orthogonal chips? Walsh Matrix NXN N = 1, 2, 4, 8, 16, 32,..., 2 i i = 0, 1, 2, 3, 4, All rows and columns are orthogonal. (See fig 12.29) Assume 4 stations S1, S2, S3, S4 D =( d 1.c 1 + d 2.c 2 +d 3.c 3 + d 4.c 4 ) At any station, in order to read the bit T x of any other station say station 2, multiply D by C 2 and divide by 4 (size of the chip sequence). (D.c 2 /4) = ( d 1.c 1.c 2 + d 2.c 2.c 2 + d 3.c 3.c 2 + d 4.c 4.c 2 ) / 4 = 4d 2 /4 = d 2 ( if d 2 = 1 0, if 1 1, if 0 silence)