# Goals of Today s Lecture. Adaptors Communicating

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3 4-bit/5-bit (100Mb/s ) Goal: address inefficiency of Manchester encoding, while avoiding long periods of no transition Solution: Use 5 bits to encode every sequence of four bits such that No 5 bit code has more than one leading 0 or two trailing 0 s Use NRZI to then encode the 5 bit codes Efficiency is 80% 5 Minute Break 4-bit 5-bit 4-bit 5-bit Questions Before We Proceed? 14 Framing Specify how blocks of data are transmitted between two nodes connected on the same physical media Service provided by the data link layer Implemented by the network adaptor Challenges Decide when a frame starts & ends How hard can that be? Simple Approach to Framing: Counting Sender: begin frame with byte(s) giving length Receiver: extract this length and count 53! Frame content! 53 bytes of data 21! Frame content! 21 bytes of data How can this go wrong? On occasion, the count gets corrupted 58! Frame content! 21! Frame 94! content! Bogus new frame length; bytes of data misdelivered desynchronization Framing: Sentinels Delineate frame with special pattern Both can be the same, e.g., Problem: what if sentinel occurs within frame? Solution: escape the special characters Sender: insert a 0 after five 1s in data portion Receiver: when it sees five 1s makes a decision on next two bits: ! Frame content! ! If next bit 0 (this is a stuffed bit), remove it If next bit, look at next bit If 0, this is end of frame (receiver has seen ) If 1 this is an error, discard frame (receiver as seen ) Example Sender: stuff bits Receiver: identify & remove stuffed bits Five 1s insert a 0 Five 1s and next bit 0 remove it Five 1s, and next two bits 10 end/start frame

6 Slotted ALOHA Slotted ALOHA Assumptions All frames same size Time divided into equal slots (time to transmit a frame) Nodes are synchronized Nodes begin to transmit frames only at start of slots No carrier sense If two or more nodes transmit, all nodes detect collision Operation When node obtains fresh frame, transmits in next slot No collision: node can send new frame in next slot Collision: node retransmits frame in each subsequent slot with probability p until success Pros Single active node can continuously transmit at full rate of channel Highly decentralized: only slots in nodes need to be in sync Simple Cons Collisions, wasting slots Idle slots Nodes may be able to detect collision in less than time to transmit packet synchronization Efficiency of Slotted Aloha CSMA (Carrier Sense Multiple Access) What is the maximum fraction of successful transmissions? Suppose N stations have packets to send Each transmits in slot with probability p Probability of successful transmission S is (very approximate analysis!): by a particular node i: S i = p (1-p) (N-1) by exactly one of N nodes S = Prob (only one transmits) = N p (1-p) (N-1) <= 1/e = 0.37 but must have p proportional to 1/N 33 Collisions hurt the efficiency of ALOHA protocol 34 Utilization 1/e 37% CSMA: listen before transmit If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don t interrupt others! CSMA Collisions CSMA/CD (Collision Detection) Collisions can still occur: propagation delay means two nodes may not hear each other s transmission in time. At time t 1, D still hasn t heard B s signal sent at the earlier time t 0, so D goes ahead and transmits: failure of carrier sense. Collision: entire packet transmission time wasted 35 CSMA/CD: carrier sensing, deferral as in CSMA 36 Collisions detected within short time Colliding transmissions aborted, reducing wastage Collision detection Easy in wired LANs: measure signal strengths, compare transmitted, received signals Difficult in wireless LANs Reception shut off while transmitting Even if on, might not be able to hear the other sender, even though the receiver can Leads to use of collision avoidance instead 6

7 CSMA/CD Collision Detection Both B and D can tell that collision occurred. This lets them (1) know that they need to resend the frame, and (2) recognize that there s contention and adopt a strategy for dealing with it. Note: for this to work, we need restrictions on minimum frame size and maximum distance 37 Three Ways to Share the Media Channel partitioning MAC protocols (TDMA, FDMA): 38 Share channel efficiently and fairly at high load Inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Taking turns protocols Eliminates empty slots without causing collisions Overhead in acquiring the token Vulnerable to failures (e.g., failed node or lost token) Random access MAC protocols Efficient at low load: single node can fully utilize channel High load: collision overhead Next Lecture + bridges/hubs/switches K&R 5.5,

Links 1 Goals of Today s Lecture Link-layer services Encoding, framing, and error detection Error correction and flow control Sharing a shared media Channel partitioning Taking turns Random access Shared

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