Islamic University of Gaza Faculty of Engineering Department of Computer Engineering ECOM 4021: Networks Discussion Chapter 2 Getting Connected Eng. Haneen El-Masry March, 2014
2.2 ENCODING Encoding the binary data that the source node wants to send into the signals that the links are able to carry and then to decode the signal back into the corresponding binary data at the receiving node. Network Adaptor: a piece of hardware that connects a node to a link. The network adaptor contains a signaling component that actually encodes bits into signals at the sending node and decodes signals into bits at the receiving node. ENCODING Techniques 1- NRZ: 1 >> High Signal, 0 >> Low Signal. 2- NRZI: 1 >> Transition, 0 >> No Transition. 3- Manchester: 1 >> High to Low Transition, 0 >> Low To High Transition. Manchester => NRZ XOR Clock. 4-4B/5B: Every 4 bits are encoded into a 5-bit code then the resulting bits encoded using NRZI. Note: the 5 bit code has at most one leading 0 and at most two trailing 0. Exercise 1: 2
Exercise 2: 2.3 FRAMING 2.3.1 Byte-Oriented Protocols (BISYNC, PPP, DDCMP) View each frame as a collection of bytes (characters) rather than a collection of bits. Sentinel-Based Approaches BISYNC and PPP use special characters known as sentinel characters to indicate where frames start and end. 3
Character Stuffing If ETX appears in the body of the frame, precede it with a DLE. Also, if DLE appears in the body of the frame, precede it with an extra DLE. Exercise 8: Exercise 9(a): Byte-Counting Approach DDCMP uses the alternative way to detect the end of a file with a sentinel value is to include the number of items in the file at the beginning of the file. 2.3.2 Bit-Oriented Protocols (HDLC) It simply views the frame as a collection of bits. Beginning and Ending Sequences are: 01111110 4
Bit Stuffing On the sending side, any time five consecutive 1s have been transmitted from the body of the message (i.e., excluding Ending Sequence 01111110 sequence ), the sender inserts a 0 before transmitting the next bit. On the receiving side, should five consecutive 1s arrive, the receiver makes its decision based on the next bit it sees (i.e., the bit following the five 1s). If the next bit is a: - 0, it must have been stuffed, and so the receiver removes it. - 1, looking at the next bit, if it sees a: - 0 then it is the end-of-frame marker. - 1 there must have been an error and the whole frame is discarded. Exercise 5: Exercise 7: Exercise 9(b): 5
2.3.3 Clock-Based Framing (SONET) SONET has been for many years the dominant standard for long-distance transmission of data over optical networks. STS-1: is the lowest speed SONET link, and runs at 51.84 Mbps. An STS-1 frame is arranged as 9 rows of 90 bytes each. Each frame is 9 90 = 810 bytes long. The first 3 bytes of each row are overhead, with the rest being available for data that is being transmitted over the link. The first 2 bytes of the frame contain a special bit pattern, and it is these bytes that enable the receiver to determine where the frame starts. The overhead bytes of a SONET frame are encoded using NRZ. The payload bytes are scrambled: this is done by calculating the exclusive OR (XOR) of the data to be transmitted and by the use of a well-known bit pattern. The bit pattern, which is 127 bits long, has plenty of transitions from 1 to 0, so that XORing it with the transmitted data is likely to yield a signal with enough transitions to enable clock recovery. For more details about SONET, see your text book. 6
2.4 ERROR DETECTION 2.4.1 Two-Dimensional Parity Exercise 11: Exercise 12: Exercise 13: 2.4.2 Internet Checksum Algorithm The transmitter adds up all the words that are transmitted and then transmit the result of that sum. The result is the checksum. The receiver performs the same calculation on the received data and compares the result with the received checksum. If any transmitted data, including the checksum itself, is corrupted, then the results will not match, so the receiver knows that an error occurred. 7
2.4.3 Cyclic Redundancy Check (CRC) Exercise 18: (a) M(x)= x 7 +x 6 +x 5 +x+1 C(x)= x 3 +1 T(x)= M(x) * x 3 = x 10 +x 9 +x 8 +x 4 +x 3 x 7 +x 6 +x 5 +x 3 +x 2 x 3 +1 x 10 +x 9 +x 8 +x 4 +x 3 x 10 +x 7 x 9 +x 8 +x 7 +x 4 +x 3 x 9 +x 6 x 8 +x 7 +x 6 +x 4 +x 3 x 8 +x 5 x 7 +x 6 +x 5 +x 4 +x 3 x 7 +x 4 x 6 +x 5 +x 3 x 6 +x 3 x 5 +x 2 x 2 Finally. P(x)= T(x)+x 2 = x 10 +x 9 +x 8 +x 4 +x 3 +x 2 Then the transmitted message should be : 11100011100 x 5 8
(b) When the LMB is inverted, the received data should be 01100011100. Divide the received message by C(x), the remainder will be: 10, the fact that the remainder is nonzero tells us a bit error occurred. 2.5 RELIABLE TRANSMISSION Frames are sometimes corrupted while in transit, corrupt frames must be discarded. A link layer protocol that wants to deliver frames reliably must recover from discarded (lost) frames. Two fundamental mechanisms for reliable delivery: Acknowledgements (ACKs): The receiver sends back an ACK (a small control frame) when it correctly receives a frame Timeouts: If the sender does not receive an ACK after a reasonable amount of time (the timeout period), it retransmits the original frame. 2.5.1 Stop-and-Wait After transmitting one frame, the sender waits for an acknowledgment before transmitting the next frame. If the acknowledgment does not arrive after a certain period of time, the sender times out and retransmits the original frame. Sequence Number Bit When the receiver correctly received and acknowledged the frame, but the Acknowledgment lost. The sender times out and retransmits the original frame, but the receiver will think that it is the next frame. To address this problem, the header for a stop-and-wait protocol usually includes a 1-bit sequence number, that is, the sequence number can take on the values 0 and 1, and the sequence numbers used for each frame alternate. Thus, when the sender retransmits frame 0, the receiver can determine that it is seeing a second copy of frame 0 and therefore can ignore it. Drawback of Stop and Wait The sender has only one outstanding frame on the link at a time, then the link capacity may not be fully utilized. 9
2.5.2 Sliding Window The sender can send more than one frame before the ACK. The Sender: SWS: The send window size, gives the upper bound on the number of outstanding (unacknowledged) frames that the sender can transmit. LAR: The sequence number of the last acknowledgment received. LFS: The sequence number of the last frame sent. The sender also maintains the following invariant: The Receiver: LFS LAR SWS SWS = Bandwidth X RTT / FrameSize (keeping the pipe full) RWS: The receive window size, gives the upper bound on the number of out of order frames that the receiver is willing to accept. LAF: The sequence number of the largest acceptable frame. LFR: The sequence number of the last frame received. The receiver also maintains the following invariant: Is MaxSeqNum SWS+ 1 sufficient? LAF LFR RWS If RWS=1, then MaxSeqNum SWS+ 1 is sufficient. If RWS=SWS, then MaxSeqNum 2*SWS is sufficient. The sliding window algorithm can be used to serve 3 roles: To reliably deliver frames across an unreliable link. To preserve frame order: the receiver does not pass a frame up to the higher layer until it has already passed up all frames with a smaller sequence number. 10
To support flow control the receiver can throttle the sender by informing the sender of how many frames it has room to receive. Exercise 23: Exercise 24: SWS = BW * RTT / FrameSize = (1*10 6 * 1.25*2) / (8*1*10 3 ) = 312.5 312 frame. When RWS=1 then MaxSeqNum SWS+ 1 MaxSeqNum 313 then the sequence number bits= 9. When RWS=SWS then MaxSeqNum 2*SWS MaxSeqNum 2*312 624 then the sequence number bits= 10. 11
Exercise 31: (a) (b) 12
Exercise 32: (a) (b) The second case reduces the total transaction time by roughly 1 RTT. 13
2.6 Ethernet and IEEE 802.3 The Ethernet is a local area network (LAN) technology. The Ethernet is a multiple access network: a set of nodes send and receive frames over a shared link. An Ethernet segment is implemented on a coaxial cable of up to 500 m. Multiple Ethernet segments can be joined together by repeaters. A repeater: is a device that forwards digital signals, it understands bits, not frames. No more than 4 repeaters may be positioned between any pair of hosts, then an Ethernet has a maximum reach of 2500 m. Types of Ethernet Cabling: coaxial, twisted pair, fiber optic. Speeds: 10Mbps, 100Mbps, 1Gbps, 10Gbps, 40Gbps, 100Gbps. Devices: repeaters, hubs. A hub is a multi-way repeater that forwards the signal on all outgoing segments. Any signal placed on the Ethernet by a host is broadcast over the entire network. Signal is propagated in both directions. A repeater forwards the signal on the outgoing segment. Terminators attached to the end of a segment absorb the signal. Ethernet adaptor receives all frames and accepts: frames addressed to its own address frames addressed to the broadcast address frames addressed to a multicast address, if programmed to listen to that address. all frames, if placed in promiscuous mode. All hosts are in the same collision domain: Data transmitted by any one host on the Ethernet reaches all the other hosts. When two hosts transmit frames at the same time, collision occurs. 14
Transmitter algorithm (CSMA/CD): Carrier Sense Multiple Access with Collision Detection. Why must an Ethernet frame be at least 64 bytes long? A host must transmit for 2*d (d=link delay) to be sure that it detects all possible collisions. The RTT of a maximally configured Ethernet (2500 m long with 3 repeaters) has been determined to be 51.2 s; on 10 Mbps Ethernet this corresponds to 512 bits (64 bytes). Ethernets work best under lightly loaded conditions. Under heavy loads, too much of the network s capacity is wasted by collisions. Why Ethernets have been so successful? Easy to administer and maintain: no switches, no routing tables, easy to add a new host. Inexpensive. 15