Mac sub layer

Editor's Notes

• #3: This presentation deals with broadcast networks and their protocols.
• #8: Whenever two frames will try to occupy the channel at the same time, there will be a collision, and both will be garbled. ALOHA is using fixed length frames.
• #11: If we are looking at the when a frame can suffer collisions, we can see that there is a vulnerable period of 2 times the frame time.
• #16: The propagation delay has an important effect on the performance of the protocol. There is a small chance that just after a station begins sending, another station will sense the channel and start sending (before the signal from the first one reached it). In this situation a collision can occur. 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 – imagine two station wanting to transmit data at the same time. But a third one is buy transmitting. Both station will wait until the third will finish is transmission, will sense idle channel and will start sending.
• #20: This protocol is widely used CSMA/CD. In particular is the basis for Ethernet LAN.
• #21: At the point marked t0, a station has finished transmitting its frame. Any other station having a frame to send may now attempt to do so. If two or more stations decide to transmit simultaneously, there will be a collision. Collision can be detected by looking at the power of the line or at the pulse width of the received signal and comparing it with the transmitted signal. After a station detects a collision, aborts its transmission, waits a random period of time and tries again latter. Therefore the CSMA/CD will consist of alternating contention and transmission periods, with idle periods occurring when all stations are quiet. The minimum time to detect a collision is two times the time it takes the signal to propagate from one station to another. In worst case scenario, the two stations can be at the ends of the cable … therefore, the minimum time to detect a collision is the round trip propagation delay for the whole cable segment. The sending station has to monitor the channel for collisions during transmission. Therefore the CSMA/CD is a half duplex system. It is important to realize that the collision detection is an analog process. The station’s hardware must listen to the cable while is transmitting. If what it reads back is different than what it sends, then a collision must have had happen. The implication is that the signal encoding must allow collisions to be detected (i.e. a collision of two 0 V signals will never be detected). For this reason, special encoding is used.
• #23: Each contention period consists of exact N slots. If station 0 has a frame to send, then it transmits a slot during zero-th contention slot. No other station is allowed to transmit during this slot. Regardless of what station 0 does, station 1 gets the opportunity to transmit a 1 bit during the contention slot 1, but only if it wants to send a frame (has a queued frame). After all N slots have passed, each station has complete knowledge of which stations whish to transmit. Al that point they begin transmitting in numerical order. Since everyone agrees who goes next, there is no collisions. Protocols like this are called RESERVATION PROTOCOLS.
• #24: A problem with bit map protocols is that the overhead per station is fixed (on bit), therefore it is not scaling well with large number of stations. In binary countdown protocol, all stations will have same length addresses. A station wanting to use the channel broadcasts its address as a binary bit string, starting with the high order bit. The bits in each address position, from each station are BOOLEAN OR-ed together. This is called binary countdown. All the stations will see the result of the OR operation instantaneously. So an station that placed an 0 on the channel sees a 1, than it will stop trying. A higher priority station wants to transmit therefore it has to stop trying this time. In other words, as soon as a station has seen that a high order bit in its address with value 0 has been overwritten with a 1, it gives up. Consider the stations 0010, 0100, 1001 and 1010 are all trying to get the channel. Station having address 1010 gets the channel (due to the fact that this protocol is giving priority to the stations with higher address).
• #25: CSMA doesn’t work because of the hidden station problem. It is important to realize that in a wireless LAN system not all stations are within the range of the others. Due to this, HIDDEN STATION PROBLEM can occur (when A transmits, C can’t sense it and it can transmit too … the frame from A to B gets garbled if C decides to transmit at the same time). Therefore a simple CSMA approach will not work for wireless LANs. EXPOSED STATION PROBLEM can also occur (the reverse situation – B transmits to A. If C senses the medium, it will hear an ongoing transmission and falsely conclude that may not send to D, when in fact such transmission would cause bad reception only in regions between B and C. In conclusion, before starting a transmission, a station wants to know if there is activity around the receiver.
• #26: MACA (Multiple Access with Collision Avoidance) – the sender stimulates the receiver to output a short packet, so the stations around it will hear it and understand to avoid transmission during the upcoming (large) data frame. In this slide we have station A wanting to send data to station B. A short exchange of packets RTS (Request to Send) and CTS (Clear To Send) takes place. RTS (short frame, less than 30 bytes) contains the length of the data that is about to be transferred (data frame). This data is copied also into the CTS frame by the receiver. When A receives the CTS frame, it begins the transmission of the data frame.
• #29: 10Base5 or Thick Ethernet. Connections onto the cable are done using some vampire taps. 10 represents the speed Base represents the transmission method (base or broadband) 5 represents the maximum length of a single, non amplified sector (500 meters or 200 meters).
• #30: In 10Base5 the transceiver and the interface board are separated. The interface board goes inside the computer, while the transceiver goes on the cable (close to the vampire tap). The transceiver contains the electronics that handle the collision detection. In 10Base2, the connection to the cable is just a passive BNC T junction. The transceiver electronics are situated onto the controller board (inside the PC). In 10BaseT there is no shared cable at all, just the hub. Each station is connected to the hub, using a dedicated, non shared cable.
• #31: Each version of Ethernet has a maximum cable length per segment. To allow larger networks, multiple cables can be connected by repeaters. A repeater is a physical layer device. It just amplifies and restores the shape of the digital signals, in both directions. A system may contain multiple cable segments and multiple repeaters, but no two transceivers can be more than 2.5 KM away. No path between any two transceivers may traverse more than four repeaters. A linear cable running through the building with stations tapped onto it A spine cable running from top of the building to the basement, with each floor connected onto it through repeaters Tree cable topology, using hubs. Most used Multiple cables connected by repeaters, to allow for larger networks.
• #32: None of Ethernet variants use binary encoding with 0V for a 0 and 5V for a 1, since it leads to ambiguities on the bus. IF one station sends the bit string 00010000 others might falsely interpret it as 10000000 or 01000000 or other combinations, because they cannot tell the difference between an idle sender (0V) and a 0 bit (0V). To overcome this, +1V could be used for 1 and -1V for 0, but we still have the problem of synchronization between the sender and the receiver, where different clock speeds could get the receiver and the sender to desynchronize. Manchester encoding (two variants) provides a way of precisely determine the start, end or middle of each bit without reference to an external clock. MANCHESTER ENCODING (b) ensures that every bit has a transition in the middle, making it easy for the receiver to synchronize with the sender: each bit period is divided into two equal intervals A binary 1 is sent by having the voltage set high during the first interval and low in the second one A binary 0 is sent by having the voltage set low during the first interval and high in the second one A disadvantage of Manchester encoding is that it needs twice as much bandwidth as binary encoding, because the pulses are half the width. DIFERENTIAL MANCHESTER ENCODING: A binary 1 is indicated by an absence of a transition at the beginning of the interval A binary 0 is indicated by the presence of a transition at the starting of the interval In any case, there is a transition in the middle as well, to help the receiver synchronize its clock. Ethernet is using Manchester encoding. Differential Manchester encoding is used by Token Ring.
• #33: There are two variants of Ethernet. The initial one (DIX, initiated by Xerox, Intel and DEC) and the IEEE 802.3 standard. The differences are minor and they are interoperable. Each frame starts with Preamble of 8 bytes, each containing the bit pattern 10101010. The Manchester encoding on this pattern produces a 10MHz square wave for 6.4 us to allow the receiver's clock to synchronize with the sender. The clock will stay in sync for the rest of the frame, using the transitions in the middle of the bit boundaries for adjustment. Ethernet addresses: High order bit distinguishes between normal and group addresses (1 for group, 0 for normal) (allow multiple stations to listen to one address, when a frame is sent to a group address, all the stations in that group receive it). Second high order bit distinguishes between local and global addresses (1 for local, 0 for global) All 1 is reserved fro broadcast address. Type field in DIX: - tells to the receiver what to do with the frame. Multiple network protocols could be supported. So this field is unique for each network protocol. It is used to dispatch the incoming frames. IEEE 802.3 changed the type field in Length field – the type is handled as part of the data itself, in a small header. The length field contains the length in bytes of the data, up to 1500 bytes. Data, up to 1500 bytes. This limit was chosen arbitrarily at the time DIX was released. It was based on the fact that a transceiver needed enough RAM to hold an entire data frame and RAM was expensive back in 1978. There is also a minimum frame length, related to the collision detection. The explanation is given in the next slide. The minimum Ethernet frame length is 64 bytes (without the preamble), and if the data portion is less than 46 bytes (64 – headers + checksum) then the PAD field is used to fill the frame to the minimum size. Checksum is a 32 bit hash code of the data, computed with the CRC algorithm presented in the Data Link Layer, having a 32 order generator polynomial. It just does error detection, no forward error correction.
• #34: The strongest reason why there is a minimum length for the Ethernet frame is to allow the CSMA/CD protocol to work properly. In other words, the collisions should be detected by the transmitting station, or preventing a transmitting station to finish the transmission of a frame until the first bit reached the far end of the cable, where it may collide with another frame. If we have a collision an time t, the station B (that detected the collision first) issues a 48 bit noise burst (known as jam signal) to warn the other stations. At about 2t time, the sender (station A) will see the noise burst and aborts its transmission too. It waits a random time until it tries again. For a 2500 meters LAN with 4 repeaters, the round trip time (the propagation time for a signal to travel from one end of the LAN to the other end of the LAN and back) is about 50us. That is equivalent to about 500 bit times (one bit time for 10Mbps network is 100ns). Therefore, a station should be in transmit mode for at least 500 bit times, which gives us the minimum Ethernet frame of about 500 bits. The standard’s choice was 512 bits or 64 bytes minimum frame. So frames with fewer than 64 bytes will be padded out to 64 bytes. As the network speed goes up, the minimum frame length goes up too.
• #36: More stations added on an Ethernet, the traffic will go up. Eventually, at one point the LAN will saturate. Switches are typically devices that have 4 to 32 plug-in cards, each with one to 8 Ethernet connectors (RJ45). When a station wants to send an Ethernet frame, the plug-in card getting the frame will check to see if the destination address is of one of the cards connected onto one of its ports. If yes, then the packet is copied onto that port. If not, then, using a multi-gigabit per second back plane, the frame is copied to the card containing the destination port.
• #38: Anyone who understands classic Ethernet already understands much about Fast Ethernet. Fast Ethernet uses the same cabling and access method as 10Base-T. With certain exceptions, Fast Ethernet is simply regular Ethernet, just ten times faster. Probably the most popular form of Fast Ethernet is 100BASE-TX . 100BASE-TX runs on UTP Category 5 unshielded twisted pair, sometimes called UTP-5. It uses the same pair and pin configurations as 10Base-T, and is topologically similar in running from a number of stations to a central hub. As an upgrade to 10Mbps Ethernet over Multimode fiber (10Base-F), 100BASE-FX is Fast Ethernet over fiber. Distances up to 2km are supported. Fast Ethernet is possible on Category 3 UTP with 100BASE-T4 . There is a popular misconception that Fast Ethernet will only run on Category 5 cable. That is true only for 100BASE-TX. If you have Category 3 cable with all four pairs (8 wires) connected between station and hub, you can still use it for Fast Ethernet by running 100BASE-T4. 100BASE-T4 sends 100Mbps over the relatively slow UTP-3 wire by fanning out the signal to three pairs of wire. This "demultiplexing" slows down each byte enough that the signal won't overrun the cable. Category 3 cable has four pairs of wire, eight wires total, running from point to point. 10Base-T only uses four wires, two pairs. Some cables only have these two pairs connected in the RJ-45 plug. If the category 3 cabling at your site has all four pairs between hub and workstation, you can use Fast Ethernet by running 100BASE-T4.
• #39: The primary difficulty with 100 Mbps transmission of data is that high-frequency signals don't propagate well over either twisted pair or fiber. 10 Mbps Ethernet uses Manchester encoding to include a clock signal with every data bit. However, the clocking almost doubles the rate of transmission, so a worst-case scenario would transmit 10Mbps of data with a 10MHz waveform. For 100 Mbps, the waveform frequency would peak at 100MHz. Category 5 UTP is only rated at 100MHz, so Fast Ethernet would be difficult to implement. 100BASE-FX uses NRZI (Non-Return-to-Zero, Invert-on-one). To decrease the frequency even further on UTP, 100BASE-TX adds a variation of NRZI at the PMD sub-layer called either MLT-3 (Multiple Level Transition - 3 levels) or NRZI-3 .
• #41: MLT-3 encodes a bit as presence or lack of transition, exactly as in NRZI . What makes MLT-3 different is that the base waveform is a 3-state alternating wave. Rather than alternating between 0 and 1 as in Manchester encoding and NRZI, MLT-3 alternates from -1 to 0 to +1, back to 0, then back to -1, repeating indefinitely. A zero is encoded as a halt in the back-and-forth progression. It may be useful to think of MLT-3 as a stop-and-go sine wave, encoding 0 as stop and 1 as go. Using MLT-3, it is therefore possible to represent four or more bits with every complete waveform, at 0, +1, 0, and -1 With 4B/5B encoding, the data rate of the cable goes up to 125Mb. After the MLT-3 encoding the base frequency of the encoded signal goes down by 4 (due to the way the encoding works), therefore the signal that is transmitted over the cable will have a maximum frequency component of about 31.25Mhz, which is handled very well by the UTP5 cable. The data being transmitted in this example is the hexadecimal byte "0E". First, the byte is broken down into four bit "nibbles", giving a nibble of "0" and a nibble of "E". Then, each nibble is looked up in a table to find the byte code associated with that hexadecimal number. The code for "0" hex is "11110" and the code for "E" hex is "11100". Finally, the byte codes are transmitted onto the wire using MLT-3 encoding.
• #49: The data being transmitted in this example is the hexadecimal byte "0E". First, the byte is broken down into four bit "nibbles", giving a nibble of "0" and a nibble of "E". Then, each nibble is looked up in a table to find the byte code associated with that hexadecimal number. The code for "0" hex is "11110" and the code for "E" hex is "11100". Finally, the byte codes are transmitted onto the wire using NRZI encoding.
• #50: It supports two modes of operation: Full Duplex mode All the stations are connected through a switch that allows traffic in both directions at the same time Since all possible connections are point to point, the CSMA/CD protocol is not employed anymore Since on the line between the computer and the switch, no other devices are present The max length of the cable is now governed by signal propagation issues rather than the round trip propagation Half duplex mode All stations are connected through a hub, that internally electrically connects all the lines, simulating a multidrop environment as with classic Ethernet electrically connects all the lines
• #51: Since 25 meters is not an acceptable radius (to maintain the minimum 64 bytes frame), two methods have been adopted by the standards committee: Carrier extension Frame bursting
• #54: Logical Link Control protocol runs in the top of all IEEE 802 protocols. It hides all the aspects of the 802 protocols, and it provides to the network layer a single format and interface. Usage of LLC: The network layer of sending machine passes a network layer packet onto the LLC layer, using the LLC layer access primitives The LLC sublayer adds the LLC header, containing sequences and acknowledgement numbers The resulting structure is then inserted into the payload filed of an 802 frame and transmitted. At receiver, the reverse process takes place. LLC provides three service options: Unreliable datagram service Acknowledged datagram service Reliable connection oriented service