Transcript File
UNIT III LAN ACCESS TECHNIQUES Introduction Network traffic must flow through some form of media, whether it is a cable, or is wireless. The most common forms of network media are twisted-pair, coaxial, and fiber-optic cable. Twisted-Pair Cable T-P cable is the most common of all of the media types in the average local area network (LAN) environment. Different categories of T-P cable exist. The different categories of cable specify the maximum data bandwidth that the cable can withstand. T-P comes in two forms, Unshielded (UTP) or Shielded (Plenum/STP). Twisted-Pair Categories Category Maximum Data Rate Usual Application CAT-1 < 1 Mbps POTS & ISDN 4 Mbps IBM Token Ring 16 Mbps Voice/Data 10baseT CAT-2 CAT-3 Twisted-Pair Categories (cont.) CAT-4 CAT-5 CAT-7 (in progress) 20 Mbps 16Mbps Token Ring Networks 100 Mbps 100baseT, 155Mb ATM 1000 Mbps 1000baseT, Gigabit Ethernet Twisted-Pair Comparison Advantages Cheap Easy to implement Easy to manage LOTS of different applications Easy to terminate Disadvantages Susceptible to EMF,RF interference Limited distance – 100 meters Twisted-Pair (cont.) Twisted-pair cable (CAT5 and up) consists of 4 separate pairs of wires, all wound separately. UTP is shown on the right. Coaxial Cable Coaxial cable (coax) is almost the same thing that carries your cable TV signal. Data coax is just held to a higher quality. Historical Tidbit: Coax cable, although not commonly seen nowadays, was how Ethernet was developed! Coax (cont.) The physical medium itself consists of an inner wire, surrounded by an insulator, which is also surrounded by a shield. Coax Applications Local Area Networks (LANs) Thinnet (10base2) – 200 meters Thicknet (10base5) – 500 meters Baseband transmissions only Wide Area Networks (WANs) T3/DS3/E3 Broadband transmissions Baseband v. Broadband Baseband is where the medium only carries one signal on the line. Broadband carries multiple signals on a single line. Coax Comparison Advantages Highly shielded from EMF,RF interference Signals propagate much farther than TP cable. Conforms to standards. More channels than TP cable. Disadvantages One cable for all computers. To add additional computers, network must be taken down. MUST properly terminate. Expensive. Low channel count compared to fiber. Fiber Optic Cable Fiber optic cable is where the future of LAN wiring exists. It is wicked fast. It is WICKED fast! Fiber Optic Cable (cont.) Fiber comes in two different types: Multimode – a channelized fiber-optic circuit. Multiple carrier frequencies. Singlemode – a “clear channel” circuit. One carrier frequency. Fiber Comparison Advantages Wicked fast! Handles lots of simultaneous B channels. Very reliable. Disadvantages Cost to implement. Splicing kit. Cable costs. Redundancy (FDDI)? When disaster strikes, it’s a major ordeal. Point-to-point only Fiber Applications High-bandwidth voice transmission. “Backbone” applications. Very fast data transfer between network devices. Network Topologies Network topology is the arrangement of the various elements (links, nodes, etc.) of a computer or biological network. Physical – actual layout of the computer cables and other network devices Logical-The way in which the data access the medium and transmits packets is the Logical Topology Factors Cost Scalability Bandwidth Capacity Ease of Installation Ease of fault finding and maintenance TOPOLOGIES There are three main local area network (LAN) topologies: Bus Star Ring Other network topologies include: Mesh &Wireless Bus Topology Bus Topology Bus Topology (2) Network maintained by a single cable Cable segment must end with a terminator Uses thin coaxial cable (backbones will be thick coaxial cable) Extra stations can be added in a daisy chain manner Bus Topology (3) Standard is IEEE 802.3 Thin Ethernet (10Base2) has a maximum segment length of 200m Max no. of connections is 30 devices Four repeaters may be used to a total cable length of 1000m Max no. of nodes is 150 Bus Topology (4) Thick Ethernet (10Base5) used for backbones Limited to 500m Max of 100 nodes per segment Total of four repeaters , 2500m, with a total of 488 nodes Bus Topology Types (5) Linear bus The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the 'bus', which is also commonly referred to as the backbone, or trunk) All data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously. Bus Topology Types (6) Distributed bus The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology (i.e., all nodes share a common transmission medium) Bus Topology (7) Advantages Inexpensive to install Easy to add stations Use less cable than other topologies Works well for small networks Disadvantages No longer recommended Backbone breaks, whole network down Limited no of devices can be attached Difficult to isolate problems Sharing same cable slows response rates Ring Topology RING TOPOLOGY The ring topology can use twisted pair or fiber optic cabling. A central device (hub) connects hubs and nodes to the network. Each node connects to its own dedicated port on the hub. You can connect multiple hubs to form a larger ring. The ring topology uses the baseband signaling method. Frames are transmitted around the ring from node to hub to node. Media Access Control (MAC) is used for token passing. Ring Topology (2) No beginning or end All devices of equality of access to media Single ring – data travels in one direction only. Each device has to wait its turn to transmit Most common type is Token Ring (IEEE 802.5) A token contains the data, reaches the destination, data extracted, acknowledgement of receipt sent back to transmitting device, removed, empty token passed on for another device to use. Ring Topology (3) Advantages Data packets travel at great speed No collisions Easier to fault find No terminators required Disadvantages Requires more cable than a bus A break in the ring will bring it down Not as common as the bus – less devices available Star Topology STAR TOPOLOGY The star topology can use coaxial, twisted pair, or fiber optic cable. A central device (hub) connects hubs and nodes to the network. Each node connects to its own dedicated port on the hub. Hubs broadcast transmitted signals to all connected devices. You can connect multiple hubs to form a hierarchical star topology. The star topology uses the baseband signaling method. Star Topology (2) Like the spokes of a wheel (without the symmetry) Centre point is a Hub Segments meet at the Hub Each device needs its own cable to the Hub Predominant type of topology Easy to maintain and expand Star Topology (3) Advantages Easy to add devices as the network expands One cable failure does not bring down the entire network (resilience) Hub provides centralised management Easy to find device and cable problems Can be upgraded to faster speeds Lots of support as it is the most used Disadvantages A star network requires more cable than a ring or bus network Failure of the central hub can bring down the entire network Costs are higher (installation and equipment) than for most bus networks Extended Star Topology A Star Network which has been expanded to include an additional hub or hubs. Extended Star Topology Mesh Topology (Web) Mesh Topology (2) Not common on LANs Most often used in WANs to interconnect LANS Each node is connected to every other node Allows communication to continue in the event of a break in any one connection It is “Fault Tolerant” Mesh Topology (3) Advantages Improves Fault Tolerance Disadvantages Expensive Difficult to install Difficult to manage Difficult to troubleshoot Types of Logical Topology Previous slides showed Physical Topologies Only two Logical Topologies (Bus or Ring) Physical Bus or Ring easy to conceptualise Logical Bus •Modern Ethernet networks are Star Topologies (physically) •The Hub is at the centre, and defines a Star Topology •The Hub itself uses a Logical Bus Topology internally, to transmit data to all segments Logical Bus Advantages A single node failure does not bring the network down Most widely implemented topology Network can be added to or changed without affecting other stations Disadvantages Collisions can occur easily Only one device can access the network media at a time Logical Ring Data in a Star Topology can transmit data in a Ring The MAU (Multistation Access Unit) looks like an ordinary Hub, but data is passed internally using a logical ring It is superior to a Logical Bus Hub – see later slide Logical Ring (2) Logical Ring (3) Advantages Disadvantages The amount of data A broken ring will that can be carried stop all in a single message transmissions is greater than on a A device must wait logical bus for an empty token There are no to be able to collisions transmit ETHERNET History The original Ethernet was developed as an experimental coaxial cable Network to operate with a data rate of 3 Mbps using (CSMA/CD) Protocol. Success with that project attracted early attention and specification and led to the 1980 joint development of the 10-Mbps Ethernet Version 1.0. History (cont) The draft standard was approved by the 802.3 working group in 1983 and published as an official standard in 1985. Since then, a number of supplements to the standard have been defined to take advantage of improvements in the technologies and to support: 1) additional network media 2) higher data rate capabilities History (cont) Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox, DEC, and Intel in 1978 LAN standards define MAC and physical layer connectivity IEEE 802.3 (CSMA/CD - Ethernet) standard – originally 2Mbps IEEE 802.3u standard for 100Mbps Ethernet IEEE 802.3z standard for 1,000Mbps Ethernet What is The Ethernet Ethernet refers to the family of local area networks (LAN) products covered by the IEEE 802.3 that operates at many speeds. It defines a number of wiring for the physical layer, through means of Network access at the Media Access Control (MAC)/Data Link Layer, and a Common addressing format. What is The Ethernet (cont) The combination of the twisted pair versions of Ethernet with the fiber optic versions largely replacing standards such as coaxial cable Ethernet. In recent years, Wi-Fi, the wireless LAN standardized by IEEE 802.11, has been used instead of Ethernet for many home and small office networks and in addition to Ethernet in larger installations. General Description Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The common cable providing the communication channel was likened to the ether and it was from this reference that the name "Ethernet" was derived. General Description (cont) Three data rates are currently defined for operation over optical fiber and twisted-pair cables: 10 Mbps—10Base-T Ethernet 100 Mbps—Fast Ethernet 1000 Mbps—Gigabit Ethernet Ethernet Network Elements Ethernet LANs consist of network nodes and interconnecting media. The network nodes fall into two major classes: 1) Data terminal equipment (DTE). 2) Data communication equipment (DCE). The current Ethernet media options include two types of copper cable: (UTP) and (STP), plus several types of optical fiber cable. Physical Layer Configurations for Physical layer configurations802.3 are specified in three parts Data rate (10, 100, 1,000) 10, 100, 1,000Mbps Signaling method (base, broad) Baseband Digital signaling Broadband Analog signaling Cabling (2, 5, T, F, S, L) 5 - Thick coax (original Ethernet cabling) F – Optical fiber S – Short wave laser over multimode fiber L – Long wave laser over single mode fiber Ethernet Standard Defines Physical Layer 802.3 standard defines both MAC and physical layer details Metcalfe’s original Ethernet Sketch Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 185 (~200) meters cable length Thin coaxial cable in a bus topology Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! 10BaseT and 100BaseT 10/100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate “star topology” Distance of any node to hub must be < 100M Ethernet Overview Most popular packet-switched LAN technology Bandwidths: 10Mbps, 100Mbps, 1Gbps Max bus length: 2500m 500m segments with 4 repeaters Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or switch Detects line state and sends/receives signals Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the medium Switches break this model Problem: Distributed algorithm that provides fair access Ethernet Overview (contd.) Ethernet by definition is a broadcast protocol Any signal can be received by all hosts Switching enables individual hosts to communicate Network layer packets are transmitted over an Ethernet by encapsulating Frame Format 64 48 48 16 Preamble Dest addr Src addr Type 32 Body CRC Data link layer divided into two functionality-oriented sublayers Taxonomy of multiple-access protocols discussed in this chapter RANDOM ACCESS In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. Topics discussed in this section: ALOHA Carrier Sense Multiple Access Carrier Sense Multiple Access with Collision Detection Carrier Sense Multiple Access with Collision Avoidance ALOHA Protocol ALOHA is developed in the 1970s at the University of Hawaii. The basic idea is simple: Let users transmit whenever they have data to be sent. If two or more users send their packets at the same time, a collision occurs and the packets are destroyed. ALOHA Protocol If there is a collision, the sender waits a random amount of time and sends it again. The waiting time must be random. Otherwise, the same packets will collide again. A Sketch of Frame Generation Note that all packets have the same length because the throughput of ALOHA systems is maximized by having a uniform packet size. Frames in a pure ALOHA network Throughput Throughput: The number of packets successfully transmitted through the channel per packet time. What is the throughput of an ALOHA channel? Assumptions Infinite population of users New frames are generated according to a Poisson distribution with mean S packets per packet time. Probability that k packets are generated during a given packet time: S k eS Pr[ k ] k! Observation on S If S > 1, packets are generated at a higher rate than the channel can handle. Therefore, we expect 0<S<1 If the channel can handle all the packets, then S is the throughput. Packet Retransmission In addition to the new packets, the stations also generate retransmissions of packets that previously suffered collisions. Assume that the packet (new + retransmitted) generated is also Poisson with mean G per packet time. G k e G Pr[ k ] k! Relation between G and S GS Clearly, At low load, few collisions: G S At high load, many collisions: G S Under all loads, S GP0 where P0 is the probability that a packet does not suffer a collision. Vulnerable Period Vulnerable time for pure ALOHA protocol Throughput Vulnerable period: from t0 to t0+2t Probability of no other packet generated during the vulnerable period is: P0 e 2 G Using S = GP0, we get S Ge 2 G Relation between G and S Max throughput occurs at G=0.5, with S=1/(2e)=0.184. Hence, max. channel utilization is 18.4%. Slotted Aloha time is divided into equal size slots (= pkt trans. time) node with new pkt: transmit at beginning of next slot if collision: retransmit pkt in future slots with probability p, until successful. Success (S), Collision (C), Empty (E) slots Slotted ALOHA Divide time up into discrete intervals, each corresponding to one packet. The vulnerable period is now reduced in half. Probability of no other packet generated during the vulnerable period is: G P0 e Hence, S Ge G slotted ALOHA Note The throughput for slotted ALOHA is S = G × e−G . The maximum throughput Smax = 0.368 when G = 1. Vulnerable time for slotted ALOHA protocol Carrier Sense In many situations, stations can tell if the channel is in use before trying to use it. If the channel is sensed as busy, no station will attempt to use it until it goes idle. This is the basic idea of the Carrier Sense Multiple Access (CSMA) protocol. CSMA Protocols There are different variations of the CSMA protocols: 1-persistent CSMA Nonpersistent CSMA p-persistent CSMA Behavior of three persistence methods 12.85 Flow diagram for three persistence methods 12.86 CSMA: Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others! CARRIER SENSE MULTIPLE ACCESS (CSMA) •CSMA protocol was developed to overcome the problem found in ALOHA i.e. to minimize the chances of collision, so as to improve the performance. •CSMA protocol is based on the principle of ‘carrier sense’. •The chances of collision can be reduce to great extent if a station senses the channel before trying to use it. • Although CSMA can reduce the possibility of collision, but it cannot eliminate it completely. •The chances of collision still exist because of propagation delay. 1-Persistent CSMA •In this method, station that wants to transmit data continuously sense the Channel to check whether the channel is idle or busy. •If the channel is busy , the station waits until it becomes idle. •When the station detects an idle channel, it immediately transmits the frame with probability 1. Hence it is called 1-persistent CSMA. •This method has the highest chance of collision because two or more station may find channel to be idle at the same time and transmit their frames. •When the collision occurs, the stations wait a random amount of time and start all over again. Drawback of 1-persistent •The propagation delay time greatly affects this protocol. Let us suppose, just after the station 1 begins its transmission, station 2 also become ready to send its data and sense the channel. If the station 1 signal has not yet reached station 2, station 2 will sense the channel to be idle and will begin its transmission. This will result in collision. Non –persistent CSMA •A station that has a frame to send senses the channel. •If the channel is idle, it sense immediately. •If the channel is busy, it waits a random amount of time and then senses the channel again. •In non-persistent CSMA the station does not continuously sense the channel for purpose of capturing it when it defects the end of precious transmission . Advantages of non-persistent •It reduces the chances of collision because the stations wait a random amount of time. It is unlikely that two or more stations Will wait for same amount of time and will retransmit at the same time. Disadvantages of non-persistent •It reduces the efficiency of network because the channel remains idle when there may be station with frames to send. This is due to the fact that the stations wait a random amount of time after the collision. p-persistent CSMA •This method is used when channel has time slots such that the time slot duration is equal to or greater than the maximum propagation delay time. •Whenever a station becomes ready to send the channel. •If channel is busy, station waits until next slot. •If the channel is idle, it transmits with a probability p. •With the probability q=1-p, the station then waits for the beginning of the next time slot. •If the next slot is also idle, it either transmits or wait again with probabilities p and q. •This process is repeated till either frame has been transmitted or another station has begun transmitting. •In case of the transmission by another station, the station act as though a collision has occurred and it waits a random amount of time and starts again. Advantages of p-persistent •it reduce the chances of collision and improve the efficiency of the network. Space/time model of the collision in CSMA 12.94 Vulnerable time in CSMA 12.95 A Comparison CSMA/CD Protocol Carrier sense multiple access with collision detection (CSMA/CD) is a Media Access Control method. a carrier sensing scheme is used. a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame. The jam signal is a signal that carries a 32-bit binary pattern sent by a data station to inform the other stations that they must not transmit. CSMA/CD CSMA/CD is a modification of pure carrier sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted. CSMA/CD collision detection 1. 2. 3. 4. 5. Main procedure Is my frame ready for transmission? If yes, it goes on to the next point. Is medium idle? If not, wait until it becomes ready Start transmitting. Did a collision occur? If so, go to collision detected procedure. Reset retransmission counters and end frame transmission. 1. 2. 3. 4. 5. Collision detected procedure Continue transmission until minimum packet time is reached to ensure that all receivers detect the collision. Increment retransmission counter. Was the maximum number of transmission attempts reached? If so, abort transmission. Calculate and wait random backoff period based on number of collisions. Re-enter main procedure at stage 1. APPLICATIONS OF CSMA/CD 1. CSMA/CD was used in now obsolete shared media Ethernet variants (10BASE5, 10BASE2) and in the early versions of twisted-pair Ethernet which used repeater hubs. Modern Ethernet networks built with switches and fullduplex connections no longer utilize CSMA/CD though it is still supported for backwards compatibility. 2. Variations of the concept are used in radio frequency systems that rely on frequency sharing, including Automatic Packet Reporting System. CSMA/CA Carrier sense multiple access with collision avoidance (CSMA/CA) in computer networking, is a network multiple access methodin which carrier sensing is used, but nodes attempt to avoid collisions by transmitting only when the channel is sensed to be "idle". It is particularly important for wireless networks. Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the channel somewhat equally among all transmitting nodes within the collision domain. Although CSMA/CA has been used in a variety of wired communication systems, it is particularly useful in wireless LANs where it is not possible to listen while sending,and therefore collision detection is not possible. The popular 802.11 based schemes use CSMA/CA. IEEE 802.11 MAC Protocol: 802.11 CSMA: sender - if sense channel idle for DIFS sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff 802.11 CSMA receiver: if received OK return ACK after SIFS CSMA/CA RTS-CTS CSMA/CA can optionally be supplemented by the exchange of a Request to Send (RTS) packet sent by the sender S, and a Clear to Send (CTS) packet sent by the intended receiver R. Thus alerting all nodes within range of the sender, receiver or both, to not transmit for the duration of the main transmission. This is known as the IEEE 802.11 RTS/CTS exchange. Implementation of RTS/CTS helps to partially solve the hidden node problem that is often found in wireless networking. Hidden Terminal effect hidden terminals: A, C cannot hear each other obstacles, signal attenuation collisions at B goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit channel reservation sender: send short RTS: request to send receiver: reply with short CTS: clear to send CTS reserves channel for sender, notifying (possibly hidden) stations avoid hidden station collisions Collision Avoidance: RTS-CTS exchange RTS and CTS short: collisions less likely, of shorter duration end result similar to collision detection IEEE 802.11 alows: CSMA CSMA/CA: reservations polling from AP IEEE 802 Subgroups and their Responsibilities 802.1 Internetworking 802.2 Logical Link Control (LLC) 802.3 CSMA/CD 802.4 Token Bus LAN IEEE 802 Subgroups and their Responsibilities (Cont.) 802.5 Token Ring LAN 802.6 Metropolitan Area Network 802.7 Broadband Technical Advisory Group 802.8 Fiber-Optic Technical Advisory Group Token Passing Standards IEEE 802.5 For the token-ring LANs IEEE 802.4 For the token-bus LANs A FDDI protocol is used on large fiber-optic ring backbones IEEE 802 Subgroups and their Responsibilities (Cont.) 802.9 Integrated Voice/Data Networks 802.10 Network Security 802.11 Wireless Networks 802.12 Demand Priority Access LANs Ex: 100BaseVG-AnyLAN INTRODUCTION Token Ring defines a method for sending and receiving data between two networkconnected devices To communicate in a token-passing environment, any client must wait until it receives an electronic token The token is a special frame that is transmitted from one device to the next A Token Bus Network Token Passing in a Token Bus Network Token Passing in a Token Bus Network TOKEN RING Token ring local area network (LAN) technology is a protocol which resides at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that travels around the ring. The Token Token Data packet that could carry data Circulates around the ring Offers an opportunity for each workstation and server to transmit data TOKEN RING Token ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access. This token passing mechanism is shared by ARCNET, token bus, and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet. Token Bus Token Client Server Client A token is distributed to each client in turn. Client TOKEN FRAME When no station is transmitting a data frame, a special token frame circles the loop. This special token frame is repeated from station to station until arriving at a station that needs to transmit data. TOKEN FRAME When a station needs to transmit data, it converts the token frame into a data frame for transmission. Once the sending station receives its own data frame, it converts the frame back into a token. TOKEN FRAME PRIORITY Token ring specifies an optional medium access scheme allowing a station with a high-priority transmission to request priority access to the token. 8 priority levels, 0–7, are used. When the station wishing to transmit receives a token or data frame with a priority less than or equal to the station's requested priority, it sets the priority bits to its desired priority. The station does not immediately transmit; the token circulates around the medium until it returns to the station. FRAME FORMAT A data token ring frame is an expanded version of the token frame that is used by stations to transmit media access control (MAC) management frames or data frames from upper layer protocols and applications. Figure 12-6 Token Bus Frame Local Area Network Technology There are two types of token-passing architectures: Token Bus is similar to Ethernet because all clients are on a common bus and can pick up transmissions from all other stations Token Ring is different from Token Bus in that the clients are set up in a true physical ring structure Token Bus Data Pickup A token is sent from one node to the other The client wanting to transmit grabs an empty token Data is attached Token leaves for the next node and its travel on the bus until it reaches the address to which the data is destined Token Bus Data Delivery Token delivers the data to the addressee Acknowledgement is returned to the sender Token is passed to the next node The process continues If there is an error in delivering the information, a request for retransmission attached to the token and it is sent to the sender Token Bus Standard and Applications IEEE 802.4 It can be used in both broadband and baseband transmission Token Passing Protocol in Operation Circulating Token A D B Workstation Server C Workstation •No collisions Comparison with CSMA/CD Absence of collision Offers a systematic method of transmitting information In theory, it is superior to CSMA/CD More sophisticated to implement Protocols used in the newer and most popular networks are, however, based on CSMA/CD The Transmitting Workstation Waits for a free token in order to be able to attach the data to be transmitted to the token On finding a free token, attach the following: Sender’s address Receiver’s address Data block to be transmitted Error checking details etc. At the Receiving End Data is received and checked for errors Outcomes at the receiving end Data received without errors Date received with errors Error-free Delivery of Data An acknowledgment is attached to the token Acknowledgment is passed to the sender Token is set free for other nodes to transmit information At this time, the next workstation on the ring will receive an opportunity Correcting Errors in Delivery A request for retransmission is attached to the token Token carries the message for retransmission to the sender The data is thus retransmitted Token Regeneration The token is regenerated at regular intervals to sustain the timing of circulation of the token Usage of Token Passing Used extensively in ring LANs Especially in the IBM token-ring LAN A version of this protocol is also used on certain types of bus LANs Token-bus networks Used in large fiber-optics backbones Used for the construction of very large networks Usage in Practice Used in backbones Uses in a number of IBM shops Overall, the usage of Ethernet surpasses the usage of Token-Ring networks that are based on the Token-Passing protocol