UNIT-IV

Local Area Network:

A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a larger geographic distance, but also generally involves leased telecommunication circuits.

LAN Applications

The variety of applications for LANs is wide. To provide some insight into the types of requirements that LANs are intended to meet, the following sections discuss some of the most important general application areas for these networks.

Personal Computer LANs

A common LAN configuration is one that supports personal computers. With the relatively low cost of such systems, individual managers within organizations often independently procure personal computers for departmental applications, such as spreadsheet and project management tools, and for Internet access.

But a collection of department-level processors won't meet all of an organization's needs; central processing facilities are still required. Some programs, such as econometric forecasting models, are too big to run on a small computer. Corporate-wide data files, such as accounting and payroll, require a centralized facility but should be accessible to a number of users. In addition, there are other kinds of files that, although specialized, must be shared by a number of users. Further, there are sound reasons for connecting individual intelligent workstations not only to a central facility but to each other as well. Members of a project or organization team need to share work and information. By far the most efficient way to do so is digitally.

Certain expensive resources, such as a disk or a laser printer, can be shared by all users of the departmental LAN. In addition, the network can tie into larger corporate network facilities. For example, the corporation may have a building-wide LAN and a wide area private network. A communications server can provide controlled access to these resources.

LANs for the support of personal computers and workstations have become nearly universal in organizations of all sizes. Even those sites that still depend heavily on the mainframe have transferred much of the processing load to networks of personal computers. Perhaps the prime example of the way in which personal computers are being used is to implement client/server applications.

For personal computer networks, a key requirement is low cost. In particular, the cost of attachment to the network must be significantly less than the cost of the attached device. Thus, for the ordinary personal computer, an attachment cost in the hundreds of dollars is desirable. For more expensive, high-performance workstations, higher attachment costs can be tolerated. In any case, this suggests that the data rate of the network may be limited; in general, the higher the data rate, the higher the cost.

1

Back-End Networks and Storage Area Networks

Back-end networks are used to interconnect large systems such as mainframes, supercomputers, and mass storage devices. The key requirement here is for bulk data transfer among a limited number of devices in a small area. High reliability is generally also a requirement. These are some typical characteristics:

High data rate. To satisfy the high-volume demand, data rates of 100 Mbps or more are required.

High-speed interface. Data transfer operations between a large host system and a mass storage device are typically performed through high-speed parallel I/O interfaces, rather than slower communications interfaces. Thus, the physical link between station and network must be high speed.

Distributed access. Some sort of distributed medium access control (MAC) technique is needed to enable a number of devices to share the medium with efficient and reliable access.

Limited distance. Typically, a back-end network will be employed in a computer room or a small number of contiguous rooms.

Limited number of devices. The number of expensive mainframes and mass storage devices found in the computer room generally numbers in the tens of devices.

Types of Network Topology

Network Topology is the schematic description of a network arrangement, connecting various nodes(sender and receiver) through lines of connection.

BUS Topology

Bus topology is a network type in which every computer and network device is connected to single cableas shown in fig4.1. When it has exactly two endpoints, then it is called Linear Bus topology.

Fig 4.1:Bus topology

2

Features of Bus Topology

1. It transmits data only in one direction.

2. Every device is connected to a single cable

Advantages of Bus Topology

1. It is cost effective.

2. Cable required is least compared to other network topology.

3. Used in small networks.

4. It is easy to understand.

5. Easy to expand joining two cables together.

Disadvantages of Bus Topology

1. Cables fails then whole network fails.

2. If network traffic is heavy or nodes are more the performance of the network decreases.

3. Cable has a limited length.

4. It is slower than the ring topology.

RING Topology

It is called ring topology because it forms a ring as each computer is connected to another computer, with the last one connected to the first. Exactly two neighbors for each deviceas shown in fig 4.2.

Fig 4.2: Ring topology

3

Features of Ring Topology

1. A number of repeaters are used for Ring topology with large number of nodes, because if someone wants to send some data to the last node in the ring topology with 100 nodes, then the data will have to pass through 99 nodes to reach the 100th node. Hence to prevent data loss repeaters are used in the network.

2. The transmission is unidirectional, but it can be made bidirectional by having 2 connections between each Network Node, it is called Dual Ring Topology.

3. In Dual Ring Topology, two ring networks are formed, and data flow is in opposite direction in them. Also, if one ring fails, the second ring can act as a backup, to keep the network up.

4. Data is transferred in a sequential manner that is bit by bit. Data transmitted, has to pass through each node of the network, till the destination node.

Advantages of Ring Topology

1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having tokens can transmit data.

2. Cheap to install and expand

Disadvantages of Ring Topology

1. Troubleshooting is difficult in ring topology.

2. Adding or deleting the computers disturbs the network activity.

3. Failure of one computer disturbs the whole network.

STAR Topology

In this type of topology all the computers are connected to a single hub through a cable. This hub is the central node and all others nodes are connected to the central node as shown in fig4.3.

4

Fig 4.3: Star topology

Features of Star Topology

1. Every node has its own dedicated connection to the hub.

2. Hub acts as a repeater for data flow.

3. Can be used with twisted pair, Optical Fibre or coaxial cable.

Advantages of Star Topology

1. Fast performance with few nodes and low network traffic.

2. Hub can be upgraded easily.

3. Easy to troubleshoot.

4. Easy to setup and modify.

5. Only that node is affected which has failed, rest of the nodes can work smoothly.

Disadvantages of Star Topology

1. Cost of installation is high.

2. Expensive to use.

3. If the hub fails then the whole network is stopped because all the nodes depend on the hub.

4. Performance is based on the hub that is it depends on its capacity

MESH Topology

It is a point-to-point connection to other nodes or devices. All the network nodes are connected to each other. Mesh has n(n-1)/2 physical channels to link n devices as shown in fig 4.4 .

There are two techniques to transmit data over the Mesh topology, they are :

1. Routing

2. Flooding

5

Fig 4.4: Mesh topology

MESH Topology: Routing

In routing, the nodes have a routing logic, as per the network requirements. Like routing logic to direct the data to reach the destination using the shortest distance. Or, routing logic which has information about the broken links, and it avoids those node etc. We can even have routing logic, to re-configure the failed nodes.

MESH Topology: Flooding

In flooding, the same data is transmitted to all the network nodes, hence no routing logic is required. The network is robust, and the its very unlikely to lose the data. But it leads to unwanted load over the network.

Types of Mesh Topology

1. Partial Mesh Topology : In this topology some of the systems are connected in the same fashion as mesh topology but some devices are only connected to two or three devices.

2. Full Mesh Topology : Each and every nodes or devices are connected to each other.

Features of Mesh Topology

1. Fully connected.

2. Robust.

3. Not flexible.

Advantages of Mesh Topology

1. Each connection can carry its own data load.

2. It is robust.

3. Fault is diagnosed easily.6

4. Provides security and privacy.

Disadvantages of Mesh Topology

1. Installation and configuration is difficult.

2. Cabling cost is more.

3. Bulk wiring is required.

TREE Topology

It has a root node and all other nodes are connected to it forming a hierarchy as shown in fig 4.5. It is also called hierarchical topology. It should at least have three levels to the hierarchy.

Features of Tree Topology

1. Ideal if workstations are located in groups.

2. Used in Wide Area Network.

Fig 4.5:Tree topology

Advantages of Tree Topology

1. Extension of bus and star topologies.

2. Expansion of nodes is possible and easy.

3. Easily managed and maintained.

4. Error detection is easily done.

7

Disadvantages of Tree Topology

1. Heavily cabled.

2. Costly.

3. If more nodes are added maintenance is difficult.

4. Central hub fails, network fails.

HYBRID Topology

It is two different types of topologies which is a mixture of two or more topologies as shown in fig 4.6. For example if in an office in one department ring topology is used and in another star topology is used, connecting these topologies will result in Hybrid Topology (ring topology and star topology).

Features of Hybrid Topology

1. It is a combination of two or topologies

2. Inherits the advantages and disadvantages of the topologies included

Fig 4.6: Hybrid topology

Advantages of Hybrid Topology

1. Reliable as Error detecting and trouble shooting is easy.

8

2. Effective.

3. Scalable as size can be increased easily.

4. Flexible.

Disadvantages of Hybrid Topology

1. Complex in design.

2. Costly.

LAN Protocol Architecture

Protocols needed to transfer data frames among stations connected on the same physical link in a local area are data link control protocols. The original specifications of the OSI's Data Link Control protocol standard, the HDLC, predates the advent of local area networks (LANs). HDLC has been designed for connecting remotely located DTEs connected with links that are relatively lower data rates and more likely to subject to distortion and noise, while the protocols for LANs need to access much higher data rates, possibly over a more reliable physical medium. To achieve this objective, the protocol standard organization divide the Data Link Control layer into two sublayers, with the physical medium access and the data transfer functions being specified as a new sublayer, the MAC (Media Access Control), that can meet the need of a LAN environment. MAC replaces the physical medium access and the data transfer functions that would otherwise be performed by HDLC. The remaining functions of HDLC are performed by another new sublayer, the LLC(Logical Link Control).

9

Fig 4.7 LAN protocol architecture

Under IEEE LAN standards, all LAN protocols share the common LLC specifications, but vary with MAC and physical layer specifications. The standard for LLC is designated as IEEE 802.2. One service being performed by the LLC is the access to the network layer. LLC is able to perform other services that are similar to HDLC's services as shown in fig 4.7. But in actual implementations, these other services are rarely employed.

Among the popular IEEE LAN standard specifications are 802.3 for the Ethernet, 802.5 for the Token ring and 802.11 for the wireless LAN.

LAN architecture

The LAN architecture consists of three layers: Physical, MAC (Medium Access Control) and LLC (Logical Link Control).

o LLC provides connection management, if needed. (For most applications, it is not needed.)

o MAC is a protocol for accessing high speed physical links and for transferring data frames from one station to another.

o Physical layer deals mainly with actual transmission and reception of bits over the transmission medium. Its specification depends on the specific physical medium and MAC protocols it interfaces with.

10

In reference to the OSI Reference Model, the Physical layer is the same. The LLC and MAC are sublayers of the DLC layer.

 LAN ARCHITECTURE --> LAN Architecture Layers

A LAN architecture consists of three layers:

1. Physical

o Encoding / decoding of signals (converting codes between those used in the user stations and those used for transmission)

o Preamble / postamble generation and removal (for synchronization)

o Bit transmission and reception

2. Media Access Control (MAC)

o Controlling access to the transmission medium (inluding frame synchronization).

o Addressing

o Error detection

3. Logical Link Control (LLC) 

o Access to the network layer

o connection management (unacknowledged connections, acknowledged connectionless, connection-oriented)

MAC Frame Format

Figure4.8 below is a MAC frame that contains MAC's key fields. Actual MAC frame format varies slightly with different LAN standards.

Variable 6 octets 6 octets Variable 4 octets

MAC Control DA SA LLC PDU CRC

MAC MAC Control Control information for MAC protocol. Variable in

11

Control length and contents. Depending on specific LAN Standards.

DA Destination Address

Destination physical attachment point on the LAN's transmission media.

SA Source AddressSource physical attachment point on the LAN's transmission media. Each MAC address is unique and burned in at the factory.

LLC PDU LLC Protocol Data Unit Data from LLC sublayer.

CRCCyclic Redundancy Check

Same as FCS (Frame Check Sequence) field in HDLC. This is used for error checking. If an error is detected, the frame is simply discarded. MAC protocol does not recover erred frames.

Fig 4.8 MAC Frame format

LLC Service

At the MAC level, when a station has a frame to send, it just sends, with no concern whether the intended destination station is ready to receive the frame. It does not know whether the frame is received correctly, is lost, or is discarded. This is how a typical connectionless service works. This is called "unacknowledged connectionless service". For most applications, this is the only service ever needed.

  If the only service needed at the data link level is the unacknowledged connectionless service, there is no need for LLC's connection management services. The only role played by LLC under this situation is to get access to the network layer.

  But there could be situations where the sender may want to know whether its frame has been received correctly, or even wants to ensure that its frames arrive at the destination correctly and in the correct sequencing order. This is where LLC could play a role.

LLC can provide HDLC level service that is not provided by MAC, should there be such a need. LLC provides three connection management services. The Unacknowledged

12

connectionless service, Acknowledged connectionless and Connection mode service are the three connection management services provided, with Unacknowledged connectionless service being the default.

LLC PDU Format

Figure 4.9 below is an LLC PDU that contains LLC's fields.

  1 octet 1 octet 1 or 2 octets variable size

LLC PDU DSAP SSAP LLC Control Information

DSAPDestination Service Access Point

DSAP is the address of the network layer entity that uses the LLC service. It identifies the network layer protocol the packet is destined for. It is also known as the port number. Multiple DSAPs may be multiplexed on a single MAC address.

SSAP Source Service Access Point

SSAP is the address of the network layer entity that uses the LLC service. It identifies the network layer protocol the packet comes from. It is also known as the port number. Multiple SSAPs may be multiplexed on a single MAC address.

LLC Control LLC Control

Similar to HDLC's Control field, but with field parameters set to the values that are applicable to the LAN environment.(See last section, "LLC Service").

Information Information PDU from LLC's users, such as IP packets.

13

Fig 4.9: LLC PDU format

The Medium Access Sublayer (MAC)

A network of computers based on multi-access medium requires a protocol for effective sharing of the media. As only one node can send or transmit signal at a time using the broadcast mode, the main problem here is how different nodes get control of the medium to send data, that is “who goes next?”. The protocols used for this purpose are known as Medium Access Control (MAC) techniques. The key issues involved here are - Where and How the control is exercised.

‘Where’ refers to whether the control is exercised in a centralized or distributed manner. In a centralized system a master node grants access of the medium to other nodes. A centralized scheme has a number of advantages as mentioned below:

Greater control to provide features like priority, overrides, and guaranteed bandwidth. Simpler logic at each node. Easy coordination.

Although this approach is easier to implement, it is vulnerable to the failure of the master node and reduces efficiency. On the other hand, in a distributed approach all the nodes collectively perform a medium access control function and dynamically decide which node to be granted access. This approach is more reliable than the former one.

‘How’ refers to in what manner the control is exercised. It is constrained by the topology and trade off between cost-performance and complexity. Various approaches for medium access control are shown in Fig. The MAC techniques can be broadly divided into four categories; Contention-based, Round-Robin, Reservation-based and. Channelization-based. Under these four broad categories there are specific techniques, as shown in Fig4.10.

14

Fig4.10 Possible MAC techniques

Goals of MACs

Medium Access Control techniques are designed with the following goals in mind.

Initialisation: The technique enables network stations, upon power-up, to enter the state required for operation.

Fairness: The technique should treat each station fairly in terms of the time it is made to wait until it gains entry to the network, access time and the time it is allowed to spend for transmission.

Priority: In managing access and communications time, the technique should be able to give priority to some stations over other stations to facilitate different type of services needed.

Limitations to one station: The techniques should allow transmission by one station at a time.

Receipt: The technique should ensure that message packets are actually received (no lost packets) and delivered only once (no duplicate packets), and are received in the proper order.

Error Limitation: The method should be capable of encompassing an appropriate error detection scheme.

Recovery: If two packets collide (are present on the network at the same time), or if notice of a collision appears, the method should be able to recover, i.e. be able to halt all the transmissions and select one station to retransmit.

Reconfigurability: The technique should enable a network to accommodate the addition or deletion of a station with no more than a noise transient from which the network station can recover.

Compatibility: The technique should accommodate equipment from all vendors who build to its specification. Reliability: The technique should enable a network to confine operating inspite of a failure of one or several stations.

Round Robin Techniques

In Round Robin techniques, each and every node is given the chance to send or transmit by rotation. When a node gets its turn to send, it may either decline to send, if it has no data or may send if it has got data to send. After getting the opportunity to send, it must relinquish its turn after some maximum period of time. The right to send then passes to the next node based on a predetermined logical sequence. The right to send may be controlled in a centralized or distributed manner. Polling is an example of centralized control and token passing is an example of distributed control as discussed below.

15

Polling

The mechanism of polling is similar to the roll-call performed in a classroom. Just like the teacher, a controller sends a message to each node in turn. The message contains the address of the node being selected for granting access. Although all nodes receive the message, only the addressed node responds and then it sends data, if any. If there is no data, usually a “poll reject” message is sent back. In this way, each node is interrogated in a round-robin fashion, one after the other, for granting access to the medium. The first node is again polled when the controller finishes with the remaining codes.

The polling scheme has the flexibility of either giving equal access to all the nodes, or some nodes may be given higher priority than others. In other words, priority of access can be easily implemented.

Figure4.11 Polling using a central controller

Polling can be done using a central controller, which may use a frequency band to send outbound messages as shown in Fig 4.11. Other stations share a different frequency to send inbound messages. The technique is called frequency-division duplex approach (FDD). Main drawbacks of the polling scheme are high overhead of the polling messages and high dependence on the reliability of the controller.

Polling can also be accomplished without a central controller. Here, all stations receive signals from other stations as shown in Fig4.12. Stations develop a polling order list, using some protocol.

16

Figure 4.12 Polling in a distributed manner

In token passing scheme, all stations are logically connected in the form of a ring and control of the access to the medium is performed using a token. A token is a special bit pattern or a small packet, usually several bits in length, which circulate from node to node. Token passing can be used with both broadcast (token bus) and sequentially connected (token ring fig 4.13) type of networks with some variation in the details as considered in the next lesson.

In case of token ring, token is passed from a node to the physically adjacent node. On the other hand, in the token bus, token is passed with the help of the address of the nodes, which form a logical ring. In either case a node currently holding the token has the ‘right to transmit’. When it has got data to send, it removes the token and transmits the data and then forwards the token to the next logical or physical node in the ring. If a node currently holding the token has no data to send, it simply forwards the token to the next node. The token passing scheme is efficient compared to the polling technique, but it relies on the correct and reliable operation of all the nodes. There exists a number of potential problems, such as lost token, duplicate token, and insertion of a node, removal of a node, which must be tackled for correct and reliable operation of this scheme.

Figure 4.13 A token ring network

17

Figure Token passing mechanism

Performance: Performance of a token ring network can be represented by two parameters; throughput, which is a measure of the successful traffic, and delay, which is a measure of time between when a packet is ready and when it is delivered. A station starts sending a packet at t = t0, completes transmission at t = t0 + a, receives the tail at t0 + 1 + a. So, the average time (delay) required to send a token to the next station = a/N. and throughput, S = 1/(1 + a/N) for a<1 and S = 1/a(1 + 1/N) for a>1.

Contention-based Approaches

Round-Robin techniques work efficiently when majority of the stations have data to send most of the time. But, in situations where only a few nodes have data to send for brief periods of time, Round-Robin techniques are unsuitable. Contention techniques are suitable for bursty nature of traffic. In contention techniques, there is no centralized control and when a node has data to send, it contends for gaining control of the medium. The principle advantage of contention techniques is their simplicity. They can be easily implemented in each node. The techniques work efficiently under light to moderate load, but performance rapidly falls under heavy load.

ALOHA

The ALOHA scheme was invented by Abramson in 1970 for a packet radio network connecting remote stations to a central computer and various data terminals at the campus of the university of Hawaii. A simplified situation is shown in Fig. 4.14. Users are allowed random access of the central computer through a common radio frequency band f1 and the computer centre broadcasts all received signals on a different frequency band f2. This enables the users to monitor packet collisions, if any. The protocol followed by the users is simplest; whenever a node has a packet to sent, it simply does so. The scheme, known as Pure ALOHA, is truly a free-

18

for-all scheme. Of course, frames will suffer collision and colliding frames will be destroyed. By monitoring the signal sent by the central computer, after the maximum round-trip propagation time, an user comes to know whether the packet sent by him has suffered a collision or not.

Figure 4.14 Collision in Pure ALOHA

It may be noted that if all packets have a fixed duration of τ (shown as F in Figure 4.15), then a given packet A will suffer collision if another user starts to transmit at any time from τ before to until τ after the start of the packet A.

Figure 4.15 Vulnerable period in Pure ALOHA

Based on this, the best channel utilisation of 18% can be obtained at 50 percent of the offered load as shown in Fig. 4.16. At smaller offered load, channel capacity is underused and at higher offered load too many collisions occur reducing the throughput. The result is not encouraging, but for such a simple scheme high throughput was also not expected.

19

Figure 4.16 Throughput versus offered load for ALOHA protocol

Figure 4.17 Slotted ALOHA: Single active node can continuously transmit at full rate of channel

Subsequently, in a new scheme, known as Slotted ALOHA, was suggested to improve upon the efficiency of pure ALOHA. In this scheme, the channel is divided into slots equal to τ and packet transmission can start only at the beginning of a slot as shown in Fig. 4.17. This reduces the vulnerable period from 2τ to τ and improves efficiency by reducing the probability of collision as shown in Fig. 4.18. This gives a maximum throughput of 37% at 100 percent of offered load, as shown in Figure 4.16.

20

Figure 4.18 Collision in Slotted ALOHA

CSMA

The poor efficiency of the ALOHA scheme can be attributed to the fact that a node start transmission without paying any attention to what others are doing. In situations where propagation delay of the signal between two nodes is small compared to the transmission time of a packet, all other nodes will know very quickly when a node starts transmission. This observation is the basis of the carrier-sense multiple-access (CSMA) protocol. In this scheme, a node having data to transmit first listens to the medium to check whether another transmission is in progress or not. The node starts sending only when the channel is free, that is there is no carrier. That is why the scheme is also known as listen-before-talk. There are three variations of this basic scheme as outlined below.

(i) 1-persistent CSMA: In this case, a node having data to send, start sending, if the channel is sensed free. If the medium is busy, the node continues to monitor until the channel is idle. Then it starts sending data.

(ii) Non-persistent CSMA: If the channel is sensed free, the node starts sending the packet. Otherwise, the node waits for a random amount of time and then monitors the channel.

(iii) p-persistent CSMA: If the channel is free, a node starts sending the packet. Otherwise the node continues to monitor until the channel is free and then it sends with probability p.

The efficiency of CSMA scheme depends on the propagation delay, which is represented by a parameter a, as defined below:

Propagation delay Packet transmission time.

The throughput of 1-persistent CSMA scheme is shown in Fig. 4.19 for different values of a. It may be noted that smaller the value of propagation delay, lower is the vulnerable period and higher is the efficiency.

CSMA/CD

CSMA/CD protocol can be considered as a refinement over the CSMA scheme. It has evolved to overcome one glaring inefficiency of CSMA. In CSMA scheme, when two packets collide the channel remains unutilized for the entire duration of transmission time of both the packets. If the propagation time is small (which is usually the case) compared to the packet transmission time, wasted channel capacity can be considerable. This wastage of channel capacity can be reduced if the nodes continue to monitor the channel while transmitting a packet and immediately cease transmission when collision is detected. This refined scheme is known as Carrier Sensed Multiple Access with Collision Detection (CSMA/CD) or Listen-While-Talk.

On top of the CSMA, the following rules are added to convert it into CSMA/CD:

21

(i) If a collision is detected during transmission of a packet, the node immediately ceases transmission and it transmits jamming signal for a brief duration to ensure that all stations know that collision has occurred.

(ii) After transmitting the jamming signal, the node waits for a random amount of time and then transmission is resumed. The random delay ensures that the nodes, which were involved in the collision are not likely to have a collision at the time of retransmissions. To achieve stability in the back off scheme, a technique known as binary exponential back off is used. A node will attempt to transmit repeatedly in the face of repeated collisions, but after each collision, the mean value of the random delay is doubled. After 15 retries (excluding the original try), the unlucky packet is discarded and the node reports an error. A flowchart representing the binary exponential back off algorithm is given in Fig 4.19.

Figure 4.19 Binary exponential back off algorithm used in CSMA/CD

22

Figure 4.20 A plot of the offered load versus throughput for the value of a = 0.01

Performance Comparison between CSMA/CD and Token ring: It has been observed that smaller the mean packet length, the higher the maximum mean throughput rate for token passing compared to that of CSMA/CD. The token ring is also least sensitive to workload and propagation effects compared to CSMS/CD protocol. The CSMA/CD has the shortest delay under light load conditions, but is most sensitive to variations to load, particularly when the load is heavy. In CSMA/CD, the delay is not deterministic and a packet may be dropped after fifteen collisions based on binary exponential back off algorithm. As a consequence, CSMA/CD is not suitable for real-time traffic.

This section deals with broadcast networks and their protocols. The basic idea behind broadcast networks is how to determine who gets to use the channel when many users want to transmit over it. The protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called MAC.

 Pure ALOHA

One of the newly discovered algorithms-protocols for allocating a multiple access channel is ALOHA. The idea is simple. Users transmit whenever they have data to be sent. Frames are destroyed when collision occurs. When a sender detects a collision waits for a random amount of time and retransmits the frame. With this method the best theoretical throughput and channel utilization we can have is 18%. Term throughput means the amount of work that a computer can do in a given time period

Slotted ALOHA

In slotted ALOHA time is divided into discrete intervals, each corresponding to one frame. A computer is not permitted to send whenever it has data to send. Instead it is required to wait for the next available slot. The best it can be achieved is 37% of slots empty, 37% success and 26% collision.

23

Nonpersistent CSMA (Carrier Sense Multiple Access)

Before sending, a station senses the channel. If no one else is sending, the station begins doing so itself. However, if the channel is already in use, waits a random time  and then repeats the algorithm.

1-Persistent CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Is an improvement of the previous techniques When a station wants to transmit listens to the cable (carrier sense). If its busy waits until it goes idle, otherwise it transmits. If two or more stations simultaneously begin transmitting on an idle cable they will collide. As soon as they detect a collision stations abort their transmission (collision detection). This is very important enhancement as it saves time and bandwidth. Then stations wait a random time and repeat the whole process al over again. CSMA/CD is widely used on LANs in MAC sublayer such as Ethernet, Token Ring, Token bus etc.

Ethernet 

We learned that a local area network (LAN) is a computer network that is designed for a limited geographic area such as a building or a campus. Although a LAN can be used as an isolated network to connect computers in an organization for the sole purpose of sharing resources, most LANs today are also linked to a wide area network (WAN) or the Internet. The LAN market has several technologies such as Ethernet, Token Ring, Token Bus, FDDI, and ATM LAN. Some of these technologies survived for a while, but Ethernet is by far the dominant technology.

IEEE STANDARDS

In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols.

STANDARD ETHERNET

The original Ethernet was created in 1976 at Xerox’s Palo Alto Research Center (PARC). Since then, it has gone through four generations.

24

In Standard Ethernet, the MAC sublayer governs the operation of the access method. It also frames data received from the upper layer and passes them to the physical layer.

Frame Format

Preamble: The first field of the 802.3 frame contains 7 bytes (56 bits) of alternating Os and 1s that alerts the receiving system to the coming frame and enables it to synchronize its input timing. The pattern provides only an alert and a timing pulse. The 56-bit pattern allows the stations to miss some bits at the beginning of the frame. The preamble is actually added at the physical layer and is not (formally) part of the frame.

Start frame delimiter (SFD): The second field (l byte: 10101011) signals the beginning of the frame. The SFD warns the station or stations that this is the last chance for synchronization. The last 2 bits is 11 and alerts the receiver that the next field is the destination address. 

Destination address (DA): The DA field is 6 bytes and contains the physical address of the destination station or stations to receive the packet. We will discuss addressing shortly.

Source address (SA) The SA field is also 6 bytes and contains the physical address of the sender of the packet.

Length or type: This field is defined as a type field or length field. The original Ethernet used this field as the type field to define the upper-layer protocol using the MAC frame. The IEEE standard used it as the length field to define the number of bytes in the data field. Both uses are common today.

Data: This field carries data encapsulated from the upper-layer protocols. It is a minimum of 46 and a maximum of 1500 bytes.

CRC: The last field contains error detection information, in this case a CRC-32.

Minimum & Maximum Length of a frame

25

Addressing

Each station on an Ethernet network (such as a PC, workstation, or printer) has its own network interface card (NIC). The NIC fits inside the station and provides the station with a 6-byte physical address. As shown in Figure 13.6, the Ethernet address is 6 bytes (48 bits), nonnally written in hexadecimal notation, with a colon between the bytes.

Unicast, Multicast, and Broadcast Addresses A source address is always a unicast address-the frame comes from only one station. The destination address, however, can be unicast, multicast, or broadcast. Figure 13.7 shows how to distinguish a unicast address from a multicast address. If the least significant bit of the first byte in a destination address is 0, the address is unicast; otherwise, it is multicast.

Physical Layer The Standard Ethernet defines several physical layer implementations, four of the most common, are shown in Figure

lOBase5: Thick Ethernet

26

The first implementation is called 10Base5, thick Ethernet, or Thicknet. The nickname derives from the size of the cable. lOBase5 was the first Ethernet specification to use a bus topology with an external transceiver (transmitter/receiver) connected via a tap to a thick coaxial cable. Figure shows a schematic diagram of a lOBase5 implementation.

 

10Base2: Thin Ethernet

The second implementation is called lOBase2, thin Ethernet, or Cheapernet. 1OBase2 also uses a bus topology, but the cable is much thinner and more flexible. The cable can be bent to pass very close to the stations. In this case, the transceiver is normally part of the network interface card (NIC), which is installed inside the station. Figure shows the schematic diagram of a IOBase2 implementation.

This implementation is more cost effective than 10Base5 because thin coaxial cable is less expensive than thick coaxial and the tee connections are much cheaper than taps. Installation is simpler because the thin coaxial cable is very flexible. However, the length of each segment cannot exceed 185 m (close to 200 m) due to the high level of thin coaxial cable.

 

lOBase-T: Twisted-Pair Ethernet

The third implementation is called lOBase-T or twisted-pair Ethernet. 1OBase-T uses a physical star topology. The stations are connected to a hub via two pairs of twisted cable, as shown in Figure. Note that two pairs of twisted cable create two paths (one for sending and one for receiving) between the station and the hub. Any collision here happens in the hub.

The maximum length of the twisted cable here is defined as 100 m, to minimize the effect of attenuation in the twisted cable.

27

lOBase-F: Fiber Ethernet

Although there are several types of optical fiber lO Mbps Ethernet, the most common is called 10Base-F. lOBase-F uses a star topology to connect stations to a hub. The stations are connected to the hub using two fiber-optic cables, as shown in Figure

Summary of Standard Ethernet implementations

Characteristics 10Base5 10Base2 10BaseT 10BaseF

Medium Thick Coaxial cable

Thin Coaxial cable

2 UTP 2 Fiber

Maximum Length

500m 185m 100m 2000m

28

Line Encoding Manchester Manchester Manchester Manchester

CHANGES IN THE STANDARD

The 10-Mbps Standard Ethernet has gone through several changes before moving to the higher data rates. These changes actually opened the road to the evolution of the Ethernet to become compatible with other high-data-rate LANs.

Bridged Ethernet

The first step in the Ethernet evolution was the division of a LAN by bridges.

Bridges have two effects on an Ethernet LAN: They raise the bandwidth and they separate collision domains. In an unbridged Ethernet network, the total capacity (10 Mbps) is shared among all stations with a frame to send; the stations share the bandwidth of the network.

If only one station has frames to send, it benefits from the total capacity (10 Mbps). But if more than one station needs to use the network, the capacity is shared. We can say that, in this case, each station on average, sends at a rate of 5 Mbps.

A bridge divides the network into two or more networks. Bandwidth-wise, each network is independent. For example, in Figure a network with 12 stations is divided into two networks, each with 6 stations.

Now each network has a capacity of 10 Mbps. The lO-Mbps capacity in each segment is now shared between 6 stations (actually 7 because the bridge acts as a station in each segment), not 12 stations.

A network with and without a bridge

  Another advantage of a bridge is the separation of the collision domain. Figure shows the collision domains for an unbridged and a bridged network. You can see that the collision domain becomes much smaller and the probability of collision is reduced .

29

Collision domains in an unbridged network and a bridged network

Switched Ethernet

The idea of a bridged LAN can be extended to a switched LAN. Instead of having two to four networks, why not have N networks, where N is the number of stations on the LAN? In other words, if we can have a multiple-port bridge, why not have an N-port switch? In this way, the bandwidth is shared only between the station and the switch (5 Mbps each). In addition, the collision domain is divided into N domains.

Full-Duplex Ethernet

One of the limitations of 10Base5 and lOBase2 is that communication is half-duplex (lOBase-T is always full-duplex); a station can either send or receive, but may not do both at the same time.

The next step in the evolution was to move from switched Ethernet to full-duplex switched Ethernet. The full-duplex mode increases the capacity of each domain from 10 to 20 Mbps. Figure 13.18 shows a switched Ethernet in full-duplex mode.

30

Note that instead of using one link between the station and the switch, the configuration uses two links: one to transmit and one to receive.

FAST ETHERNET

Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps.

Fast Ethernet topology

Fast Ethernet implementations

31

Encoding for Fast Ethernet implementation

Summary of Fast Ethernet implementations

Characteristics 100Base-Tx 100Base-Fx 100Base-T4

Number of wires 2 2 4

Maximum Length

100m 100m 100m

32

Block Encoding 4B/5B 4B/5B

Line Encoding MLT-3 NRZ-I 8B/6T

GIGABIT ETHERNET

The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the standard 802.3z.

In the full-duplex mode of Gigabit Ethernet, there is no collision; the maximum length of the cable is determined by the signal attenuation in the cable.

Topologies of Gigabit Ethernet

Gigabit Ethernet implementations

33

Encoding in Gigabit Ethernet implementations

Summary of Gigabit Ethernet implementations

Characteristics 1000Base-SX 1000Base-LX 1000Base-CX 1000Base-T

Media Fiber Short-Wave

Fiber Long-wave STP Cat 5 UTP

Number of Wires 2 2 2 4

Maximum Length

550m 5000m 25m 100m

34

Block Encoding 8B/10B 8B/10B 8B/10B 8B/10B

Line Encoding NRZ NRZ NRZ 4D-PAM5

Summary of TenGigabit Ethernet implementations

Characteristics 10GBase-S 10GBase-L 10GBase-E

Media Short-Wave 850-nm multimode

Long-wave 1310-nm single- mode

Extended 1550-mm single- mode

Maximum Length

300m 10km 40km

Token ring:

In the token-passing method, the stations in a network are organized in a logical ring. In other words, for each station, there is a predecessor and a successor. The predecessor is the station which is logically before the station in the ring; the successor is the station which is after the station in the ring. The current station is the one that is accessing the channel now. The right to this access has been passed from the predecessor to the cur-rent station. The right will be passed to the successor when the current station has no more data to send.

But how is the right to access the channel passed from one station to another? In this method, a special packet called a token circulates through the ring. The posses-sion of the token gives the station the right to access the channel and send its data. When a station has some data to send, it waits until it receives the token from its pre-decessor. It then holds the token and sends its data. When the station has no more data to send, it releases the token, passing it to the next logical station in the ring. The sta-tion cannot send data until it receives the token again in the next round. In this process, when a station receives the token and has no data to send, it just passes the data to the next station.

Token management is needed for this access method. Stations must be limited in the time they can have possession of the token. The token must be monitored to ensure it has not been lost or destroyed. For example, if a station that is holding the token fails, the token will disappear from the network. Another function of token management is to assign priorities to the stations and to the types of data being transmitted. And finally, token management is needed to make low-priority stations release the token to high-priority stations.

Logical Ring

35

In a token-passing network, stations do not have to be physically connected in a ring; the ring can be a logical one. Figure 12.20 show four different physical topologies that can create a logical ring.

In the physical ring topology, when a station sends the token to its successor, the token cannot be seen by other stations; the successor is the next one in line. This means that the token does not have to have the address of the next successor. The problem with this topology is that if one of the links-the medium between two adjacent stations-fails, the whole system fails.

The dual ring topology uses a second (auxiliary) ring which operates in the reverse direction compared with the main ring. The second ring is for emergencies only (such as a spare tire for a car). If one of the links in the main ring fails, the system automatically combines the two rings to form a temporary ring. After the failed link is restored, the auxiliary ring becomes idle again. Note that for this topology to work, each station needs to have two transmitter ports and two receiver ports. The high-speed Token Ring networks called FDDI (Fiber Distributed Data Interface) and CDDI (Copper Distributed Data Interface) use this topology.

In the bus ring topology, also called a token bus, the stations are connected to a sin-gle cable called a bus. They, however, make a logical ring, because each station knows the address of its successor (and also predecessor for token management purposes). When a station has finished sending its data, it releases the token and inserts the address of its successor in the token. Only the station with the address matching the destination address of the token gets the token to access the shared media. The Token Bus LAN, standardized by IEEE, uses this topology.

In a star ring topology, the physical topology is a star. There is a hub, however, that acts as the connector. The wiring inside the hub makes the ring; the stations are con-nected to this ring through the two wire connections. This topology makes the network less prone to failure because if a link goes down, it will be bypassed by the hub and the rest of the stations can operate. Also adding and removing stations from the ring is easier. This topology is still used in the Token Ring LAN designed by IBM.

36

Fiber channel:Fibre Channel is designed to combine the best features of both technologies— the simplicity and speed of channel communications with the flexibility and inter-connectivity that characterize protocol-based network communications. This fusion of approaches allows system designers to combine traditional peripheral connec-tion, host-to-host internetworking, loosely coupled processor clustering, and multi-media applications in a single multiprotocol interface. The types of channel-oriented facilities incorporated into the Fibre Channel protocol architecture include

Data-type qualifiers for routing frame payload into particular interface buffersLink-level constructs associated with individual I/O operationsProtocol interface specifications to allow support of existing I/O channel architectures, such as the Small

Computer System Interface (SCSI)

The types of network-oriented facilities incorporated into the Fibre Channel protocol architecture include

Full multiplexing of traffic between multiple destinationsPeer-to-peer connectivity between any pair of ports on a Fibre Channel networkCapabilities for internetworking to other connection technologies

Depending on the needs of the application, either channel or networking approaches can be used for any data transfer. The Fibre Channel Industry Associa-tion, which is the industry consortium promoting Fibre Channel, lists the following ambitious requirements that Fibre Channel is intended to satisfy [FCIA01]:

Full-duplex links with two fibers per linkPerformance from 100 Mbps to 800 Mbps on a single line (full-duplex 200 Mbps to 1600 Mbps per link)Support for distances up to 10 kmSmall connectorsHigh-capacity utilization with distance insensitivityGreater connectivity than existing multidrop channelsBroad availability (i.e., standard components)Support for multiple cost/performance levels, from small systems to super-computersAbility to carry multiple existing interface command sets for existing channel and network protocolsThe solution was to develop a simple generic transport mechanism based on point-to-point links and a switching network. This underlying infrastructure sup-ports a simple encoding and framing scheme that in turn supports a variety of chan-nel and network protocols.

Fibre Channel ElementsThe key elements of a Fibre Channel network are the end systems, called nodes, and the network itself, which consists of one or more switching elements. The collection of switching elements is referred to as a fabric. These elements are interconnected by point-to-point links between ports on the individual nodes and switches. Com-munication consists of the transmission of frames across the point-to-point links.Each node includes one or more ports, called N_ports, for interconnection. Similarly, each fabric-switching element includes multiple ports, called F_ports. Interconnection is by means of bidirectional links between ports. Any node can communicate with any other node connected to the same fabric using the services of the fabric. All routing of frames between N_ports is done by the fabric. Frames may be buffered within the fabric, making it possible for different nodes to connect to the fabric at different data rates.

37

A fabric can be implemented as a single fabric element with attached nodes (a simple star arrangement) or as a more general network of fabric elements, as shown in Figure 16.8. In either case, the fabric is responsible for buffering and for routing frames between source and destination nodes.The Fibre Channel network is quite different from the IEEE 802 LANs. Fibre Channel is more like a traditional circuit-switching or packet-switching network, in contrast to the typical shared-medium LAN. Thus, Fibre Channel need not be con-cerned with medium access control issues. Because it is based on a switching network, the Fibre Channel scales easily in terms of N_ports, data rate, and distance covered.

This approach provides great flexibility. Fibre Channel can readily accommodate new transmission media and data rates by adding new switches and F_ports to an existing fabric. Thus, an existing investment is not lost with an upgrade to new technologies and equipment. Further, the layered protocol architecture accommodates existing I/O interface and networking protocols, preserving the preexisting investment.

Fibre Channel Protocol ArchitectureThe Fibre Channel standard is organized into five levels. Each level defines a func-tion or set of related functions. The standard does not dictate a correspondence between levels and actual implementations, with a specific interface between adja-cent levels. Rather, the standard refers to the level as a “document artifice” used to group related functions. The layers are as follows:

FC-0 Physical Media: Includes optical fiber for long-distance applications, coaxial cable for high speeds over short distances, and shielded twisted pair for lower speeds over short distancesFC-1 Transmission Protocol: Defines the signal encoding schemeFC-2 Framing Protocol: Deals with defining topologies, frame format, flow and error control, and grouping of frames into logical entities called sequences and exchangesFC-3 Common Services: Includes multicastingFC-4 Mapping: Defines the mapping of various channel and network proto-cols to Fibre Channel, including IEEE 802, ATM, IP, and the Small Computer System Interface (SCSI)Fibre Channel Physical Media and TopologiesOne of the major strengths of the Fibre Channel standard is that it provides a range of options for the physical medium, the data rate on that medium, and the topology of the network (Table 16.4).

38

Transmission Media The transmission media options that are available under Fibre Channel include shielded twisted pair, video coaxial cable, and optical fiber. Standardized data rates range from 100 Mbps to 3.2 Gbps. Point-to-point link dis-tances range from 33 m to 10 km.

39