data fundamentals

FUNDAMENTALS OF DATA COMMUNICATION 4/18/2011

RELIANCE INFOCOMM For internal use only Page - 1



Data and Voice communication have several similarities but are different in a few ways:

1. Voice communication is tolerance of minor failures and noise, as the transmitting and receiving parties are human being (highly intelligent as compared to machines that communicate). Data communication needs to be Fault sensitive as the M/c’s can’t make much error correction.

2. Voice Communication on the other hand is Delay sensitive. Human being feel very uncomfortable if there is variable delay in transmitting successive parts of the speech. Human senses are used to hearing only in the way they hear to direct voice. Machines however have no such inhibitions and that reconstruct a communicated information from various parts of the same.

3. From the above points one would intuitively realise that Voice communication is best suited to Circuit switching and Data to Packet switching. One a circuit is established between the two parties there would be no variation in transmission delays. The only delay is that of electrical signal traveling over the circuit, which is in negligible. No fault correction mechanism is implemented. But in case of Data, information in sent in packets with suitable error detecting codes and transmitted along with several other packets. Different packets could be of different sizes, thereby producing differential delays in receiving subsequent packets for a node. But that’s not the problem.

4. Lastly for Voice, we are all bound by the 64 kbps bandwidth, even during silences. While in Data communication the bandwidth for each channel can be set differently depending on the need of the parties.



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The OSI model divides the tasks of computer communications into a series of seven layers, with the lowest layer providing an interface to the physical medium and the highest layer providing an interface to the user application.

Each layer communicates with the immediately higher and lower layers by placing information in headers added to the data. These headers often contain fields called service access points (SAPs), which act like mailboxes where a communication program can leave information for a program running at a higher or lower layer.

The application, presentation, and session layers format the data for presentation to the user application and establish a communication session between the local and remote applications.

The transport layer typically guarantees the reliability of the transmissions between two end stations by providing facilities for disassembly, assembly, error checking, sequencing, and retransmission. This layer also often provides a way to identify the application that generated the message, using a special SAP (called a socket in TCP/IP).

The network layer provides addresses for the sending and receiving stations that allow routing of the message over a network composed of various physical networks.

The data link layer provides addresses for the sending and receiving stations that allow delivery from one station to the next station on the same physical network. The data link trailer usually includes some form of checksum (an arithmetic sum used to verify data integrity).

The physical layer converts the bits of data hi each frame into electrical or optical signals, depending on the physical medium, and sends it through the medium.



Lets take the example of an e-banking solution. The Users (bank and it’s customer) would “communicate” to each other only through some application, lets say some “e-banking” application available to the customer and some ERP solution used by the bank. As you might have observed these are two different applications, but would have an understanding of what support is provided by each one to it’s user. So these to apparently different applications can communicate with each other. This understanding that allows them to communicate is the Application layer Protocol.These Applications are serviced with necessary information (like Ac no, Amount, ID, Password, etc.) by the Presentation layer. As the data is going to be broken down to the last byte and then into bits to communicate through the physical layer, it is essential to know how these bits & bytes are to be re-constituted to form meaning information to the User. These may be inform of Field type & length (in this example) or Colour code (in a bit-map transmission). This is the Presentation layer protocol.The presentation layer exchange information between themselves (like what is the Balance left, Order is confirmed, etc.) through communication Sessions. These layer essentially controls the start, continue and stop of any communication between to end-points.The Transport layer takes care of reliable transmission, so that even if the complete information is sent in multiple packets, which could be out of sequence, some of which could be lost/ dropped, the session layer still receives the complete information, as was transmitted.These layers are generally built in the terminating DCE’s through some software. The lower three are built into all the nodes that form the network path. These are by the way of Hardware (like Cable, Hub, Switches, Routers) and embedded software (like Routing protocols).



Important to note at this point that while the two end systems need all the 7 layers to communicate the in-between elements (Network Elements/ Routers) need to use upto Layer three. The Lower three layers therefore are also called “NETWORK LAYERS” while the upper four are called “APPLICATION LAYERS”.Also note the Physical layer or Data/Link Layer protocol is applicable from one Node to another and not all across the network.



Coaxial cablesA coaxial cable is one that consists of two conductors that share a common axis. Theinner conductor is typically a straight wire, either solid or stranded and the outerconductor is typically a shield that might be braided or a foil.Coaxial cable is a cable type used to carry radio signals, video signals, measurementsignals and data signals. Coaxial cables exists because we can't run open-wire linenear metallic objects (such as ducting) or bury it. We trade signal loss for convenienceand flexibility. Coaxial cable consists of an insulated ceter conductor which is coveredwith a shield. The signal is carried between the cable shield and the center conductor.This arrangement give quite good shielding agains noise from outside cable, keepsthe signal well inside the cable and keeps cable characteristics stable.Coaxial cables and systems connected to them are not ideal. There is always somesignal radiating from coaxial cable. Hence, the outer conductor also functions as ashield to reduce coupling of the signal into adjacent wiring. More shield coveragemeans less radiation of energy (but it does not necessarily mean less signalattenuation).

Unshielded Twisted PairThe quality of UTP may vary from telephone-grade wire to extremely high-speedcable. The cable has four pairs of wires inside the jacket. Each pair is twisted with adifferent number of twists per inch to help eliminate interference from adjacent pairsand other electrical devices. The tighter the twisting, the higher the supportedtransmission rate and the greater the cost per foot. The EIA/TIA (Electronic IndustryAssociation/Telecommunication Industry Association) has established standards ofUTP and rated five categories of wire.



BANDWIDTH-DISTANCE PRODUCT:

The product of bandwidth (thereby the baud-rate) & the distance between transmitter & receiver is generally constant for any particular type of cable.

So while the type of cable determines the bandwidth-distance product, for any cable the bandwidth is inversely proportional to the distance.

While bandwidth determines speed, distance gives coverage, thereby these two are very important for any network design.



• Uses FDM system

• Bandwidth divided three parts

0 < f < 4 kHz POTS (Channel 0)

30 < f < 138 kHz Upstream ( Channel 6 -32 )

138 < f < 1104 kHz Downstream

4 kHz 30 kHz 138 kHz 1104 kHz



European std.s Synchronous Digital Hierarchy (SDH) and it’s North American counterpart SONET proposes a transport system with highly synchronised network elements and OFC as the physical media. Thereby the concept of bit-interleaving is replaced by a Byte interleaved system. Also the bandwidths are defined upto much higher range making it suitable for modern data & broadband communication.

SDH/ SONET defines to types of “packaging” – one for the electrical network called Synchronous Transmission Module/ System (STM-n/ STS-n) and another for the optical network called Transport Unit (TU-n)/ Optical Carrier (OC-n).

STM-n has now been defined from STM-1 (63 E1’s) to STM-256 (16128 E1’s). Proposal for STM-1024 is under examination for standardising. That would take us to an amazing 160 Gbps.

Technically there is no difference between SDH and SONET. Some terms differ and some details in Overhead definitions defer but that doesn’t come in the way of making these to standards compatible to each other.



Standard: ETSI EN 301 021 (TM4) Frequency Band: 10.15- 10.65 GHzFDD Separation: 350 MHzRange: 10KmReceiver Sensitivity: -87dbm @ BER 10-9Emitted Power:

Terminal Station: 15 dbmBase Station: 15 dBm (per carrier)RFU: 27 dBm

Antenna Characteristics:Terminal Station 80, 25 dBiBase Station 900, 15. dBi, 4 sectors

600, 18 dBi



Flags: Delimits the beginning and end of the frame. The value of this field is always the same

and is represented either as the hexadecimal number 7E or as the binary number 01111110.

Address: Contains the following information:

DLCI—The 10-bit DLCI is the essence of the Frame Relay header. This value

represents the virtual connection between the DTE device and the switch.

Extended Address (EA)—The EA is used to indicate whether the byte in which the

EA value is 1 is the last addressing field. If the value is 1, then the current byte is

determined to be the last DLCI octet.

C/R—The C/R is the bit that follows the most significant DLCI byte in the Address field.

The C/R bit is not currently defined.

Congestion Control—This consists of the 3 bits that control the Frame Relay

congestion-notification mechanisms. These are the FECN, BECN, and DE bits, which

are the last 3 bits in the Address field.

Data: Contains encapsulated upper-layer data. Each frame in this variable-length field includes

a user data or payload field that will vary in length up to 16,000 octets.

Frame Check Sequence: Ensures the integrity of transmitted data.



A drawing of the first Ethernet system by Bob Metcalfe.

„ Invented by Metcalf at Xerox in 1973 and patented in 1976„ Xerox convinced Digital and Intel to join in making products (hence the group called DIX)„ IEEE standard in 1989All the stations are connected to a bus (Half-duplex). All transmissions are in broadcast mode. Every frame contains source address and destination address. A station before commencing transmission, checks if bus is being used. If not, it transmits. Ifanother station also commences transmission at the same time Collision occurs. While transmitting a frame, the station keeps check on the bus for any other transmission. If it detects, it discontinues, backs off and takes another chance for retransmission of the frame.

http://www.ethermanage.com/ethernet/metcalfe-drawing.html



• 802.3 Now encompasses– Original 802.3: 10BASE-T 10BASE-5 10BASE-2 10BROAD-36– 802.3u Fast Ethernet: 100BASE-TX 100BASE-FX 100BASE-T4– 802.3x: Flow Control– 802.3z Gigabit Ethernet: 1000BASE-SX / -LX / -CX• 802.3ab Copper Gigabit Ethernet: 1000BASE-T• 802.3ac Frame Tagging for VLAN support• 802.3ad Link Aggregation• 802.3ae 10 Gigabit Ethernet: Completion by March 2002• 802.3af DTE Power via MDI: Completion by Sept 2001



CSMA/CD

Multiple nodes may share a physical cable, or carrier. As shown in figure, when a node is ready to transmit, it checks for the presence of a signal already on the cable. If there is no signal, the carrier is available and the node begins transmission. However, if the carrier is in use, the node defers transmission until the cable is idle. This is the Carrier Sense Multiple Access (CSMA) process.

If two nodes sense an idle cable and transmit data at the same time, a collision occurs, which renders the colliding packets unreadable. CSMA/CD provides a scheme for monitoring these collisions in a network. This scheme is called Collision Detection (CD).

In a star-wired unshielded twisted pair (UTP) network, if the transceiver of the sending station detects activity on both its transmit and its receive wire pairs before it completes the transmission, it determines that a collision has occurred. In a network using a coaxial cable, collisions are detected when the DC signal level on the cable equals or exceeds the combined signal level of the two transmitters.

On busy Ethernet networks, frames may collide frequently. After a random delay interval, they are retransmitted. However, the retransmissions may also collide. After 15 consecutive collisions, the transmitting station discards the frame. Excessive collisions usually indicate an overloaded LAN or a faulty device “jabbering” or sending noise onto the carrier.



• Preamble (PRE)—Consists of 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.• Start-of-frame delimiter (SOF)—Consists of 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.• Destination address (DA)—Consists of 6 bytes. The DA field identifies which station(s) should receive the frame. The left-most bit in the DA field indicates whether the address is an individual address (indicated by a 0) or a group address (indicated by a 1). The second bit from the left indicates whether the DA is globally administered (indicated by a 0) or locally administered (indicated by a 1). The remaining 46 bits are a uniquely assigned value that identifies a single station, a defined group of stations, or all stations on the network.• Source addresses (SA)—Consists of 6 bytes. The SA field identifies the sending station. The SA is always an individual address and the left-most bit in the SA field is always 0.• Length/Type—Consists of 4 bytes. This field indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format. If the Length/Type field value is less than or equal to 1500, the number of LLC bytes in the Data field is equal to the Length/Type field value. If the Length/Type field value is greater than 1536, the frame is an optional type frame, and the Length/Type field value identifies the particular type of frame being sent or received.• Data—Is a sequence of n bytes of any value, where n is less than or equal to 1500. If the length of the Data field is less than 46, the Data field must be extended by adding a filler (a pad) sufficient to bring the Data field length to 46 bytes.



MAC Address Format

Ethernet MAC addresses are 48 bits in length. The IEEE assigns the first 24 bits to organizations requesting them, typically, equipment vendors. Vendors are then responsible for placing a unique value in the remaining 24 bits, yielding a globally unique MAC address for every physical interface.

Unicast Address:

A Unicast address is simply a MAC address with the Multicast bit set to zero. Unicast addresses must be unique within the network.

Broadcast address:

A Broadcast address at the MAC level is simply an address of all 1's. In hexadecimal notation this would be expressed as FF-FF-FF-FF-FF-FF.

Multicast Address:

If the Multicast bit is set to 1 then the MAC address represents a multicast destination. There are several reserved multicast addresses that are used by well known protocols.



A. Links are primarily point-to-point and can be any speed of Ethernet.B. Nodes can be either switches or routers, depending on their location in the MEN, the nature of the services being provisioned, and the level of service resilience (network protection). Nodes are meshed to whatever degree necessary to provide the desired connectivity, services, and protection.C. WAN links connect MENs together across large distances.D. Ethernet services can be topologically classified into point-to-point (as shown in this illustration), multipoint-to-multipoint, or point-to-multipoint. Services are then further classified according to the bandwidth provisioned and used. This bandwidth usage can be exclusive or shared across multiple users. Bandwidth is provisioned on demand from 1 Mbps to 1 Gbps, in increments as fine as 1 Mbps.E. Varying degrees of service resilience are obtained by implementing a combination of network protection techniques. Protection can be end-to-end (as shown in this illustration) or node-to-node.F. Quality of Service (QoS) is realized using a combination of various techniques to provide both ‘hard’ and ‘soft’ bandwidth and packet-loss guarantees. QoS can be end-to-end (as shown in this illustration) or node-to-node. From an enterprise end-customer perspective, QoS is visible as a technical/operational Service Level Specification (SLS), which is underwritten by a commercial Service Level Agreement (SLA).



WLANs can be used either to replace wired LANs, or as an extension of the wired LAN infrastructure. The basic topology of an 802.11 network is shown in Figure 1. A Basic Service Set (BSS) consists of two or more wireless nodes, or stations (STAs), which have recognized each other and have established communications. In the most basic form, stations communicate directly with each other on a peer-to-peer level sharing a given cell coverage area. This type of network is often formed on a temporary basis, and is commonly referred to as an ad hoc network, or Independent Basic Service Set (IBSS).In most instances, the BSS contains an Access Point (AP). The main function of an AP is to form a bridge between wireless and wired LANs. The AP is analogous to a basestation used in cellular phone networks. When an AP is present, stations do not communicate on a peer-to-peer basis. All communications between stations or between a station and a wired network client go through the AP. AP’s are not mobile, and form part of the wired network infrastructure. A BSS in this configuration is said to be operating in the infrastructure mode.The Extended Service Set (ESS) shown in Figure 2 consists of a series of overlapping BSSs (each containing an AP) connected together by means of a Distribution System (DS). Although the DS could be any type of network, it is almost invariably an Ethernet LAN. Mobile nodes can roam between APs and seamless campus-wide coverage is possible.Radio TechnologyIEEE 802.11 provides for two variations of the PHY. These include two (2) RF technologies namely Direct Sequence Spread Spectrum (DSSS), and Frequency Hopped Spread Spectrum (FHSS). The DSSS and FHSS PHY options were designed specifically to conform to FCC regulations (FCC 15.247) for operation in the 2.4 GHz ISM band, which has worldwide allocation for unlicensed operation.Region Allocated Spectrum



The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA). This requires each station to listen for other users. If the channel is idle, the station may transmit. However if it is busy, each station waits until transmission stops, and then enters into a random back off procedure. This prevents multiple stations from seizing the medium immediately after completion of the preceding transmission.Packet reception in DCF requires acknowledgement as shown in Fig. The period between completion of packet transmission and start of the ACK frame is one Short Inter Frame Space (SIFS). ACK frames have a higher priority than other traffic. Fast acknowledgement is one of the salient features of the 802.11 standard, because it requires ACKs to be handled at the MAC sublayer. Transmissions other than ACKs must wait at least one DCF inter frame space (DIFS) before transmitting data. If a transmitter senses a busy medium, it determines a random back-off period by setting an internal timer to an integer number of slot times. Upon expiration of a DIFS, the timer begins to decrement. If the timer reaches zero, the station may begin transmission. However, if the channel is seized by another station before the timer reaches zero, the timer setting is retained at the decremented value for subsequent transmission.



VCI: Virtual Channel Id PTI: Payload Type Indicator

VPI: Virtual Path Id CLP: Cell Loss Priority

UNI: User Network Interface HEC: Header Error Check

GFC: Generic Flow Control (undefined )

Allows for 256 VPs and 65,536 VCs

Byte1

2

3

4

5

GFC VPI

VPI VCI

VCI

VCI PTI CLP

HEC

8 7 6 5 4 3 2 1 Bits

- Octets sent in increasing order(start at Octet 1)

- Within the Octet,the MSB (8) is sentfirst

Byte1

2

3

4

5

VPI

VPI VCI

VCI

VCI PTI CLP

HEC

8 7 6 5 4 3 2 1 Bits

VCI: Virtual Channel Id PTI: Payload Type Indicator

VPI: Virtual Path Id CLP: Cell Loss Priority

NNI: Network Network Interface

Allows for 4,096 VPs and 65,536 VCs

• No GFC Field

• More VPI bits for “trunking”



Higher Layers

ATM AdaptationLayer

ATM Layer

Phy

Higher Layers

ATM AdaptationLayer

ATM Layer

Phy

ATM Endpoint ATM Endpoint

ATM Network Interfaces

ATM Switching ATM Switching

ATM Layer

Phy Phy

ATM Layer

Phy Phy

Peer Layer Communication



•Virtual Channel

Assigned at call-setup time

Has only local significance

May be used for multi-component services e.g. video telephony with separate voice and video VCI streams

• Virtual Path

Carry a bundles of Virtual Channels

Virtual Path service from Carriers allows reconfiguration of virtual channels without service order changes to the Carrier



Repeaters let you extend the distance between remote nodes and increase the total number of nodes on a network. The physical layer defines the rules for the media (such as electrical or fiber optic cable) that interconnect network devices. Electrical signals become weaker over distance (called attenuation) and may become distorted by exposure to interference (EMI and RFI). The rules provided by the physical layer protocol ensure that the signal remains strong enough for the most remote end nodes to exchange data.

A repeater amplifies the signal and removes any accumulated distortion. The only possible values for a digital signal are 0 or 1, so it is easy to restore it to its original condition. Because a repeater deals with signal reproduction, it is considered to operate at the OSI physical layer.

Besides limiting the length of the segment, physical layer specifications also state the maximum number of nodes per segment for different media types. For instance, a maximum of 30 attachments are permitted on an 802.3 network using 10BASE2 (thin Ethernet) coaxial cable.

Repeaters allow you to interconnect multiple segments of the same media type and the same data link layer protocol. This interconnection increases the number of nodes allowed and extends the maximum distance permitted between the most distant end nodes in the network.



Problems with Bus topology implemented with simple cable connections:



In telecommunication networks, a bridge is a product that connects a local area network (LAN) to another local area network that uses the same protocol (for example, Ethernet or token ring).

You can envision a bridge as being a device that decides whether a message from you to someone else is going to the local area network in your building or to someone on the local area network in the building across the street. A bridge examines each message on a LAN, "passing" those known to be within the same LAN, and forwarding those known to be on the other interconnected LAN (or LANs).

In bridging networks, computer or node addresses have no specific relationship to location. For this reason, messages are sent out to every address on the network and accepted only by the intended destination node. Bridges learn which addresses are on which network and develop a learning table so that subsequent messages can be forwarded to the right network.

Bridging networks are generally always interconnected local area networks since broadcasting every message to all possible destinations would flood a larger network with unnecessary traffic. For this reason, router networks such as the Internet use a scheme that assigns addresses to nodes so that a message or packet can be forwarded only in one general direction rather than forwarded in all directions.

A bridge works at the data-link level of a network, copying a data frame from one network to the next along the communications path.

A bridge is sometimes combined with a router in a product called a brouter.



Source Route Bridge

• Each workstation must determine the route to use to transmit each frame. Source route bridges require some manual configuration and depend on each workstation having the right hardware and software. Source route bridges do not require as much processing power as transparent bridges.

Transparent Bridge

The bridge listens to all messages transmitted on the networks to which it is connected. A bridge learns device addresses by building a forwarding table mapping the source address from each frame it receives to the port on which it is received. A bridge filters traffic, ignoring frames with destination addresses mapped to the same port on which they are received. (The bridge can be configured to perform additional filtering based on various criteria.) Finally, a bridge forwards traffic to the appropriate ports to allow data to reach its destination.

A transparent bridge examines the destination address of all messages transmitted on each network to which it is connected. If the destination address is associated with the port on which it was received, the message is not forwarded to transmitting ports. If the destination address is known and is not associated with the port on which it was received, the bridge forwards the frame to the port associated with the destination. For instance a message with the destination address AA received on port 1 is ignored, but one destined for BB is forwarded on port 2.

With Ethernet's CSMA/CD access method, performance decreases as the size of the collision domain increases. Filtering lets bridges improve network performance by reducing the size of the Ethernet collision domain. Since traffic for destinations on the local segment remains local, the number of users in a single collision domain is reduced.



Bridges are often compared to routers, because both kinds of devices are used to interconnect different physical networks. Although these devices are sometimes used interchangeably, bridges work at the data link layer and routers work at the network layer. As a result, there are situations in which each kind of device is more efficient. A bridge will tend to be more useful than a router under the following circumstances:

• A small number of LAN segments is required.

• No separation is desired between LAN segments.

• Fast response time or very high bandwidth is required across different physical networks.

• Technical expertise required to administer routers is scarce.

• Nodes using LAT or other nonroutable protocols need to be interconnected.

• More than four protocols need to be interconnected.

• Communication is between a central network and branch networks, with little communication among the branches.

Local bridges interconnect LANs directly, while remote bridges connect LANs over WAN links. Although bridges can be configured to filter broadcasts over WAN links, this requires a detailed knowledge of the protocols in use on the LANs that are being connected. Otherwise, essential broadcasts may be filtered out. This, and the fact that routers provide better local administrative control, means that remote routers are usually a better choice for establishing WAN connections.



BPDUs contain the following information:

• Protocol ID—Defines this packet as a BPDU.

• Version—The current version used by this BPDU packet.

• Message Type—Indicates the stage of the negotiation.

• Flags.Used to indicate a topology change.

• Root ID.A number composed of the assigned bridge priority (most significant two octets) followed by the bridge MAC address.

• Root cost.Cost of the total path to the root bridge from the bridge sending the BPDU.

• Bridge ID.ID of the bridge sending the BPDU; actually composed of the bridge priority (2 bytes) and the bridge MAC address (6 bytes).

• Port ID.Made up of the configured port priority (most significant octet) and the interface number (the order of circuit configuration).

• Message Age.Timers used for message aging and other configuration information.

• Max Age.The message age value at which a stored configuration message is judged “too old” and is discarded.

• Hello Time.The time that elapses between the generation of configuration messages by a bridge that assumes itself to be the root.

• Forward Delay.A parameter that temporarily prevents a bridge from starting to forward data packets to and from a link until news of a topology change has spread to all parts of a bridged network. This should give all links that need to be turned off in the new topology time to do so before new links are turned on.



To connect several Segments, we can use a Switch. A Switch is a Layer 2 device, it forms bridge between various segments and also does basic Data layer error checking. Addressing is limited to Media Access Codes (MAC) of the DTE’s. MAC’s are unique codes provided in the Network Interface Cards (NIC) of each DTE. MAC is unique all over the world.

The Ethernet switch is different from a bridge because it does not need to read the entire frame before selecting the destination port. This means that the switch can begin transmitting the frame before it has been completely received. This results in much smaller delay times, compared to a bridge. An Ethernet switch reads only the first 6 bytes, which contain the data link layer destination address, before it begins transmitting the frame.

The Ethernet input controllers (also called packet processors) learn the network's topology by reading the source address of incoming packets and by storing these addresses in a resource table. The packet processors refer to this table when determining whether to forward a packet or not. The control processor, called the system module, maintains address tables in each packet processor.

Switches do not normally use store/forward technology; as a result, they provide the lowest latency of all networking devices. Switches do provide buffering, since this is necessary when the output port is busy. However, as long as the output port is available, no buffering is required.

Like a bridge, when the Ethernet switch starts, all address tables are empty. Initially, each packet received by the switch is sent to the system module with the port of entry marked, so that the system module knows the packet port of origin. The system module reads the packet source address and sends data to each packet processor to set up its lookup table. Future packets are then directly routed through the cross-point matrix, without passing through the system module.

Whenever the packet processor receives a packet with an unknown source or destination address, it marks the packet with the port of entry and sends it to the system module. The system module verifies that the new packet is fully formed. If the packet destination is known, the system module sends the packet to all output ports. As with bridges, the reply to flooding provides the switch with the port connected to the destination station.



When we need to cover the entire Globe, it is not possible to have one unique network covering all the machines. So M/c’s connected in a LAN is treated as a Network, identified by a Nw ID. All M/C’s inside a Network (say LAN) are distinctly identified by Host ID. However the Network Layer concentrates on the Nw ID’s and focuses on transfer from one Network to another. Internal distribution inside a Network can be handled by Layer 2.Now Nw ID and Host ID need to be Logical Address as the same M/c (say your Laptop) could be sometime in Nw 1 and sometime in Nw 23 (or any other Network). So we need Logical Addresses (Layer 3 Address) on top of Physical Address (L2 address) like MAC.Also because the WAN spans over multiple Continents/ Countries/ Cities, the physical connections, which are long distance and passes through several Network Elements, are less reliable than the LAN connections. Hence redundant paths are maintained, although the cost a lot of money. This gives rise to situation where you may find multiple routes possible from Source Network to destination. The Network Layer has to maintain all the relevant info about these alternate routes and also find out which one is the best route.



Once a LAN is established would come the need to interconnect them. That is done by a Router. Routers provide Layer 3 services, i.e. they serve to packetise data and send them across the network. This they do by following some Addressing philosophy like the IP (Internet Protocol).



Routers base their routing decisions on the network number within the network layer address. Routers route to networks, while bridges direct traffic using the physical addresses of specific devices. Since there are fewer networks in the network than stations, the routing table is smaller and the forwarding decision is simpler. The routing table maps each of the router's ports to the networks that can be reached through each port. When the router receives a packet, it transmits the packet on the appropriate port.

A routing table generally provides some information about each path, such as the number of hops required to reach each network by means of each port. This allows the router to choose the best path to a particular network, based on various criteria such as hop count, configured cost, or bandwidth. If a path becomes unavailable for some reason, a router will direct traffic to the most efficient path that remains available.

If a port provides a connection directly to the destination network, routing is complete. Otherwise, if another router receives the packet, the process is repeated. Each router transmits the packet toward the destination network on the most efficient path it knows about. The packet proceeds, hop by hop, until it reaches its destination network.

Each time a router forwards a packet, it increments the hop counter in the network layer header. Many routers (such as IP routers) will discard a packet when the hop count reaches 16. This prevents packets from roaming endlessly about the network. Routing table entries with a path cost of 16 hops are generally considered unreachable by such routers, and packets for those addresses are discarded.

Routers do not listen to every packet on the network; only those packets explicitly addressed to the router are monitored. For this reason, end stations must be aware



FPC: Flexible PIC ConcentratorPIC: Physical Interface Card



Routing is the process of creating and maintaining Routing Table. Essential to this is the information of Physical Connectivity. This information can be stored and updated either Statically or Dynamically.

To make the Routing process simple, Physical connection that a Router remembers is limited to it’s immediate neighbours only. Example: Router A remembers that it has a physical link with Router C. So also does B, D and E. D and E also remember that they have physical links with F and G, respectively in addition to a link between themselves. Similarly F remember it’s links with H,I and J in addition to one with D and G remembers it’s link with J in addition to the one with E.

Routers A and B can now be Logically linked to rest of the Routers through their physical links with C. Like A is logically linked to B through C and visa-versa. This addition information is also stored in the Routing Table but only after these information is received from the neighbouring routers. This exchange of information can be done by the Routers in a iterative process of transfer of Link-state information between themselves or can be feed into it manually.



Static Routing

The first routers had to be manually configured with the numbers of each network that could be reached by each of a router's ports. This kind of routing is called static routing. In a large network, performing this configuration and maintaining its accuracy is a time-consuming task. However, under some circumstances, this is still the only way to provide a router with the information it needs to reach certain networks.

Routing Protocols

To overcome the difficulty with static routing, most routers provide a routing protocol. Routing protocols allow routers to broadcast information to each other about the network topology. Using a routing protocol allows the status of links and networks to be updated dynamically. Depending on the protocol, routers may inform each other of error rates, link failures, network configuration, or other information.

There are two basic types of routing protocols in common use today:

• Distance vector

• Link state



1970 23 hosts connectedARPANET starts using NCP (Network Control Program)Host-to-host communicationReliable end-to-end communication

Sent one packet at a time; waited for acknowledgementDealt with packet format and format of data within the packet

BBN, MIT, Rand, SDC, and Harvard join ARPANET1972 Internetworking Working Group (INWG) formed

Goal was to establish protocolsVint Cerf named first chairman, known as “Father of the Internet”First e-mail program designed Abhay

BhushanBasic send-read software for e-mail Ray Tomlinson“to list”, “file”, “forward”, “respond” capabilities Lawrence

Roberts1978 Last iteration of TCP is produced

Internet Protocol (IP) proposedVint Cerf, Jonathan Postel, and Danny CohenSegregation of data addressing and error/congestion detection

1979 USENET newsgroups developedDuke University & University of North CarolinaUtilized Unix-to-Unix Copy Protocol (UUCP)

Early 80s CSNET formed (Computing Science Network)Non-ARPANET users make dial-up connectionsSend/receive e-mailBITNET (Because It’s Time Network)

Connected IBM systems together



There 42,759 routes listed in the GIRT – Global Internet Routing TableData transported “between” ISP networks

PeeringISPs exchanging each other’s trafficBi-lateral or Multi-lateral agreements

Private peeringTwo ISPs exchanging IP packets with each other

Occurs in a co-location facilityPublic PeeringMultiple ISPs exchanging IP packets with each other

Metropolitan Area Exchange (MAE®)Network Access Point (NAP)



Version—Indicates the version of IP currently used.•IP Header Length (IHL)—Indicates the datagram header length in 32-bit words.•Type-of-Service—Specifies how an upper-layer protocol would like a currentdatagram to be handled, and assigns datagrams various levels of importance.•Total Length—Specifies the length, in bytes, of the entire IP packet, including thedata and header.•Identification—Contains an integer that identifies the current datagram. This field isused to help piece together datagram fragments.•Flags—Consists of a 3-bit field of which the two low-order (least-significant) bitscontrol fragmentation. The low-order bit specifies whether the packet can befragmented. The middle bit specifies whether the packet is the last fragment in aseries of fragmented packets. The third or high-order bit is not used.•Fragment Offset—Indicates the position of the fragment's data relative to thebeginning of the data in the original datagram, which allows the destination IPprocess to properly reconstruct the original datagram.•Time-to-Live—Maintains a counter that gradually decrements down to zero, atwhich point the datagram is discarded. This keeps packets from looping endlessly.•Protocol—Indicates which upper-layer protocol receives incoming packets after IPprocessing is complete.•Header Checksum—Helps ensure IP header integrity.•Source Address—Specifies the sending node.•Destination Address—Specifies the receiving node.•Options—Allows IP to support various options, such as security.•Data—Contains upper-layer information.



Address Block Date Registry - Purpose Notes or

Reference

000/8 Sep 81 IANA - Reserved



003/8 May 94 General Electric Company

004/8 Dec 92 Bolt Beranek and Newman Inc.

005/8 Jul 95 IANA - Reserved

006/8 Feb 94 Army Information Systems Center

007/8 Apr 95 IANA - Reserved

008/8 Dec 92 Bolt Beranek and Newman Inc.

009/8 Aug 92 IBM

010/8 Jun 95 IANA - Private Use See [RFC1918]

011/8 May 93 DoD Intel Information Systems

012/8 Jun 95 AT&T Bell Laboratories

013/8 Sep 91 Xerox Corporation

014/8 Jun 91 IANA - Public Data Network

015/8 Jul 94 Hewlett-Packard Company

016/8 Nov 94 Digital Equipment Corporation

017/8 Jul 92 Apple Computer Inc.

018/8 Jan 94 MIT

019/8 May 95 Ford Motor Company

020/8 Oct 94 Computer Sciences Corporation



Address Block Date Registry - Purpose Notes or

Reference

021/8 Jul 91 DDN-RVN

022/8 May 93 Defense Information Systems Agency

023/8 Jul 95 IANA - Reserved

024/8 May 01 ARIN - Cable Block (Formerly IANA - Jul 95)

025/8 Jan 95 Royal Signals and Radar Establishment

026/8 May 95 Defense Information Systems Agency


028/8 Jul 92 DSI-North

029/8 Jul 91 Defense Information Systems Agency

030/8 Jul 91 Defense Information Systems Agency


Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine. [You can test this: you should always be able to ping 127.0.0.1, which points to yourself] Class D addresses are reserved for multicasting. Class E addresses are reserved for future use. They should not be used for host addresses.

Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N) and which part belongs to the node (n).

•Class A -- NNNNNNNN.nnnnnnnn.nnnnnnn.nnnnnnn

•Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn

•Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn



Class-full & Classless NetworksWhile Class A, B & C defines three specific sizes of network, not all requirements fit

into their sizes. If someone needs about 600 IP’s he cannot do with just 1 Class Cnetwork, while getting a Class B would be not only costly but also a gross underutilisation of IP addresses.In such case a Classless network can be evolved.



How Subnet Masks are Used to Determine the Network Number

The router performs a set process to determine the network (or more specifically, thesubnetwork) address. First, the router extracts the IP destination address from theincoming packet and retrieves the internal subnet mask. It then performs a logicalAND operation to obtain the network number. This causes the host portion of the IPdestination address to be removed, while the destination network number remains.The router then looks up the destination network number and matches it with anoutgoing interface. Finally, it forwards the frame to the destination IP address.Specifics regarding the logical AND operation are discussed in the following section.

Logical AND Operation

Three basic rules govern logically "ANDing" two binary numbers. First, 1 "ANDed"with 1 yields 1. Second, 1 "ANDed" with 0 yields 0. Finally, 0 "ANDed" with 0 yields 0.The truth table provided in table 30-4 illustrates the rules for logical AND operations.



How Subnet Masks are Used to Determine the Network NumberThe router performs a set process to determine the network (or more specifically, thesubnetwork) address. First, the router extracts the IP destination address from theincoming packet and retrieves the internal subnet mask. It then performs a logicalAND operation to obtain the network number. This causes the host portion of the IPdestination address to be removed, while the destination network number remains.The router then looks up the destination network number and matches it with anoutgoing interface. Finally, it forwards the frame to the destination IP address.Specifics regarding the logical AND operation are discussed in the following section.Logical AND OperationThree basic rules govern logically "ANDing" two binary numbers. First, 1 "ANDed"with 1 yields 1. Second, 1 "ANDed" with 0 yields 0. Finally, 0 "ANDed" with 0 yields 0.The truth table provided in table 30-4 illustrates the rules for logical AND operations.



Examples of Sub-netting in Class B operatorNumber Subnet Mask Number of Number of Hostsof Bits Subnets2 255.255.192.0 2 163823 255.255.224.0 6 81904 255.255.240.0 14 40945 255.255.248.0 30 20466 255.255.252.0 62 10227 255.255.254.0 126 5108 255.255.255.0 254 2549 255.255.255.128 510 12610 255.255.255.192 1022 6211 255.255.255.224 2046 3012 255.255.255.240 4094 1413 255.255.255.248 8190 614 255.255.255.252 16382 2



In order for IP hosts within the same network to communicate, they must know

each others MAC address in order to correctly address the packet to the

appropriate NIC. The destination MAC address is used by the NIC to

determine if the current packet is destined for that host.

Address resolution is the process of mapping a Network Layer address to a

Data Link Layer address, such as a MAC address. IP addresses are assigned

to hosts and are logically independent of their physical address. The higher-

layer software must depend on the Data Link Layer to deliver data to a host on

the same LAN or VLAN. Therefore, the IP address must be mapped to the

physical address of the host.

Address Resolution Protocol (ARP) is the protocol used to bind an Internet

address to a physical hardware, MAC, address.

A node ARPs another node when it determines that the destination address is

in the same network as the sender. It does this by comparing the network

portion of its own address with the target address. If they match, the hosts can

ARP each other and communicate directly. If they are different, the packets

must go through a router. In that case, the host would ARP its default gateway

and send the packets there for routing. Remember that router ports have MAC

addresses too.



The Internet Control Message Protocol (ICMP) is a network-layer Internet protocol that provides message packets to report errors and other information regarding IP packet processing back to the source. ICMP is documented in RFC 792.

ICMP MessagesICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP messages.When an ICMP destination-unreachable message is sent by a router, it means that the router is unable to send the package to its final destination. The router then discards the original packet. Two reasons exist for why a destination might be unreachable. Most commonly, the source host has specified a nonexistent address. Less frequently, the router does not have a route to the destination.Destination-unreachable messages include four basic types: network unreachable, host unreachable, protocol unreachable, and port unreachable. Network-unreachable messages usually mean that a failure has occurred in the routing or addressing of a packet. Host-unreachable messages usually indicates delivery failure, such as a wrong subnet mask. Protocol-unreachable messages generally mean that the destination does not support the upper-layer protocol specified in the packet. Port-unreachable messages imply that the TCP socket or port is not available.An ICMP echo-request message, which is generated by the ping command, is sent by any host to test node reachability across an internetwork. The ICMP echo-reply message indicates that the node can be successfully reached.An ICMP Redirect message is sent by the router to the source host to stimulate more efficient routing. The router still forwards the original packet to the destination. ICMP redirects allow host routing tables to remain small because it is necessary to know the address of only one router, even if that router does not provide the best path. Even after receiving an ICMP Redirect message, some devices might continue using the less-efficient route.An ICMP Time-exceeded message is sent by the router if an IP packet's Time-to-Live field (expressed in hops or seconds) reaches zero. The Time-to-Live field prevents packets from continuously circulating the internetwork if the internetwork contains a routing loop. The router then discards the original packet.



DNSTranslates between IP addresses and hostnames

Hierarchical name structure with variable levels

13 root name servers

Country Code Domains (.AF)

Generic Domains (.com, .net, .org)

International Domains (.int)

US Domains (.edu, .gov, .mil)

Other (.arpa)

Proposed (.firm, .web, .shop, .rec, .arts, .nom, .info)

DNS utilizes a label tree



IPv6One of the newest major standards on the horizon is IPv6. Although IPv6 has not officially become a standard, it isworth some overview. It is very possible that this information will change as we move closer to IPv6 as a standard,so you should use this as a guide into IPv6, not the definitive information.A number of books are now being published that cover in detail this emerging standard; if you are looking formore details you should refer to these books. All the RFCs available on the Internet have the raw details on howthis standard is developing. However, these documents are difficult to interpret at first glance and require somecommitment to going through any number of RFCs pertaining to many subjects all related to IPv6 development.Internet Protocol Version 4 is the most popular protocol in use today, although there are some questions aboutits capability to serve the Internet community much longer. IPv4 was finished in the 1970s and has started toshow its age. The main issue surrounding IPv6 is addressing—or, the lack of addressing—because many expertsbelieve that we are nearly out of the four billion addresses available in IPv4. Although this seems like a very largenumber of addresses, multiple large blocks are given to government agencies and large organizations. IPv6 couldbe the solution to many problems, but it is still not fully developed and is not a standard—yet!Many of the finest developers and engineering minds have been working on IPv6 since the early 1990s. Hundredsof RFCs have been written and have detailed some major areas, including expanded addressing, simplified headerformat, flow labeling, authentication, and privacy.Expanded addressing moves us from 32-bit address to a 128-bit addressing method. It also provides newer unicast and broadcasting methods, injects hexadecimal into the IP address, and moves from using "." to using ":" as delimiters. Next slide shows the IPv6 packet header format.



RIP Characteristics

RIP is a distance vector protocol that uses the Bellman-Ford algorithm to control the decisions involved in dynamically updating routing tables. RIP makes its routing decisions based solely on distance (hops). RIP allows a maximum of 15 router hops between networks because of the time it takes for all routers to converge (stabilize their routing tables).

Some implementations may allow the use of extended RIP (up to 127 hops) in the network topology. It is not recommended to increase the network diameter beyond the 15-hop limit. Doing so will cause a significant increase in network convergence time. RIP does not take into consideration such things as congestion, line speed, and cost.

A list of routes presently known to the router is broadcast to each interface every 30 seconds. Routing tables are exchanged at the following times:

• Initial broadcast (router entering the network)

• Every 30 seconds (unsolicited)



Routing Updates

RIP sends routing-update messages at regular intervals and when the networktopology changes. When a router receives a routing update that includes changes toan entry, it updates its routing table to reflect the new route. The metric value for thepath is increased by 1, and the sender is indicated as the next hop. RIP routersmaintain only the best route (the route with the lowest metric value) to a destination.After updating its routing table, the router immediately begins transmitting routingupdates to inform other network routers of the change. These updates are sentindependently of the regularly scheduled updates that RIP routers send.

RIP Routing Metric

RIP uses a single routing metric (hop count) to measure the distance between thesource and a destination network. Each hop in a path from source to destination isassigned a hop count value, which is typically 1. When a router receives a routingupdate that contains a new or changed destination network entry, the router adds 1to the metric value indicated in the update and enters the network in the routingtable. The IP address of the sender is used as the next hop.



RIP Stability Features

RIP prevents routing loops from continuing indefinitely by implementing a limit onthe number of hops allowed in a path from the source to a destination. Themaximum number of hops in a path is 15. If a router receives a routing update thatcontains a new or changed entry, and if increasing the metric value by 1 causes themetric to be infinity (that is, 16), the network destination is considered unreachable.The downside of this stability feature is that it limits the maximum diameter of a RIPnetwork to less than 16 hops.

RIP includes a number of other stability features that are common to many routingprotocols. These features are designed to provide stability despite potentially rapidchanges in a network's topology. For example, RIP implements the split horizon andholddown mechanisms to prevent incorrect routing information from being



When a router starts, it initializes OSPF and waits for indications from lower-level

protocols that the router interfaces are functional.

The router then uses the OSPF hello protocol to acquire neighbors, by sending and

receiving hello packets.

On broadcast or non-broadcast multi-access networks the OSPF hello protocol elects

a designated router for the network.

This router is responsible for sending link-state advertisements that describe the

network, which reduces the amount of network traffic and the size of the routers’

topological databases.



The topology of an area is hidden from the rest of the AS, thus significantly reducing routing traffic in the AS. Also, routing within the area is determined only by the area’s topology, providing the area with some protection from bad routing data. All routers within an area have identical topological databases. Routers that belong to more than one area are called area border routers. They maintain a separate topological database for each area to which they are connected.

Internal Routers

An internal router is a router with all directly connected networks belonging to the same area. Routers with only backbone interfaces also belong to this category. These routers run a single copy of the basic routing algorithm and maintain one Shortest Path First (SPF) for that area.

Area Border Routers

An area border router is a router with interfaces in multiple areas. Area border routers maintain multiple link state databases (LSDBs), one copy for each attached area including the backbone. Area border routers must be connected to the backbone ( Area 0.0.0.0 ).

Backbone Routers

A backbone router is a router with an interface to the backbone. This router can also be an area border router or an internal router. Area border routers are, by definition, also backbone routers.

AS Boundary Routers

OSPF views non-OSPF networks as outside its autonomous system (AS) and, therefore, external to it. An OSPF router connected to such networks, Routing



A BGP system shares network reachability information with adjacent BGP systems, which are referred to as neighbors or peers.

BGP systems are arranged into groups. In an internal BGP group, all peers in the

group—called internal peers—are in the same AS. Internal peers can be anywhere in the local AS and do not have to be directly connected to each other. Internal groups use routes from an IGP to resolve forwarding addresses. They also propagate external routes among all other internal routers running internal BGP, computing the next hop by taking the BGP next hop received with the route and resolving it using information from one of the interior gateway protocols.

In an external BGP group, the peers in the group—called external peers—are in different AS’s and normally share a subnet. In an external group, the next hop is computed with respect to the interface that is shared between the external peer and the local router.

The route to each destination is called the AS path, and the additional route information is included in path attributes.

BGP uses the AS path and the path attributes to completely determine the network topology, detect and eliminate routing loops and it can enforce administrative preferences and routing policy decisions.

BGP supports two types of exchanges of routing information: exchanges between different ASs and exchanges within a single AS. When used between ASs, BGP is called external BGP (eBGP) and BGP sessions perform inter-AS routing.

When used within an AS, BGP is called internal BGP (iBGP) and BGP sessions perform intra-AS routing. Next slide illustrates ASs, IBGP, and EBGP.



The Border Gateway Protocol (BGP) is an exterior gateway protocol (EGP) that is used

to exchange routing information among routers in different autonomous systems

(ASs).

BGP routing information includes the complete route to each destination. BGP uses

the routing information to maintain a database of network reachability information,

which it exchanges with other BGP systems.

BGP uses the network reachability information to construct a graph of AS

connectivity, thus allowing BGP to remove routing loops and enforce policy decisions

at the AS level.

BGP allows for policy-based routing. You can use routing policies to choose among

multiple paths to a destination and to control the redistribution of routing

information.

BGP uses the Transmission Control Protocol (TCP) as its transport protocol, using port

179 for establishing connections.

Running over a reliable transport protocol eliminates the need for BGP to implement

update fragmentation, retransmission, acknowledgment, and sequencing.

BGP Version 4 also supports aggregation of routes, including the aggregation of AS

paths.



A BGP system shares network reachability information with adjacent BGP systems, which are referred to as neighbors or peers.

BGP systems are arranged into groups. In an internal BGP group, all peers in the

group—called internal peers—are in the same AS. Internal peers can be anywhere in the local AS and do not have to be directly connected to each other. Internal groups use routes from an IGP to resolve forwarding addresses. They also propagate external routes among all other internal routers running internal BGP, computing the next hop by taking the BGP next hop received with the route and resolving it using information from one of the interior gateway protocols.

In an external BGP group, the peers in the group—called external peers—are in different AS’s and normally share a subnet. In an external group, the next hop is computed with respect to the interface that is shared between the external peer and the local router.

The route to each destination is called the AS path, and the additional route information is included in path attributes.

BGP uses the AS path and the path attributes to completely determine the network topology, detect and eliminate routing loops and it can enforce administrative preferences and routing policy decisions.

BGP supports two types of exchanges of routing information: exchanges between different ASs and exchanges within a single AS. When used between ASs, BGP is called external BGP (eBGP) and BGP sessions perform inter-AS routing.

When used within an AS, BGP is called internal BGP (iBGP) and BGP sessions perform intra-AS routing. Next slide illustrates ASs, IBGP, and EBGP.



Suggested Reading:

1. Internetworking TutourialCisco www.cisco.com

2. LAN/ WAN Tutorial www.itpapers.com

3. ATM, MPLS Tutorial www.iec.org

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