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Networks notes

Apr 12, 2017



Q) Explain about DQDBIEEE 802.6 is standards for MANs. IEEE 802.6 standard uses the Distributed Queue Dual Bus (DQDB) network form.DQDB uses two unidirectional buses and are labelled Bus A and Bus B as shown in the below figure The direction of traffic is the opposite of traffic on other. Bus A traffic moves from right to left.Bus B traffic moves from left to right. All the station ahead of the station on a bus are said to be downstream stations to that station. Example:For station 3 on Bus A, station 4 and station 5 are said to be downstream station and station 2 and station 1 are said to be upstream stations.Data travels in each slot in 53 byte slots. To send data on one bus, a station must make a reservation on other bus. To make and track these reservations, each station stores two queues one for each bus. Each station has one queue for Bus A, called queue A and one queue for Bus B called queue B.For station X to make reservation, it must know how many of its neighbours had already made reservations. To track these reservations, it uses virtual tokens. It adds a token at the rear of queue each time a slot passes on Bus B with reservation bit set.When the station X needs to make reservation for itself, it sets one of the reservation bits in a slot passing on Bus B. The Station X then inserts special token into its queue A.Station X watches the unoccupied slots passing in Bus A. For each empty slot that passes, it removes and discards one token from the front of the queue A . When it sees an empty slot and finds its own token at the front of the queue A, it discards the token and inserts its data in the empty slot.DQDB operates in physical and the MAC sublayer. DQDB can also be implemented as ring topology.Q) Explain about Subscriber access to ISDNAn ISDN provides digital services to users over integrated digital networks.ISDN incorporates all the connections from home or building into a single interface.Digital subscriber loops are 2 types.1. Basic rate interface (BRI)Consists of 2 B channels (64 Kbps each) +one 16Kbps D channel+48 Kbps operating overhead = 192 KbpsBRI is designed to meet the needs of residential and small office customers

2. Primary rate interface (PRI)Consists of 23 B channels (64 Kbps each) +one 64Kbps D channel+8 Kbps operating overhead = 1.544 Mbps

Each digital subscriber loop has multiple channels. Channels are 3 types as shown in table belowChannel Data rate (Kbps)

Bearer channel (B channel)64Used to carry digital data, digital voice or other low data rate information

Data channel (D channel)16, 64Carries control signals. Rarely D channel is used to for low rate data transfer.

Hybrid channel (H channel)384, 1536, 1920Used to carry high data rate applications such as video, teleconferencing etc..

A subscriber uses the D channel to connect to a network and secure a B channel connection. The subscriber then uses the B channel to send data to another user. All the devices attached to a given subscriber loop use the same D channel for signalling but each sends data over dedicated B channel to that exchange.Functional groupings: defines the functions of each type of equipment used in the ISDN.Functional groupings include 1. Network termination type 1 (NT1):connects users system to ISDN

2. Network termination type 2 (NT2):connects multi user system to NT1. NT2 can be a LAN, PBX etc..

3. Terminal Equipment type 1 (TE1)is any DTE that supports ISDN standard. TE1 can be digital telephone, digital fax machine etc..

4. Terminal Equipment type 2 (TE2)is any DTE that do not supports ISDN standard. Hence it requires TA to connect to ISDN network. TE2 can be regular telephone, host computer, workstation etc.

5. Terminal adapter (TA)Converts non-ISDN format to ISDN format.

Reference points define how two network elements must be connected and the format of traffic between them.Q) Differences between broad band and narrow band ISDNNarrow band ISDN (N-ISDN)Broadband ISDN (B-ISDN)

Data rates64 kbps to 1.544 Mbps600 Mbps

Cables usedUsed telephone wiring (Twisted pair)Fibre optics cable

Q) Explain about Broad band ISDNBroadband ISDN offers data rates in the range of 600 Mbps and uses fibre optics cable at all levels of telecommunications.Broadband ISDN provides 2 types of services1. Interactive services are bidirectional services between subscriber and service provider (or) between 2 subscribers. These services are 3 typesa. Conversational services are real time services Ex: Telephony, video conferencing etc.b. Messaging services are store and forward services. Ex: voice mail, video mail etc.c. Retrieval services allows users to retrieve information on demand. Ex: videotex that allows subscribers to select video data from online library

2. Distributive services are unidirectional services sent from provider to subscriber. These services can bea. Without User Control : Example : Commercial TV . Programming content and times are decided by the provider alone.b. With User Control: Example: Pay TV. A program is made available in a limited number of time slots. A user must activate his TV at any time slot of his choice to see.Physical SpecificationsPhysical layer of B-ISDN is same as ATM but differs in access methods, functional groupings and reference points.B- ISDN defines 3 access methods1. Symmetrical 155.520 Mbps full duplex is designed to full the needs of residential users.2. Asymmetrical 155.520 Mbps output / 622.080 Mbps input : is used when the input needs of the subscribers are far greater than their output needs3. 622.080 Mbps full duplex : is designed for businesses that provide and receive distributed servicesFunctional groupings are B-NT1, B-NT2, B-TE1, B-TE2 and B-TAReference Points: R, S, T and U

Q) Explain about Networking and Internetworking Devices1. Networking devicesRepeaters: operates in physical layer of OSI model. A repeater does not amplify the signal; it regenerates it. When it receives a weakened or corrupted signal, it creates a copy bit for bit at the original strength.Repeater extends the length of the network.

Bridges: operate in both physical and data link layer of OSI modelBridges divide a large network into smaller segments.Bridges keep the traffic for each segment separate.If source and destination stations are on the same segment, bridge will not broadcast the frame to other segments.A bridge maintains a look up table that contain the physical addresses of every station connected to it.A bridge connect LANs using different data link layer protocols such as Ethernet LAN to Token Ring LAN. Inorder to do so there are many issues to be considered and they are1. Frame format2. Payload size3. data rate4. address bit order5. other issues such as ack, priority and collsion

2. Internetworking devicesRoutersRouters operate in physical, data link and network layers of OSI model.Routers access the network layer address. Routers do not broadcast like bridges.If more than one path exists between source and destination network, router chooses the best path using least cost routing algorithms (Distance vector and Link state routing algorithms).

GatewaysGateways operate in all seven layers of OSI model. Gateway is usually software installed within a router.Gateway is a protocol converter. a packet formatted in one protocol (AppleTalk) is converted to a packet formatted in TCP/IP protocol.

Q) Explain different types of routingRouting is classified as 1. non adaptive routing: the router sends all packets along one route2. adaptive routing: router may select a new route for each packet

Q) What is Packet lifetime?To avoid packet being circulated in infinite loops, packet life time or Time to live (TTL) field is added in IP. Each router that receives the packet subtracts 1 from packet lifetime value. When this value becomes zero, the packet is destroyed.

Q) Explain types of bridges1. Simple Bridge: are least expensive. Entering and updation of physical addresses is done by user.2. Multiport Bridge: are used to connect more than 2 LANs. In the below figure, bridge has 3 tables for each LAN.3. Transparent Bridge (or) Learning Bridge: builds look up table on its own. Initially the table is empty. As the frames are sent through the bridge, it understands the segment to which source stations belongs to; by the direction from which frame is received. It then updates its tables.

Q) How does a transparent bridge avoids loopsWhen transparent bridges are used, they may create loops, to avoid this we can use1. Spanning tree algorithm2. Source routing: source station will decide through which LANs and bridges, the frame has to go.

Q) Explain about Link state least cost routing algorithm1. Each router sends a short greeting packet to its neighbors to find out the state of each link.2. It then prepares a Link state packet (LSP) based on the results of greeting.An LSP contains 4 fieldsAdvertiserNetworkCost Neighbor

3. It then floods the network with LSP. This is known as Advertising.4. Every router receives every LSP and each router stores this information in its Link state database.5. Every router has exactly the same Link state database.6. To calculate its routing table, each router applies Dijkstras algorithm to its link state database.

Q) what are the functions of network layer in TCP/IP1. Routing: decides the best path from source to destination2. Packetizing: encapsulates the data from upper layers in its packets.3. Logical addressing: provides unique logical address to each device 4. Internetworking: translates logical addresses to physical address and vice versa5. Fragmentation: divides larger packets into smaller fragmentsAt network layer TCP/IP supports Internetwork protocol (IP). IP supports 4 supporting protocols1. ARP2. RARP3. ICMP4. IGMP

Q) What are the funct

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