ATM QoS

The five ATM service classes are:

1.    constant bit rate (CBR)

2.    variable bit rate—real-time (VBR-rt)

3.    variable bit rate—non-real-time (VBR-nrt)

4.    available bit rate (ABR)

5.    unspecified bit rate (UBR)

Constant Bit Rate

Sources of CBR traffic are expected to send at a constant rate.

The source’s peak rate and average rate of transmission are equal.

CBR class is designed for customers who need real-time audio or video services. CBR is a relatively easy service for implementation

Variable Bit Rate

The VBR class is divided into two subclasses: real-time (VBR-rt) and non-real-time (VBR-nrt).

VBR-rt is designed for users who need real-time services (such as voice and video transmission) and use compression techniques to create a variable bit rate.

The traffic generated by the source is characterized by a token bucket, and the maximum total delay required through the network is specified.

VBR-nrt bears some similarity to IP’s controlled load service. The source traffic is specified by a token bucket.

VBR-nrt is designed for users who do not need real-time services but use compression techniques to create a variable bit rate

Unspecified Bit Rate

UBR class is a best-effort delivery service that does not guarantee anything. UBR allows the source to specify a maximum rate at which it will send.

o   Switches may make use of this information to decide whether to admit or reject or negotiate with the source for a less peak rate.

Available Bit Rate

ABR apart from being a service class also defines a set of congestion-control mechanism.

The ABR mechanisms operate over a virtual circuit by exchanging special ATM cells called resource management (RM) cells between the source and destination.

ABR allows a source to increase or decrease its allotted rate as conditions dictate.

ABR class delivers cells at a minimum rate. If more network capacity is available, this minimum rate can be exceeded.

ABR is suitable for applications that are bursty in nature. 

Multicast routing protocols

Explain Multicast routing protocols in detail.

A host places a multicast address in the destination address field to send packets to a set of hosts belonging to a group.

Internet multicast can be implemented on physical networks that support broadcasting by extending forwarding functions. The extended ones are:

o  Link-State Multicast

o  Distance-Vector Multicast

o  Protocol Independent Multicast (PIM)

Link-State Multicast

Multicasting is added to the existing link-state routing.

o  Each router knows entire topology by way of update messages.

o   Dijkstra's algorithm is used to compute shortest path spanning tree to reach all destinations.

Each router determines which groups have members on which LAN by monitoring the periodical announcements.

o   If a host does not advertise periodically, then it has left the group.

Equipped with group and membership knowledge, each router computes shortest path multicast tree from any source to any group using Dijkstra's algorithm.

 Link-state routing is expensive as each router must store a multicast tree from every source to every group.

Distance-Vector Multicast

Multicasting is added to existing distance-vector routing in two stages.

o   Each router maintains a table of (Destination, Cost, NextHop) for all destination through exchange of distance vectors.

o  Reverse Path Broadcast mechanism that floods packets to other networks

o   Reverse Path Multicasting that prunes end networks that do not have hosts belonging to a multicast group.

Reverse-Path Broadcasting

A router when it receives a multicast packet from source S to a Destination from NextHop, then it forwards the packet on all out-going links.

The drawbacks are:

o  It floods a network, even if it has no members for that group

o   Duplicate flooding, i.e., packets are forwarded over the LAN by each router connected to that LAN.

Duplicate flooding is avoided by

o   Designating a router on the shortest path as parent router.

o   Only parent router is allowed to forward multicast packets from source S to that LAN.

Reverse-Path Multicasting

Multicasting is achieved by pruning networks that do not have members for a group G. Pruning is achieved by identifying a leaf network, which has only one router (parent). The leaf network is monitored to determine if it has any members for group G.

The router then decides whether or not to forward packets addressed to G over that LAN. The information "no members of G here" is propagated up the shortest path tree.

Thus routers can come to know for which groups it should forward multicast packets. Including all this information in a routing update is expensive.

interdomain routing

What is an autonomous system?

Internet is so large that no one routing protocol can handle the task of updating the routing tables of all routers. Internet is divided into autonomous systems.

An autonomous system is a group of networks and routers under the authority of a single administration.

Routing inside an autonomous system is referred to as intra-domain routing. Routing between autonomous systems is referred to as inter-domain routing.

What is interdomain routing?

The interdomain routing involves AS sharing their reachability information with each other AS.

The goal of interdomain routing is reachability and not optimality.

The two major interdomain routing protocols are Exterior Gateway Protocol (EGP) and Border Gateway Protocol (BGP).

What are the problems in interdomain routing?

An internet backbone must be able to route packets to any destination, i.e., there should be a match in the routing/forwarding table.

Each AS has its own intradomain routing protocols and chooses the metric assigns to path. This varies from one AS to another.

Autonomous systems may not trust each other.

Write short notes on BGP.

Border Gateway Protocol (BGP) is an inter-domain routing protocol using path vector routing

Traffic on the internet can be classified into two types:

o   local traffic that starts/ends on nodes within an AS

o   transit traffic that passes through an AS AS can be classified into three types

o   Stub AS has only a single connection to one other AS. This AS can carry local traffic only, such as Small corporation.

o   Multihomed AS has connections to more than one other AS but refuses to carry transit traffic, such as Large corporation.

o   Transit AS has connections to more than one other AS and is designed to carry both transit and local traffic, such as the backbone providers

Each AS selects one of its nodes to be the BGP speaker.

Speaker node creates a routing table for that AS and advertises it to other BGP speakers in the neighboring ASs.

State the drawbacks of IPv4?

Despite all short-term solutions, such as subnetting, classless addressing, and NAT, address depletion is still a long-term problem in the Internet.

The Internet must accommodate real-time audio and video transmission that requires minimum delay strategies and reservation of resources, which are not provided in IPv4.

The Internet must provide encryption and authentication of data for some applications. No encryption or authentication is provided by IPv4.

circuit and packet switching

What is a switch and its function?

A switch is a multi-input, multi-output device, which transfers packets from an input to one or more outputs.

Large networks can be built by interconnecting a number of switches. Hosts are connected to the switch using point-to-point link.

A switch receives packets on one of its links and transmits them on one or more other links. This is known as switching or forwarding.

List the different types of switched networks.

Circuit switched networks Packet switched networks o  Datagram networks

o Virtual-circuit networks Message switched networks

Bring out the differences between circuit and packet switching.

Circuit switching	Packet switching
Source  and  destination  host  are  physically	No such physical connection exists
connected
Switching takes place at the physical layer	Switching takes place at network (datagram) or
	data link layer (VCN)
Resources such as bandwidth, switch buffer &	Resources are allocated on demand
processing time, are allocated in advance.
Resources  remain  allocated  for  the  entire	Resources can be reallocated when idle.
duration of data communication.
There is no delay during data transfer.	Delay exists at each switch during data transfer
Data transferred between the two stations is a	Data is transferred as discrete packets
continuous flow of signal
Example: Telephony	Example: Internet

CSMA/CD

Explain how collision is handled using CSMA/CD over a wired medium.

The Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method augments CSMA algorithm to handle collisions when detected over a wired medium.

In this method, a station monitors the medium after it sends a frame

The channel is sensed before frame transmission by using one of the persistence methods. If there is a collision, the frame is sent again, otherwise successful transmission.

Transmission and collision detection is a continuous process. The station transmits and receives simultaneously.

For CSMA/CD to work, the sending station must detect a collision before sending the last bit of the frame, since sender does not keep a copy of the frame.

Frames should be long enough to allow collision detection prior to end of transmission

If a collision is detected during transmission, the station transmits a brief jamming signal to ensure that all stations know that there has been a collision and aborts transmission.

The station waits for a random amount of time and attempts retransmission. Retransmission is given after 15 attempts.

Example

At time t1, station A starts sending the bits of its frame.

At time t2, station C has not yet sensed the first bit sent by station A.

Station C starts sending bits in its frame, which propagate in both directions.

Station C detects a collision at time t3 when it receives the first bit of A's frame. Station C sends a jamming signal and immediately aborts transmission.

Station A detects collision at time t4 and aborts transmission.

various physical medium

Classify the various physical medium and highlight their merits and demerits.

Network links are implemented on media that is either guided (wired) or unguided (wireless).

GUIDED MEDIA

The guided media is broadly classified into Twisted-pair, Coaxial and Fiber-optic cable

Twisted-Pair Cable

The least-expensive and most commonly-used transmission medium is twisted-pair. The copper wires are twisted together to reduce the electrical interference.

 

Twisted-pair cables are either shielded (STP) or unshielded (UTP). The EIA has classified UTP cable into various categories. Some are:

Category	Data Rate in Mbps	Usage
CAT-1	< 0.1	Telephone
CAT-4	20	Token Ring
CAT-5	100	LAN

UTP cable is commonly used for LANs such as 10Base-T and 100Base-T. The most common UTP connector is RJ45.

UTP cable suffers from attenuation and need repeaters for long distance transmission (every 100m in case of CAT-5).

Twisted-pair cables are also used in telephone lines to provide voice and data channels. DSL lines also use the high-bandwidth capability of UTP cables.

Coaxial Cable

 

It has a central core conductor enclosed in an insulating sheath, which in turn is encased in an outer conductor of metal foil.

Coaxial cable carries signals of higher frequency ranges than those in twisted-pair cable To connect coaxial cable to devices, coaxial connectors such as BNC connector, BNC T

connector and BNC terminator are used.

Coaxial cables are categorized by their radio government (RG) ratings. Some are:

Category	Usage	Bandwidth	Distance
RG-59	Cable TV
RG-58	Thin Ethernet	10–100 Mbps	200 m
RG-11	Thick Ethernet	10–100 Mbps	500 m

The attenuation is much higher in coaxial cables than in twisted-pair cable. The signal weakens rapidly and requires the frequent use of repeaters.

Coaxial cable was widely used in analog telephone networks and in cable TV networks, but was later replaced by optical fiber.

Coaxial cable is used to provide connection to user premises as it requires narrow bandwidth and is cost-effective.

Fiber-Optic Cable

A fiber-optic cable transmits signals in the form of light and use property of reflection.

 

The outer jacket is made of PVC/Teflon. Inside the jacket are Kevlar strands to strengthen the cable. Below the Kevlar is another plastic coating to cushion the fiber. 

Fiber-optic supports two modes: multimode and single mode.
	Cable	Bandwidth	Distance
	Multimode fiber	100 Mbps	2 km
	Single-mode fiber	100–2400 Mbps	40 km

The attenuation is much lesser and few (10 times less) repeaters are required. Supports higher bandwidth and longer transmission distance.

Fiber-optic cables are immune to interference and corrosive-resistant.

Installation and maintenance require expertise and fiber-optic cables are more expensive. Two fiber-optic cables are required for duplex communication (light is unidirectional)

Cable TV companies use optical fiber to provide the backbone structure. LANs such as 100Base-FX network and 1000Base-X also use fiber-optic cable.

UNGUIDED MEDIA

Unguided media transport signals through free space.

It can be broadly classified as Radio waves, Micro waves and Infrared. Signals can travel through ground, sky or line-of-sight propagation.

The spectrum pertaining to radio waves and microwaves is divided into bands, regulated by government authorities. It ranges from very low to extremely high frequency.

 

Radio Waves

The frequency range is 3 kHz – 1 GHz.

Radio waves are omni-directional and hence antennas need not be aligned.

The waves transmitted by one antenna are susceptible to interference by another.

Radio waves that propagate in the sky mode, can travel long distances and is used for long-distance broadcasting such as AM radio.

Radio waves of low and medium frequencies can penetrate walls and used in FM radio, maritime radio, cordless phone, paging, etc.

Microwaves

Microwaves have frequencies between 1 and 300 GHz.

Microwaves are unidirectional. Hence sending and receiving antennas should be aligned. A pair of antennas can be aligned without interfering with another aligned pair.

Microwave propagation is line-of-sight.

Very high-frequency microwaves cannot penetrate walls.

They are used in cellular phone networks, satellite networks, and wireless LANs.

Infrared

Infrared waves frequencies ranges from 300 GHz to 400 THz.

It can be used for short-range communication only within a closed space. Infrared waves have high frequencies and cannot penetrate walls.

IrDA has established standards for communication between devices such as keyboards, mouse, PCs, and printers.

Infrared signals transmit through line of sight with very high data rate of transmission.

Last mile links

If two nodes to be connected are on opposite sides of the country/town, then only option is to lease a dedicated link from the telephone company which is expensive. The less expensive options available are last-mile links that span from the home to a service provider. Some common services and their bandwidth are:

Service	Bandwidth
POTS	28.8–56 Kbps
ISDN	64–128 Kbps
xDSL	16 Kbps–55.2 Mbps
CATV	20–40 Mbps