Answer sheet of switching & routing

1. How can you define contention network?
Contention network describe the situation where two or more nodes attempt to transmit a message
across the same wire at the same time.
 In contention networks, any node that has a packet to send, merely sends the packet
 It is clear that this type of network frequently experiences collisions
 The more nodes trying to communicate, the higher the chance of collisions
 Thus, contention networks are severely limited in the number of hosts possible
2. What is carrier sense multiple access? Define carrier sensing hardware with diagram?
Carrier Sense Multiple Access (CSMA) is a network protocol that listens to or senses network signals on
the carrier/medium before transmitting any data. CSMA is implemented in Ethernet network and is a
part of the Media Access Control (MAC) protocol.
 To reduce the number of collisions, the medium is tested for a signal before each transmission
 If a signal exists, the node waits
 Signal testing can be anything from detection of an electrical signal, to testing for photons
 Collisions can still occur (although less often)
 If a node tests for a signal before a transmission from another node, and transmits after, a
collision occurs
3. Define the components of a router with diagram?
Hardware components of a router:
 Network interfaces
 Interconnection network
 Processor with a memory and CPU
Commercial routers:
 Interconnection network and interface cards are sophisticated
 Processor is only responsible for control functions (route processor)
 Almost all forwarding is done on interface cards

Functional Components:
4. “Router has modularized network interfaces” Explain?
Often, routers have modularized network interfaces
a. One can add/remove/replace network interfaces as needs change
b. Some routers can accept network interface modules of different types (e.g. Ethernet,
Token Ring)
c. Each network interface would have its own:
i. Input buffer
ii. Output buffer
iii. Routing processor (in high-end routers)
5. What are the differences between hub, layer2 and layer3 switches? Explain with necessary
diagram?
Interface Card
Interconnection Network
Interface Card Interface Card
Processor
CPUMemory

Hub:
 Transmission from a station received by central hub and retransmitted on all outgoing lines
 Only one transmission at a time
Layer 2 Switch:
 Incoming frame switched to one outgoing line
 Many transmissions at same time
 Flat address space
 Broadcast storm
 Only one path between any 2 devices
 Solution 1: sub networks connected by routers
 Solution 2: layer 3 switching, packet-forwarding logic in hardware
Layer 3 Switch:
 Uses layer 3 routing to determine a path for packets
 Once a path is found, subsequent packets are switched
 This switching typically occurs on layer 2
6. Define packet tunneling in Internet with necessary diagram?
Tunneling is the transmission of data in such a way that the routing nodes in the network are unaware
that the transmission is from a different network.
Working principle:
 Source sends packets to an intermediate gateway
 Intermediate gateways put the whole packet into the payload field (don't interpret it).
 The destination will understand the packet

Tunneling a packet from Paris to London
7. What is shortest path routing? Find the shortest path from a source node to destination node?
Shortest Path Routing is a static routing algorithm that just finds the shortest path.
A graph is used to represent the network.
a. Each node of the graph represents a router.
b. Each arc of the graph represents a communication link.
c. To choose the route between a given pair of routers, the algorithm just finds the shortest
path between them on the graph.
Metric used in the shortest path.
a. Number of hops
b. Geographic distance in miles/kilometers
c. Transmission delay  fastest path

The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working
node.
8. Explain different types of packet fragmentation with diagram? Suppose a 19 byte packet to be
transmitted along with 3 byte maximum segment length network. Show the segment numbering
systems of the packet. The packet number is 15.
Transparent Fragmentation
With transparent fragmentation, end hosts (sender and receiver) are unaware that fragmentation has
taken place.
A router fragments a packet, and the next-hop router on the same network reassembles the fragments
back into the original packet.
Non-Transparent Fragmentation:
As before, routers fragment packets when needed. Routers along the path do not reassemble.
Destination hosts perform re-assembly (if needed).
(a) Transparent fragmentation. (b) Nontransparent fragmentation.
9. Explain briefly the functions of routing processor.
• Routing processors have two functions:
1. Maintain and exchange routing data with other routers in the network
Often this involves computing the forwarding table from data received by other routers
2. Use the forwarding table data to configure the switching fabric to forward the packet to the
correct output port

10. Describe briefly the construction of routers of different generation with necessary diagram.
1st
Generation
• This architecture is still used in low end routers
• Arriving packets are copied to main memory via direct memory access (DMA)
• Interconnection network is shared bus
• All IP forwarding functions are performed in the central processor.
• Routing cache at processor can accelerate the routing table lookup.
2nd
Generation
• Keeps shared bus architecture, but offloads most IP forwarding to interface cards
• Interface cards have local route cache and processing elements
Fast path: If routing entry is found in local cache, forward packet directly to outgoing interface
Slow path: If routing table entry is not in cache, packet must be handled by central CPU
3rd
Generation
• Interconnection network is a switch fabric (e.g., a crossbar switch)
• Distributed architecture:
Memory
Shared Bus
DMA
MAC
DMA
MAC
Interface
Card
DMA
MAC
Route Processor
Interface
Card
Interface
Card
CacheCPU
Shared Bus
Interface
Cards
DMA
MAC
DMA
MAC
DMA
MAC
Route Cache
Memory
Route Cache
Memory
Route Cache
Memory
Route Processor
slow path
fast path
MemoryCacheCPU
slow path
fast path

– Interface cards operate independently of each other
– No centralized processing for IP forwarding
• These routers can be scaled to many hundred interface cards and to aggregate capacity of > 1
Terabit per second
11. Find the minimum spanning tree for the weighted graph in figure.
https://www.youtube.com/watch?v=3isltkMwxEs
12. Describe the routing protocol for mobile host with necessary diagram.
All hosts are assumed to have a permanent home location (home address) that never changes.
Each area has one or more foreign agents (FA), keeping track of all mobile hosts (MH) visiting the area.
Each area has a home agent (HA), which keep track of hosts whose home is in the area but are
currently visiting another area.
A WAN to which LANs, MANs, and wireless cells are attached
• When a new host enters an area, it registers with the FA.
– Each FA periodically announces its existence and address. The newly-arrived mobile host
(MH) waits for one of these messages. If no message is received, it broadcasts a message
and asks for FAs.
– The MH sends its home address, link layer address, and some security info to the FA.
Interface
Cards CPU
Memory
Route
Processor
Memory
Route
Processing
MAC
Switch
Fabric
Interface
Switch
Fabric
Memory
Route
Processing
MAC
Switch
Fabric
Interface

– The FA contracts the HA.
– The HA examines the security info and records the temporary location of the MH.
– The FA gets ACK from HA, and informs MH that it has been registered.
Packet routing for mobile users
13. How can you define TCP/IP tunneling over ATM? Explain the categories of services that ATM
provides.
 TCP/IP tunneling allows transmission of TCP/IP packets over ATM (and other non-TCP/IP) networks
 The use of TCP/IP over these networks allows applications which normally only run on TCP/IP
networks to operate on all networks
 ATM provides 4 service categories:
o CBR (Constant Bit Rate): A constant bandwidth is reserved and guaranteed by the
network
o UBR (Unspecified Bit Rate): Data is transmitted when bandwidth is available, and not
when bandwidth is not available
o ABR (Available Bit Rate): The network will provide feedback about network
congestion, under the assumption the node will adjust its transmission to meet the
current availability of network bandwidth
o VBR (Variable Bit Rate): A minimum bandwidth is reserved and guaranteed by the
network, although available bandwidth may increase/peak above this value
 RT (Real Time): Fluctuation of bandwidth is minimized
 Used for non-guaranteed streaming audio/video
 NRT (Non Real Time): Any bandwidth available is used
 Used for downloads
14. What is distance vector routing? Explain with example.
Distance Vector Routing algorithm – Dynamic routing
a. Each router maintains a table (vector), giving the best known distance to each destination
and the outgoing line to get there.
b. These tables are updated by exchanging information with the neighbors.
c. The metric used might be the number of hops, the time delay, or the number of queued
packets.

d. The router is assumed to know the “distance” to each of its neighbors.
(a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.
15. Explain the link state routing construction routing packets for a subnet.
Link State Routing is a dynamic routing.
Each router must do the following:
a. Discover its neighbors, learn their network address.
b. Measure the delay or cost to each of its neighbors.
c. Construct a packet telling all it has just learned.
d. Send this packet to all other routers.
e. Compute the shortest path to every other router.
(a) A subnet. (b) The link state packets for this subnet
16. Explain the hierarchical routing with necessary diagram.
With the increase of network/routers, it is infeasible to have an entry for each router. The
hierarchical routing is required.
a. Divide the routers into regions.
b. The router only knows details to route packets to the destination within the same region.

c. But may not be optimal (e.g., the best route from 1A to 5C is via region 2, but since the
route via region 3 is better for most nodes in region 5.
Hierarchical routing
17. Name and define the hardware components of a router. Show the routers functional components
with diagram.
Check number 3 answer.
18. What is ATM network? Define the logical connections used in ATM networks.
ATM networks are packet-switched, but still create a (virtual) circuit through the network
a. Before transfer can occur, the network must create a path (called a virtual circuit) between
the two machines
b. Once the virtual circuit (VC) has been established, packets can be transferred between the
machines
Use optical fiber similar to that used for FDDI networks
- ATM runs on network hardware called SONET
ATM cells (packets) are 53 octets long
- 5 bytes of header information
- 48 bytes of data
Logical Connections:
- VCC (Virtual Channel Connection): a logical connection analogous to virtual circuit in X.25
- VPC (Virtual Path Connection): a
bundle of VCCs with same
endpoints

19. Explain ATM protocol architecture. Define ATM cell format.
Fixed-size packets called cells
Streamlined: minimal error and flow control
2 protocol layers relate to ATM functions:
a. Common layer providing packet transfers
b. Service dependent ATM adaptation layer (AAL)
AAL maps other protocols to ATM
Protocol model has three planes:
• User
• Control
• management
ATM Cell Format: Data Cells
• Generic flow control: Control traffic flow.
• Virtual path identifier: An identifier for the virtual path/circuit
• Virtual channel identifier: An identifier to identify which channel within the specified virtual
path/circuit
• Payload Type: 3 flag bits
• Cell loss priority: Should the cell be discarded in the event of a congested switch?
• Header error check: Cyclical redundancy check for the cell header
20. Explain the way of creating ATM virtual circuits
Creating a virtual circuit has been compared to making a telephone call
A network node sends a request to the ATM switch specifying the destination

The switch interacts with any other switches necessary to align themselves to form a complete path
When communication is complete, the node sends a disconnect message to the switch
The switch will then notify all switches involved to release the connection
21. What do you mean by segmentation & reassembly in ATM network? Define ATM switching with
necessary diagram.
Segmentation:
• Segmentation is the process of turning a chunk of data into a group of ATM cells
– For example, the chunk of data might be a packet from another type of network
• e.g. An Ethernet frame
• Since ATM cells travel on the same virtual circuit, they do not arrive out of order
– Thus sequencing information is not necessary
• Each sequence of 48 octets is sent in its own cell
Reassembly:
• Reassembly is the process of recombining ATM cells into the original data chunk
• As the destination node receives cells, it removes the 48 octets and appends it to the end of a
buffer
• One of the configuration parameters, called ‘Payload Type’ is used to indicate the final cell
ATM Switching:
• As can be seen in the diagram, ATM switches exist entirely in the hardware layer
• As a result, they are much faster than routers, which require software execution
– Routers must read packets from electronic signals into a memory buffer (which is slow)
– Routers then convert packets back into electronic signals onto a new network connection
22. Write the drawbacks of ATM network.
Small, finite sized cells provide faster transmission speeds
- However, 53 octet cells are incompatible with other technologies which are in widespread use

ATM addressing also differs significantly from other forms of addressing
- For example the TCP/IP protocol suite is the most common network protocol system
- Most Internet applications are based on TCP/IP
ATM networks are not broadcast networks
- Each cell only arrives at its intended destination
- Broadcasting & multicasting are not directly supported
23. Show the internetworking using concatenated virtual circuits and connectionless internetworking.
Concatenated Virtual Circuits:
a. A connection to a remote host is set up by concatenating virtual circuits in all networks it
passes by.
b. Gateways response for converting packet format and maintaining VC.
c. Work best when all network have the same properties.
i. All reliable or all unreliable.
Can also be done on transport layer
Internetworking using concatenated virtual circuits
Connectionless internetworking:
– inject datagram’s into subnets and hope for the best
– packets may not follow the same route
– Also works on VC subnet.
A connectionless internet

24. Name and define the network properties.
Scope: A network should provide services to several applications
Scalability: A network should operate efficiently when deployed on a small-scale as well as on a large-
scale
Robustness: A network should operate in spite of failures or lost data
Self-Stabilization: A network, after a failure or other problem, should return to normal (or near
normal) without human intervention
Auto configurability: A network should optimize its own parameters in order to achieve better
performance
Safety: A network should prevent failures as well as prevent failures from affecting other areas of the
network
Configurability: A network’s parameters should be configurable to improve performance
Determinism: Two networks with identical conditions should yield identical results
Migration: It should be possible to add new features to a network without disruption of network
service
25. Explain the TCP/IP protocol in action using a simplified network route.
The TCP/IP Protocol in Action
 Consider the following simplified network
route
 The source (S) and destination (D) are
separated by two routers (R1, R2)
S DR1 R2
51
 Let’s consider a web browser, using HTTP
 The web browser on S sends a packet to the web
server on D
 The application layer (i.e. the browser) provides the
logical (IP) addresses for S (IPS) and D (IPD)
 The application layer also provides the port numbers
for the source (PortS) and destination (PortD)
S DR1 R2
HTTP Req
52
 The Transport layer (TCP) uses the port
numbers (e.g. 2765 and 80) to create a TCP
packet (sometimes called a segment):
S DR1 R2
Source Port: 2765
Destination Port: 80
HTTP Req 53
Source IP: 137.207.140.71
Dest IP: 24.87.204.16
 The Internet (i.e. IP) layer uses the IP
addresses specified by the application layer
to create an IP datagram
 e.g. 137.207.140.71, 24.87.204.16
 Next, a route is determined for the packet,
using S’s routing table
 S only needs one router’s address (R1)
S DR1 R2
TCP Segment
HTTP Req 54

Source MAC: MACS
Dest MAC: MACR1
IP Datagram
 The MAC addresses of S and R1 (MACS and
MACR1) are used to create a network frame
 If the MAC address of R1 is not known, ARP
(address resolution protocol) is used
S DR1 R2
TCP Segment
HTTP Req
55
Source MAC: MACS
Dest MAC: MACR1
IP Datagram
 Let’s simplify the picture (for clarity)
 In subsequent steps the IP datagram and its
contents will not change very much
S DR1 R2
56
Source MAC: MACS
Dest MAC: MACR1
IP Datagram
 The network frame is transmitted on the
network to R1
 This is possible since S and R1 are both
members of the same network
S DR1 R2
57
IP Datagram
 R1 will extract the IP datagram from the
payload of the network frame
 R1 looks up the destination IP address (IPD) in
it’s routing table, to determine which router
should get the datagram next (R2)
S DR1 R2
58
Source MAC: MACR1
Dest MAC: MACR2
IP Datagram
 R1 uses its own MAC address (MACR1) and
R2’s MAC address (MACR2) to create another
network frame
S DR1 R2
59
Source MAC: MACR1
Dest MAC: MACR2
IP Datagram
 The network frame is received by R2, and the
IP datagram is extracted from it’s payload
 R2 uses its routing table to lookup IPD
 In this case, R2 is directly connected to D
 This is called direct routing
S DR1 R2
60
ARP Request
IP: 24.87.204.16
MAC: ?
IP Datagram
 Most likely, R2 does not have the MAC
address of D (MACD)
 The address resolution protocol (ARP) is used
to determine the MAC address:
S DR1 R2
61
ARP Response
IP: 24.87.204.16
MAC: 08-7F-3C-90-0C-DF
IP Datagram
 D recognizes it’s IP address and responds
with its MAC address (MACD)
 e.g. 08-7F-3C-90-0C-DF
S DR1 R2
62

Source MAC: MACR2
Dest MAC: MACD
IP Datagram
 A network frame is created by R2 now that
the MAC address is known
 The frame is sent directly to D
S DR1 R2
63
Source MAC: MACR2
Dest MAC: MACD
IP Datagram
 D extracts the IP datagram from the network
frame (which is discarded)
 The IP datagram’s payload is passed to the
transport layer
S DR1 R2
64
 The Transport layer (within D’s operating
system), will use the port numbers specified
in the TCP segment to determine to which
application it should send the segment
 In this case, to the application bound to port
80 (the web server)
S DR1 R2
Source Port: 2765
Destination Port: 80
HTTP Req65
 Now, the web server on D has the HTTP
request, and it processes it
 An HTTP response is sent back using the
same process
 The web server uses the same IP addresses
and logical addresses as the last message
S DR1 R2
HTTP Req
66
26. Name & define different types of network bridges.
There are four types of network bridges:
1. Transparent basic bridge
2. Source routing bridge
3. Transparent learning bridge
4. Transparent spanning bridge
The Transparent Basic Bridge
The simplest type of bridge is the transparent basic bridge. It stores the traffic until it can transmit it to
the next network. The amount of time the data is stored is very brief. Traffic is sent to all ports except the
port from which the bridge received the data. No conversion of traffic is performed by a bridge. In this
regard, the bridge is similar to a repeater.
Source Routing Bridge
The route through the LAN internet is determined by the source (originator) of the traffic hence this bridge
is called as source routing bridge. The routing information field (RIF) in the LAN frame header, contains the
information of route followed by the LAN network.
The Transparent Learning Bridge
The transparent bridge finds the location of user using the source and destination address. When the
frame is received at the bridge it checks its source address and the destination address. The destination
address is stored if it was not found in a routing table. Then the frame sent to all LAN excluding the LAN
from which it came. The source address is also stored in the routing table. If another frame is arrived in
which the previous source address is now its destination address then it is forwarded to that port.

The Transparent Spanning Tree Bridge
These bridges use a subnet of the full topology to create a loop free operation.
27. Short notes on routers input buffer, output buffer, processor, switching fabric.
Input Buffer:
The incoming packets of a network interface are placed in input buffers
 These are banks of very high speed memory for packet queuing prior to processing
 The packet is stored here until the routing processor is available
The network interface may have a routing processor, which would:
a. have a copy of the forwarding table (to prevent concurrent access)
b. lookup the destination address in this forwarding table, to determine the correct output port
c. configure the switching fabric to forward the packet to the correct output buffer
Low-end routers would share one routing processor
Output Buffer:
The switching fabric gets the packet to the right output port
 However, that port’s network may not be immediately available
 The packets are stored in the output buffer until the network is available
Routing Processor:
• A routing processor is software which executes on a CPU:
– Off-the-shelf CPU
• These are very inexpensive
• However, the performance of these CPUs is low since they are not optimized for
the types of operations a router typically needs to perform
– Application-Specific Integrated Circuit (ASIC)
• These are expensive to design (time and money)
• They are optimized for typical routing operations
• High-end routers use these to achieve higher performance levels
Switching Fabric:
• Switching fabric’s job is to move packets from the input buffer into the correct output buffer
– The routing processor determines the correct output port, using the forwarding table
• Switching fabric comes in 3 major types:
– In-memory switching fabric:
• The packets are input into the routing processor’s memory, and output into the
correct output buffer
– Bus-based switching fabric:

• The packets move along a shared bus (similar to a network bus) to the correct
output buffer
– Crossbar switching fabric:
• The packets move along a grid of redundant buses
• If any bus fails, alternate paths exist so that forwarding can continue
28. What is switching? Write the reasons for switching in communication.
The establishing, on-demand, of an individual connection from a desired inlet to a desired outlet within
a set of inlets and outlets for as long as is required for the transfer of information.
 Switching implies directing of information flows in communications networks based on known
rules
 Switching takes place in specialized network nodes
 Data switched on bit, octet, frame or packet level
 Size of a switched data unit is variable or fixed
Reasons for switching in communication:
 Switches allow reduction in overall network cost by reducing number and/or cost of transmission
links required to enable a given user population to communicate
 Limited number of physical connections implies need for sharing of transport resources, which
means
 better utilization of transport capacity
 use of switching
 Switching systems are central components in communications networks
29. Describe the main building block of a switch. Define basic type of switching networks.

30. Explain different types of switching modes. Define label switching.
Types of switching modes:
• Circuit switching
• Cell switching
• Packet switching
– Routing
– Layer 3 - 7 switching
– Label switching
Circuit Switching:
 End-to-end circuit established for a connection
 Signaling used to set-up, maintain and release circuits
 Circuit offers constant bit rate and constant transport delay
 Equal quality offered to all connections
 Transport capacity of a circuit cannot be shared
 Applied in conventional telecommunications networks (e.g.
PDH/PCM and N-ISDN)
Cell switching:
• Virtual circuit (VC) established for a connection
• Data transported in fixed length frames (cells), which carry information needed for routing cells along
established VCs
• Forwarding tables in network nodes
• Signaling used to set-up, maintain and release VCs as well as update forwarding tables
• VCs offer constant or variable bit rates and transport delay
• Transport capacity of links shared by a number of connections (statistical multiplexing)
• Different quality classes supported
• Applied, e.g. in ATM networks
Packet switching:
 No special transport path established for a connection
 Variable length data packets carry information used by network nodes in making forwarding
decisions
 No signaling needed for connection setup

 Forwarding tables in network nodes are updated by routing protocols
 No guarantees for bit rate or transport delay
 Best effort service for all connections in conventional packet switched networks
 Transport capacity of links shared effectively
 Applied in IP (Internet Protocol) based networks

Label Switching:
 Evolved from the need to speed up connectionless packet switching and utilize L2-switching in
packet forwarding
 A label switched path (LSP) established for a connection
 Forwarding tables in network nodes
 Signaling used to set-up, maintain and release LSPs
 A label is inserted in front of a L3 packet (behind L2 frame header)
 Packets forwarded along established LSPs by using labels in L2 frames
 Quality of service supported
 Applied, e.g. in ATM, Ethernet and PPP
 Generalized label switching scheme (GMPLS) extends MPLS to be applied also in optical networks,
i.e., enables light waves to be used as LSPs

Answer sheet of switching & routing

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