Data Communications and Networking 2.9 : Questions and Answers
List the layers of the Internet model.
Physical, Data Link, Network, Transport, Application.
Which layers in the Internet model are the network support layers?
Physical, Data Link, Network.
Which layer in the Internet model is the user support layer?
Application.
What is the difference between network layer delivery and transport layer delivery?
Transport layer is responsible for process (source) - to - process (destination) delivery of entire message, whereas network layer oversees host (source) - to - host (destination) delivery of individual packets across multiple links.
What is a peer-to-peer process?
The processes at each machine that communicate at a given layer. Physical Layer has a direct link between 2 devices, while other layers have to pass the information down to the lower layers on the sender device by adding extra bits at each layer, and the receiver device unwraps the message at each layer moving upwards till it finally reaches the corresponding communicating layer.
How does information get passed from one layer to the next in the Internet
model?
At the physical layer, communication is direct between devices. At the higher layers, however, communication
must move down through the layers on sending device, over to receiving device, and then back up through the layers. Each layer in the sending device adds its own information
to the message it receives from the layer just above it and passes the whole package to
the layer just below it.
At layer I the entire package is converted to a form that can be transmitted to the
receiving device. At the receiving machine, the message is unwrapped layer by layer,
with each process receiving and removing the data meant for it.
What are headers and trailers, and how do they get added and removed?
Additional information wrapped with the data unit at each layer. Usually, a trailer is added at data link layer. Header and trailer contain information such as source/destination address, control bits, error correction bits etc. These extra bits are added at the layer at sender's side, and removed at the corresponding layer at receiver's side.
What are the concerns of the physical layer in the Internet model?
The physical layer is concerned with actual transfer of data bits across a transmission medium between 2 devices. The physical layer coordinates the functions required to carry a bit stream over a physical
medium. It deals with the mechanical and electrical specifications of the interface and
transmission medium. It also defines the procedures and functions that physical devices
and interfaces have to perform for transmission to occur. Physical characteristics of interfaces and medium, Representation of bits, data rate, synchronization of bits, line configuration, physical topology, transmission mode.
What are the responsibilities of the data link layer in the Internet model?
Transforms physical layer to reliable link, framing, physical addressing, flow, error and access control.
What are the responsibilities of the network layer in the Internet model?
Source to destination delivery of packet across multiple links, logical addressing, routing.
What are the responsibilities of the transport layer in the Internet model?
Process - to - process delivery of entire message, service point addressing, segmentation and reassembly, connection, flow and error control.
What is the difference between a port address, a logical address, and a physical
address?
Port address - transport layer, logical address - network layer, physical address - data link and physical layer. Port address is the address of a process on a host. A logical address (IP) in the Internet
is currently a 32-bit address that can uniquely define a host connected to the Internet. Physical address is address of node as defined by it's LAN or WAN.
Name some services provided by the application layer in the Internet model.
The application layer enables the user, whether human or software, to access the network.
It provides user interfaces and support for services such as electronic mail,
remote file access and transfer, shared database management, and other types of distributed
information services. Network virtual terminal, file transfer, access and management, mail services, directory services.
How do the layers of the Internet model correlate to the layers of the OSI model?
The TCPIIP protocol suite was developed prior to the OSI model. Therefore, the layers
in the TCP/IP protocol suite do not exactly match those in the OSI model. The
original TCP/IP protocol suite was defined as having four layers: host-to-network,
internet, transport, and application. However, when TCP/IP is compared to OSI, we can
say that the host-to-network layer is equivalent to the combination of the physical and
data link layers. The internet layer is equivalent to the network layer, and the application
layer is roughly doing the job of the session, presentation, and application layers
with the transport layer in TCP/IP taking care of part of the duties of the session layer.
So in this book, we assume that the TCP/IP protocol suite is made of five layers: physical,
data link, network, transport, and application. The first four layers provide physical
standards, network interfaces, internetworking, and transport functions that correspond
to the first four layers of the OSI model. The three topmost layers in the OSI model,
however, are represented in TCP/IP by a single layer called the application layer.
How are OSI and ISO related to each other?
ISO is the organization (International Standards Organization), and OSI (Open Systems Interconnection) is its model.
Match the following to one or more layers of the OSI model:
a. Route determination - Network
b. Flow control - Transport and Data Link
c. Interface to transmission media - Physical
d. Provides access for the end user - Application
Match the following to one or more layers of the OSI model:
a. Reliable process-to-process message delivery - Transport
b. Route selection - Network
c. Defines frames - Data Link
d. Provides user services such as e-mail and file transfer - Application
e. Transmission of bit stream across physical medium - Physical
Match the following to one or more layers of the OSl model:
a. Communicates directly with user's application program - Application
b. Error correction and retransmission - Data Link and Transport
c. Mechanical, electrical, and functional interface - Physical
d. Responsibility for carrying frames between adjacent nodes - Data Link
Match the following to one or more layers of the OSI model:
a. Format and code conversion services - Presentation
b. Establishes, manages, and terminates sessions - Session
c. Ensures reliable transmission of data - Data Link and Transport
d. Log-in and log-out procedures - Session
e. Provides independence from differences in data representation - Presentation
Suppose a computer sends a frame to another computer on a bus topology LAN.
The physical destination address of the frame is corrupted during the transmission.
What happens to the frame? How can the sender be informed about the situation?
If the corrupted destination
address does not match any station address in the network, the packet is lost.
If the corrupted destination address matches one of the stations, the frame is
delivered to the wrong station. In this case, however, the error detection
mechanism, available in most data link protocols, will find the error and
discard the frame. In both cases, the source will somehow be informed using one
of the data link control mechanisms discussed in Chapter 11.
Suppose a computer sends a packet at the network layer to another computer
somewhere in the Internet. The logical destination address of the packet is corrupted.
What happens to the packet? How can the source computer be informed of
the situation?
Before using the destination address in an intermediate or the destination node, the packet goes through error checking that may help the node find the corruption (with a high probability) and discard the packet. Normally the upper layer protocol will inform the source to resend the packet.
Suppose a computer sends a packet at the transport layer to another computer
somewhere in the Internet. There is no process with the destination port address
running at the destination computer. What will happen?
Most protocols issue a special
error message that is sent back to the source
in this case.
If the data link layer can detect errors between hops, why do you think we need
another checking mechanism at the transport layer?
The errors between the nodes can be detected by the data link layer control, but the error at the node (between input port and output port) of the node cannot be detected by the data link layer.
