EPFO EO/AO 2023 General Ability Test · Question 50 of 120

Correct — C, (c) Session layer. The word the examiner has built the question around is 'managing'. Several layers carry data between two computers; only one of them manages the conversation in which that data moves. The session layer is the fifth of the seven layers of the OSI reference model, sitting above transport and below presentation, and the standard formulation describes it as the network dialogue controller: it establishes, maintains and synchronises the interaction between two communicating systems, and closes it down in an orderly way when the exchange is over. Three functions are usually listed under it. Dialogue control decides whether the two sides may speak at once or must take turns, and where turns are taken, token management decides who holds the right to transmit, so that two parties do not attempt the same critical operation simultaneously. Synchronisation inserts checkpoints into a long transfer, so that a failure part-way through a large file resumes from the last checkpoint instead of starting again from the beginning. Set against that, the layers below have narrower jobs that stop short of managing anything. The data link layer moves a frame across one link. The network layer chooses a route and delivers a packet from the source host to the destination host across intervening networks. The transport layer delivers data reliably and in order between the two end processes, dividing it into segments and controlling flow and errors. Each of them is concerned with getting bits to the far end; none of them is concerned with the structure of the conversation those bits belong to. That structure — when the session opens, how the two sides take turns, where it can be resumed after a failure, and when it ends — is the session layer's charge, and it is what the stem's 'managing the communication' names. One practical footnote. The OSI model is a reference framework rather than a description of the software actually deployed: the TCP/IP suite that runs the internet has no separate session layer, and such session functions as it needs are handled inside applications. Questions of this kind are asked about the OSI model on its own terms, and are answered from its seven-layer scheme.

• (a)Network layer — The network layer, the third, is responsible for the source-to-destination delivery of a packet across multiple networks — logical addressing, which is where the internet address of a host belongs, and routing, which is the choice of path from one network to the next. It answers the question 'which way does this packet go ?', not the question 'how is this conversation organised ?'. A candidate who reads 'between computers in the network' as a reference to the network's own layer is answering the noun in the stem rather than its verb.
• (b)Transport layer — The closest of the three, and the one worth spending a sentence on. The transport layer, the fourth, provides end-to-end delivery between PROCESSES rather than merely between machines — it segments data and reassembles it, addresses the right application through a port number, and supplies flow control and error control so that what arrives is complete and in order. It does open and close connections, which is why it attracts the answer. But its connection exists to make delivery reliable, whereas the session layer's dialogue exists to organise the exchange itself, with turn-taking and checkpoints for resumption. Delivery against dialogue is the line the examiner is drawing.
• (d)Data link layer — The data link layer, the second, is concerned with node-to-node delivery over a single link. It groups bits into frames, attaches the physical or hardware address of the next node, detects and often corrects transmission errors on that link, and controls access to a shared medium. Its horizon is one hop; it has no knowledge of the far end of the path, still less of the conversation being conducted across it. Being the layer that carries the hardware address, it is a frequent wrong answer to any question that mentions computers by name.

The Open Systems Interconnection reference model, published by the International Organization for Standardization, divides the task of communication into seven layers so that each can be designed and changed independently: physical, data link, network, transport, session, presentation and application, counting upward. The lower three move data — bits on a medium, frames across a link, packets across networks. The transport layer bridges the two halves by making delivery between end processes reliable. The upper three serve the user's process: the session layer organises the dialogue, the presentation layer translates between the sender's and receiver's representations and handles encryption and compression, and the application layer offers the services the user actually invokes, such as file transfer, mail and remote login. Data descending the stack acquires a header at each layer and sheds it again on the way up, which is the process called encapsulation. The practical suite that runs the internet, the TCP/IP model, collapses the top three OSI layers into a single application layer and the bottom two into a network access layer, which is why session functions have no separate home there. Two mnemonics carry the order: taking the layers from the top, All People Seem To Need Data Processing.

Networking questions in EO/AO papers are almost always about which layer does what, because that single table answers a dozen possible items. The examiner's method here is to describe a function in ordinary words — 'managing the communication between computers' — rather than to use the model's own vocabulary, so the candidate has to translate before matching. The habit rewarded is learning each layer by its characteristic OBJECT: bits, frames, packets, segments, sessions, formats, services. Once the objects are attached to the layers, a description in plain English can be routed to the right one.

• The OSI reference model has seven layers: physical, data link, network, transport, session, presentation and application.
• The session layer is the fifth, and it establishes, maintains, synchronises and terminates the dialogue between two communicating systems.
• Its functions include dialogue control, token management for the right to transmit, and synchronisation through checkpoints that allow a long transfer to resume after failure.
• The data link layer handles node-to-node delivery of frames on one link and uses physical addresses.
• The network layer handles source-to-destination delivery of packets across networks, using logical addressing and routing.
• The transport layer handles process-to-process delivery of segments, with port addressing, flow control and error control.
• The presentation layer handles translation, encryption and compression; the application layer provides services to the user's process.
• The TCP/IP model has no separate session layer — its application layer covers the OSI session, presentation and application layers.

• Matching the word 'network' in the stem to the network layer, when the stem is describing a function rather than naming a layer.
• Choosing the transport layer because it also opens and closes connections; its connection serves delivery, not dialogue.
• Assuming the layer numbering runs downward — the physical layer is the first and the application layer the seventh.
• Looking for a session layer in the TCP/IP suite, which does not have one.

Networking items in EO/AO papers ask for the layer responsible for a named function, the layer at which a named device operates, or the expansion of a protocol acronym. The single most productive preparation is a table of the seven layers against their functions, data units, addresses and example protocols; nearly every question in this family is a lookup in that table once the stem has been translated into the model's own vocabulary.

No directly related past PYQ was found.

• practice — not a real PYQ

  In the OSI reference model, routing of packets from the source host to the destination host across different networks is the responsibility of the :

  • (a)Data link layer
  • (b)Network layer
  • (c)Session layer
  • (d)Presentation layer

  Answer(b) Network layer

• practice — not a real PYQ

  Encryption and compression of data are functions of which OSI layer ?

  • (a)Transport layer
  • (b)Session layer
  • (c)Presentation layer
  • (d)Application layer

  Answer(c) Presentation layer