FL 2 Kommunikationssystem
Application/app/user beskriv funktionenen och ge exempel
uses network ex. spotify netflix
host/end system/end node/sourse(sink/edge device ge funktion och ex
runs software to support applications ex. mobil, dator, server
router;switch/hub/node beskriv funktion och ex
relays messages between incominh and outgoing links ex wifi
link/channel beskriv funktion och ex
connecting nodes ex kabel, radio kanal
what is the host, protocol and url in this
http://www.chalmers.se/en/Pages/default.aspx
http = protocol
www.chalmers.se = host
en/Pages/default.aspx = path
how to connect to a server
-The browser opens a connection to the server using Transmission Control Protocol (TCP)
-Applications are addressed using port numbers
-The web server software always listens to port 80
-The web browser use a temporary (ephemeral) port number
http
hypertext transfere protocol
DNS
DNS (domain name system)
TCP
TCP (transport control protocol)
wht is and happens in protocals
• A protocol is a set of rules that determines how two or more communicating entities in the same layer interact
– Messages are sent and received
– Actions are based on events (e.g., sending or receiving messages, expiration of timers)
what is the purpose with protocals
The purpose of a protocol is to provide a service to the layer above
node(layer(protocal)))
vilka är application layer och protocals
• Both HTTP and DNS are application layer protocols
• This means that HTTP and DNS provide communication services to application programs, e.g., web browsers, email clients, etc.
• HTTP and DNS use transport layer service (i.e., TCP) to provide services to application programs
vad gör en layer
• A layer
– provides a service to the layer above by using the services of the layer below
• Each layer operates according to a number of protocols
• A set of layers with protocols is called a protocol stack
säg 2 referens modeller och vrf d är bra
– Open System Interconnection (OSI) model
• 7 layers, very influential, but not used in practice
– Internet reference model
• 5 (or 4) layers, empirical, heavily used in practice
OSI Reference Model har 7 application program, beskriv den
7 6 5 4 3 2 1
7. Application Program = Provide services frequently needed by application programs
6. Presentation = Conversions between different data representations
5. Session = Dialog management, error recovery
4. Transport = End-to-end transfer of messages
3. Network = Transfer of packets over several links
2. Data Link = Transfer of frames over a single link
1. Physical = Transfer of bits over a physical channel
beskriv förenklad OSI Reference Model där de är absorberade i respektive lager
7. application, presentation, session
4. transport
3. network
2. data link
1. phusical
Protocol messages typically consist of
1. header with control information: addresses, sequence numbers, length fields, etc.
2. data field (payload) carrying the data from the layer above
3. sometimes a trailer with check bits for error control or other control information
vad är peer processes
Communicating entities on the same layer but at different nodes are called peer processes
hur skapas PDU
Layer-n peer processes communicate by exchanging Protocol Data Units (PDUs)
how is the Communication between peer processes (osi unified view on service)
Layer n + 1 transfers information by invoking the services provided by the layer just below it,
i.e., layer n
where are these services available
Services are available at Service Access Points (SAPs), which have unique identifiers
– Ex: an HTTP process passes information to/from a TCP process via a specific transport layer SAP called port 80
how does this connect with the oayers
Each layer passes data and control information to the layer below it until the physical layer is reached and transfer of bits occurs
• The data received from the layer above is called a Service Data Unit (SDU)
what is a SDU except when the data is recieved for the lauer abowe
SDUs are encapsulated in PDUs
what is corresponding name for PDU and SDU for the layer application
message,
what is corresponding name for PDU and SDU for the layer transport
segment, message
what is corresponding name for PDU and SDU for the layer network
packet, segment
what is corresponding name for PDU and SDU for the layer link
frame, packet
what is corresponding name for PDU and SDU for the layer physical
bits, frame
Consider a service provided by layer n to layer (n + 1)
• It can be either connection-oriented or connectionless
give example for Connection-oriented ( Connection-Oriented and Connectionless Services)
(example TCP)
1. Create a logical connection between the layer-n SAPs at the end nodes: allocate buffers, agree
on transmission parameters, etc.
2. Transmit layer (n + 1) data, i.e., n-SDUs
3. Tear down connection when communication session is done: release buffers and allocated resources
Consider a service provided by layer n to layer (n + 1)
• It can be either connection-oriented or connectionless
give example for Connectionless ( Connection-Oriented and Connectionless Services)
(example IP)
– Simply transmit n-SDUs, without first connecting to the destination or even checking that it is
ready to receive data •
Important: the service type is not necessarily dependent on the lower layer service type. Ex: TCP, which is connection-oriented, runs on top of IP, which is connectionless
Fragmentation and Reassembly
• A layer may impose an upper limit on the size of a data
block that it can transfer for implementation or other
reasons
• Thus, a layer-n PDU may be too large to be handled as
a single unit by layer-(n − 1) n-PDU 1 n-PDU 2
• Sender side: a SDU is fragmented over multiple PDUs
• Receiver side: the SDU is reassembled from a
sequence of PDUs
• Layer n fragmentation is transparent to layer n + 1
(Aggregation and Deaggregation)
• A layer may impose a lower limit on the size of a data
block that it can transfer for efficiency, implementation
or other reasons
• Thus, a layer-n PDU may be too small to be handled as
a single unit by layer-(n − 1)
• Sender side: multiple SDUs are aggregated into a single PDUs
• Receiver side: the SDUs are deaggregated from a single
PDUs
• Layer n aggregation is transparent to layer n + 1
what is multiplexing
• A layer n service is shared by multiple layer-(n + 1) entities
• A multiplexing tag or ID is required in the n-PDU to determine which layer-(n + 1) user the SDU belongs to
Splitting (Inverse Multiplexing)
• A layer-(n + 1) entity may use multiple layer-n services
• The parallel n-SDU streams are combined to a single stream of SDUs at the receiving end
• Sequence number may be required to order SDUs correctly
describe Layer 1: Physical Layer
• Service provides
– Transfer of frames across a physical channel (wires, fibers, etc.)
• Specification of the physical aspects of a communications link
– Mechanical: cable, plugs, pins...
– Electrical/optical: modulation, signal strength, voltage levels, etc.
– Functional/procedural: how to activate, maintain, and deactivate physical link
describe Layer 2: Data Link Layer
• Service provides
– transfer blocks of bits, called packets over a link that connects two nodes directly or via a transparent switch
• Implements (depending on service provided)
– framing, i.e., a way to insert markers in a bit stream such start and end of a frame can be detected
– flow control, i.e., regulate transmission speed to avoid overwhelming a slow receiver
– error control via error-detecting and error-correcting codes – medium access, i.e., rules for sharing a medium that is common to many transmitters (radio,LAN)
– multiplexing of network layer entities – link layer addressing, typically flat (MAC address)
– activation, maintenance, deactivation of data link connections
describe Layer 3: Network Layer
• Service provides
– transfer of segments over multiple links or multiple networks
• Implements (depending on service provided)
– fragmentation and reassembly of segments
– transfer of packets over multiple links and networks
– a routing algorithm that together with other nodes determines routes across the network
– forwarding of packets across a node
– a scalable addressing scheme (typically hierarchal)
– flow control
– multiplexing of network layer entities
– congestion control (avoid too much traffic inside the network)
– connection setup, maintenance, and release of network connections
– internetworking, i.e., routing over several, possibly different, networks
describe Layer 4: Transport Layer
• Service provided
– end-to-end transfer of application layer messages, e.g.,
• reliable stream transfer
• unreliable datagram transfer
• Implements (depending on service provided)
– message fragmentation and reassembly
– end-to-end transfer of segments
– multiplexing of application layer entities by port numbers
– sequence control to deliver messages in correct order
– error control to provide reliable transmission
– flow and congestion control
– connection setup, maintenance, and release
describe Layer 5, 6, 7: Session, Presentation, Application
• Session layer: controls in which manner data is transmitted, e.g., certain application might require half-duplex dialogs (take turns transmitting)
• Presentation layer: provides machine-independent representations of data (e.g., different character codes)
• Typically, the session and presentation layers are absorbed into the application layer
• The application layer provides useful services for application programs, e.g.,
– File transfer (e.g., FTP)
– Remote login (e.g., SSH)
– Transfer of emails (SMTP)
– Information about Internet names (DNS)