Kommunikationssysteme Flashcards
Vorlesung 4
Kommunikationssystem - Bestandteile
Hardware,
Software und
Transmission protocols (erlauben nformationsaustausch)
Schichtmodelle (Layer Models)
- Kommunikationsnetzwerke nutzen verschiedene Schichten, um die Komplexität zu reduzieren,
- jede Schicht stellt Dienste für die Schicht darüber bereit, ohne Details zur Implementierung oder Bereitstellung der Dienste weiterzuleiten
- Kommunikation pro Layer erfolgt über jeweiliges Protokoll (Protokoll legt Kommunikationsregeln fest)
Das OSI Referenz Modell
OSI = Open System Interconnection
Besteht aus 7 Ebenen
- Ebene 1-4 sind transportorientiert
- Ebene 5-7 sind Anwendungsorientiert
Das OSI Referenz Modell - Ebenen (Nennung)
- Physical
- Data Link
- Network
- Transport
- Session
- Presentation
- Application
Das OSI Referenz Modell - Layer 1
- Physical Layer
- Defines mechanical, electrical and time-related specifications
- Deals with the transmission of bits (over a communication channel)
Das OSI Referenz Modell - Layer 2
- Data Link Layer
- Contains algorithms for efficient and reliable communication
- Tasks: Error and Flow control, Error detection and correction through
1. segmentation (allocate data in small packages) and confirmation (that package is received),
2. management of simultaneous access (manage colliding of messages from multiple users) - Media access control (MAC) address: globally unique identifier for a network (hardware) address
Das OSI Referenz Modell - Layer 3
- Network Layer
- Transmission from packets from the source to the destination (including passing through network segments on the way between two routers)
- Main task: Routing –> choose adequate path for each packet
- Uses the internet protocol (IP)
- Best path: low transport costs, fast, bug-proof (abhörsicher) and tamper-resistant (manipulationssicher) transmission, optimized network load
- Routing algorithm: Dijkstra Algorithm –> Finds shortest path between two routers (router = point, transmission line = line)
Das OSI Referenz Modell - Layer 4
- Transport Layer
- Ensures reliable and cost-effective transport of data from the source to the destination, independent from physical networks (logical end-to-end connection)
- Service: Setup, Coordination and termination of connections (3-way-handshake), flow control, multiplexing
- Protocols: Transmission Control Protocol (TCP) and User Data Protocol (UDP)
Das OSI Referenz Modell - Layer 5
- Session Layer
- Enables users to start sessions from different computers, data exchange is structured
- Tasks:
1. Dialogue control (which user is allowed to transfer data at which time),
2. Token Management (preventing two users from performing a critical operation at the same time),
3. Synchronization (restauration points to resume transmission after an interruption)
Das OSI Referenz Modell - Layer 6
- Presentation Layer
- Define data in an abstract way so computers with different representations of data can communicate
- Tasks: Data compression, encryption and conversion of data representation
Das OSI Referenz Modell - Layer 7
- Application Layer
- Contains multiple protocols so applications can work (ex. E-Mail, HTTP, Skype, DNS)
- Provides task for the user (in contrast to the other layers)
- Example: DNS (Domain Name System), distributed data base
Transmission Control Protocol (TCP)
- designed to provide a reliable and connection-oriented transportation of a byte-stream through unreliable networks –> end-to-end, information is sent directly to an application
- Reliable: Data communication is repeated until the remote station acknowledges the receipt
- Connection-oriented: During setup of TCP-connection by 3-way-handshake, a logical end-to-end connection between sender and receiver is provided
User Data Protocol (UDP)
- connectionless, insecure transport protocol, no assurance if data-packet was received
- Reduced protocol, used for DNS (Domain Name System)
Domain Name System (DNS)
Distributed Database
- Located in application layer
- Translates names into addresses and vice versa, provides information about IP address of a mail server of a domain
- 3 types of DNS servers:
Local (within the organization, your own),
root (Root DNS of a domain) and
authoritative (responsible for particular domain names –> certain website)
- DNS queries involve all layers of the network stack (Layer 1-4)
