End-systems (a.k.a. hosts):

• PCs, workstations
• dedicated computers
• network components

Multiaccess vs. Point-to-point

• Multiaccess means shared medium.
  • many end-systems share the same physical communication resources (wire, frequency, ...)
  • There must be some arbitration mechanism.
• Point-to-point
  • only 2 systems involved
  • no doubt about where data came from !

LAN - Local Area Network

• connects computers that are physically close together ( < 1 mile).
• high speed
• multi-access
• Technologies:
  • Ethernet 10 Mbps
  • Token Ring 16 Mbps
  • FDDI 100 Mbps

WAN - Wide Area Network

• connects computers that are physically far apart. (long-haul network).
• typically slower than a LAN.
• typically less relaiable than a LAN.
• point-to-point
• Technologies:
  • telephone lines
  • Satellite communications

MAN - Metropolitan Area Network

• Larger than a LAN and smaller than a WAN
• Technologies:
  • coaxial cable
  • microwave

Internetwork

• Connection of 2 or more distinct (possibly dissimiliar) networks.
• Requires some kind of network device to facilitate the connection.

OSI Reference Model

• 7. Application
• 6. Presentation
• 5. Session
• 4. Transport
• 3. Network
• 2. Data Link
• 1. Physical

The Physical Layer

• Responsibility:
  • transmission of raw bits over a communication channel.
• Issues:
  • mechanical and electrical interfaces
  • time per bit
  • distances

The Data Link Layer - Data Link Control

• Responsibility:
  • provide an error-free communication link
• Issues:
  • framing (dividing data into chunks)
  • header & trailer bits
  • addressing

The Data Link Layer - The MAC sublayer

• Medium Access Control - needed by mutiaccess communications.
• MAC provides DLC with Òvirtual wiresÓ on multiaccess networks.

The Network Layer

• Responsibilities:
  • path selection between end-systems (routing).
  • subnet flow control.
  • translation between different network types.
• Issues:
  • packet headers
  • virtual circuits

The Transport Layer

• Responsibities:
  • provides virtual end-to-end links between peer processes.
  • fragmentation & reassembly
  • end-to-end flow control
• Issues:
  • headers
  • error detection
  • reliable communication

The Session Layer

• Responsibilities:
  • establishes, manages, and terminates sessions between applications.
  • service location lookup
• Many protocol suites do not include a session layer.

The Presentation Layer

• Responsibilities:
  • data encryption
  • data compression
  • data conversion
• Many protocol suites do not include a Presentation Layer.

The Application Layer

• Responsibities:
  • anything not provided by any of the other layers
• Issues:
  • application level protocols
  • appropriate selection of Òtype of serviceÓ

Layering & Headers

• Each layer needs to add some control information to the data in order to do it's job.
• This information is typically prepended to the data before being given to lower layers.
• Once the lower layers deliver the the data and control information - the peer layer uses the control information.

Headers

• Physical: no header - just a bunch of bits.
• Data Link:
  • address of the receiving endpoints
  • address of the sending endpoint
  • length of the data
  • checksum.

Network layer header

• protocol suite version
• type of service
• length of the data
• packet identifier
• fragment number
• time to live

Connecting Networks

• Repeater: physical layer
• Bridge: data link layer
• Router: network layer
• Gateway: network layer and above.

Repeater

• Copies bits from one network to another
• Does not look at any bits
• Allows the extension of a network beyond physical length limitations

Bridge

• Copies frames from one network to another
• Can operate selectively - does not copy all frames (must look at data-link headers).
• Extends the network beyond physical length limitations.

Router

• Copies packets from one network to another.
• Makes decisions about what route a packet should take (looks at network headers).

Gateway

• Operates as a router
• Data conversions above the network layer.
• Conversions:
  • encapsulation - use an intermediate network
  • translation - connect different application protocols
  • encrpyption - could be done by a gateway

Encapsulation Example

• Provides service connectivity even though intermediate network does not support protocols.

Translation

• Translate from red protocol to brown protocol

Encryption gateway

Hardware vs. Software

• Repeaters are typically hardware devices.
• Bridges can be implemented in hardware or software.
• Routers & Gateways are typically implemented in software so that they can be extended to handle new protocols.
• Many workstations can operate as routers or gateways.

Byte Ordering

• Different computer architectures use different byte ordering to represent multibyte values.

Network Byte Order

• Conversion of application-level data is left up to the presentation layer.
• But hold on !!! How do lower level layers communicate if they all represent values differently ? (data length fields in headers)
• A fixed byte order is used (called network byte order) for all control data.

Multiplexing

• Many processes sharing a single network interface.
• A single process could use multiple protocols.
• More on this when we look at TCP/IP.

Modes of Service

• connection-oriented vs. connectionless
• sequencing
• error-control
• flow-control
• byte stream vs. message based
• full-duplex vs. half-duplex.

Connection-Oriented vs. Connectionless

• A connection-oriented service includes the establishment of a logical connection between 2 processes.
  • establish logical connection
  • transfer data
  • terminate connection.
• Connectionless services involve sending of independent messages.

Sequencing

• Sequencing provides support for an order to communications.
• A service that includes sequencing requires that messages (or bytes) are received in the same order they are sent.

Error Control

• Some services require error detection (it is important to know when a transmission error has occured).
• Checksums provide a simple error detection mechanism.
• Error control sometimes involves notification and retransmission.

Flow Control

• Flow control prevents the sending process from overwhelming the receiving process.
• Flow control can be handled a variety of ways - this is one of the major research issues in the development of the next generation of networks (ATM).

Byte Stream vs. Message

• Byte stream implies an ordered sequence of bytes with no message boundaries.
• Message oriented services provide communication service to chunks of data called datagrams.

Full- vs. Half-Duplex

• Full-Duplex services support the transfer of data in both directions.
• Half-Duplex services support the transfer of data in a single direction.

End-to-End vs. Hop-toHop

• Many service modes/features such as flow control and error control can be done either: between endpoints of the communication. -or- between every 2 nodes on the path between the endpoints.

End-to-End

Hop-by-Hop

Buffering

• Bufferring can provide more efficient communications.
• Buffering is most useful for byte stream services.

Addresses

• Each communication endpoint must have an address.
• Consider 2 processes communicating over an internet:
  • the network must be specified
  • the host must be specified
  • the process must be specified.

Addresses

• Physical Layer - no address necessary
• Data Link Layer - address must be able to select any host on the network.
• Network Layer - address must be able to provide information to enable routing.
• Transport Layer - address must identify the destination process.

Broadcasts

• Many networks support the notion of sending a message from one host to all other hosts on the network.
• A special address called the Òbroadcast addressÓ is often used.
• Some popular network services are based on broadcasting (YP/NIS, rup, rusers )

Bridge Software

• A bridge connects networks and forwards frames from one network to another. Selective Forwarding
• If A sends a frame to E - the frame must be forwarded by the bridge.
• If A sends a frame to B - there is no reason to forward the frame.

Bridge Database

• The bridge needs a database that contains information about which hosts are on which network.
• The realistic options are:
  • The system administrator can create and maintain the database.
  • The bridge can acquire the database on the fly.

Learning the host mapping

• The bridge forwards packets for which it does not know which network the destination is on.
• Everytime the bridge forwards a packet it can record the network on which the sender is located.
• Each host mapping expires when it is unused for a ÒlongÓ period of time.

Learning Bridge

• A host can be moved to another network.
• New hosts can be added at any time.
• Requires no setup information from humans.
• One MAJOR flaw: What is the problem ?