Basic Data Communication Network Terms

(See lecture notes for CS/ECpE 5516 for more details.)

Protocol

A set of rules for computers to exchange data

Network

Internet

Circuit versus packet switching

Circuit switching:

Used in the telephone system: network resources in the telephone system are reserved from your phone to the phone you call when you place the call; they're released when you hang up.

Packet switching:

Internet is packet switched: data you wish to send is put into packets, which are then sent through the network. Network resources are only used during the time it takes to transmit each packet.
In the diagram below, 4 packets arrive at various times. The times marked x1, x2, ... denote the busy times of the network for these packets. Therefore delivery of packets from many users can be multiplexed on a single network link.

Connections and Datagrams

Service at the Transport Layer can be

connection-oriented (UDP) or
connection-less (TCP)

The second form is often called datagram traffic.

Connection-oriented is exemplified by the phone system: you dial a phone number, wait for the call to be established, then talk, then hang up. Similarly TCP will first connect to a remote host by sending one or more packets, then transfer data, then terminate the connection. HTTP uses TCP, so the overhead of establishing and terminating connections is overhead for HTTP.
In contrast, datagram service simply includes the destination host address (and port number, essentially naming a process to receive the data on the destination host), so each datagram packet can be independently routed through the Internet.

Summary of Circuit, Packet Switching

Two forms:

VC: Virtual circuit: simulates circuit switching
DG: Datagram: every man/women for him/herself

Feature	Circuit Switching	Packet Switching
Data sent as packets?	no	yes
Packets follow same route?	N/A	may or may not (may for VC)
Resources reserved in network?	yes	may or may not (not for DG)
Data send can experience variable latency	no	yes
Connection establishment done?	yes	VC: yes DG: no
State information stored at network nodes	?	VC: yes (tables to route virtual circuits) DG: no
Good for connection-less service	no	VC: yes DG: no
Impact of node/switch crash	all circuits through switch fail	VC: all virtual circuits through node fail DG: only packets at node are lost
Addressing info needed needed?	only when call is set up	Every packet needs... VC: virtual circuit number DG: full source, destination address
Congestion control	unnecessary	VC: easy if sufficient buffers allocted DG: hard

Store and forward

Routers in packet switched networks usually store incoming packets in buffers before sending them to their destination. This complicates delivering video, voice.

Performance Measures

Throughput

Response time (latency)

Utilization

Quality of Service (QOS)

Traffic

Protocol stack

A protocol is usually specified as a set of layers with well defined interfaces. Here is a typical representation, following the infamous seven layer Open Systems Interconnection (OSI) protocol stack of the International Standards Organization (ISO):

Layer	Functions
Physical	Example: modems, phone lines Provides virtual bit pipe
Data link	Example: Ethernet, FDDI, and ATM Provides asynchronous, virtual packet pipe Possibly provides reliability: error detection and recovery Flow control Multiaccess (if multiple access media in layer 1) Addressing (if multiple access in layer 1)
Network	Example: IP and X.25 Routing Flow control Perhaps congestion control Network management Possibly virtual circuits (hence connections)
Transport	Example: TCP and OSI TP4 Reliable, end-to-end delivery Breaks messages into packets and reassembles at receiver Multiplex low data rate sessions onto one network connection Split high data rate transport session into multiple network sessions.
Session	Note: functions vary from protocol-to-protocol Possibly connection establishment, release Data transfer Token management Synchronization (roll back uuper layers on lower layer error) Partitioning of message into records Exception reporting
Presentation	Example: Sun's External Data Representation (XDR) routines Translation to/from universal data represenation Encryption, decryption Data compression, decompression
Application	Example: HTTP, FTP There are many app-level protocols, customized to the needs of particular app

Abstract representation of above diagram::

Frame, packet, and message

A frame is transmitted between the Data Link Layers (layer 2).

Frame is simply a sequence of bits transmitted on a wire or over a wireless medium.
Special method is used to delineate frame boundaries.
Some frame types are Ethernet frames and FDDI frames.

Here is a generic frame:

The Data Link Layer's job is to serve, logically, as a pipe through which packets are sent:

Shown below is the full view of how layer 2 interacts with layer 1, and provides a logical connection between two computers that are physically connected through a third computer (or switching node in a network):

A packet is the transfer unit at the Network Layer (layer 3). It is the data field of a frame.

A message is the sequence of bits passed by a higher layer to the the Transport Layer (layer 4).

A message is arbitrarily long
It is broken into fixed sized packets by the Transport Layer for delivery by the Network Layer.

The diagram below relates Transport Layer messages, Network Layer packets, and Data Link Layer frames:

Last modified on 4 February 1998.

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