CS 268 - Class Notes (Introduction)

Communication of Computing Devices

Terms and Conventions (Jargon)

Data is sent across a network as a "stream" of bits (1 or 0).

Encapsulation: "wrapping" data in information (or context):

Header: information appended to the front of a stream.
Trailer: information appended to the back of a stream.

Some Conventions:

Mb: Mega-bit
MB: Mega-byte
Kb: Kilo-bit
KB: Kilo-byte
octet: 8 bits (1byte)

Example of Encapsulation: WWW Page Download over Ethernet



                              +-------------------------------+
                              |      WWW Page Contents        |
                              +-------------------------------+
                              V                               V
                       +--------------------------------------+
                       | HTTP |                               |
                       |Header|                               |
  Application          +--------------------------------------+
---------------------- V ------------------------------------ V
  Network/OS    +---------------------------------------------+
                | TCP  |                                      |
                |Header|                                      |
                +---------------------------------------------+
                V                                             V
         +----------------------------------------------------+
         | IP   |                                             |
         |Header|                                             |
         +----------------------------------------------------+
         V                                                    V
+-------------------------------------------------------------+--------+
|Ethernet| IP   | TCP  | HTTP |      WWW Page Contents        |Ethernet|
|Header  |Header|Header|Header|                               | Trailer|
+-------------------------------------------------------------+--------+

Protocol: a formula for passing messages, specifying the details of message formats, and describing how to handle error conditions. Protocols exist at various levels:

Application Protocols: SMTP (email), HTTP (www), telnet/rlogin, etc.
Network Protocols: TCP, IP, UDP, ARP, RARP, Ethernet, etc.

Basic Types of Networks

Circuit-Switched: form a dedicated connection (circuit) between two points. The telephone system is a good example. A connection has a guaranteed capacity indepdent of traffic load. We will not be covering these types of networks in this class.
Packet-Switched: traffic divided into small pieces (packets) that are multiplexed onto high capacity intermachine connections. Each packet contains information to get it to its final destination (headers/trailers). We will be covering these types of networks in this class.

Network Technologies

Local Area Network (LAN):
- High speeds (4 Mbps - 2 Gbps)
- Small areas (hundreds of yards)
- Low delay (sub-millisecond ranges)
- Shared medium for multiple systems
- Ex: Ethernet, IEEE 802.3, Token Ring (IEEE 802.5), etc.
Metropolitan Area Network (MAN):
- Medium-High speeds (56 Kbps - 100 Mbps)
- Slightly more delay than LANs (mostly due to distance)
- City areas ("metro" areas)
- Ex: usually combination of WAN and LAN technologies
Wide Area Network (WAN):
- Low speeds (but there are exceptions) (9.6 Kbps - 45 Mbps)
- Large delay (tens to hundreds of milliseconds)
- Arbitrary distances (usually large)
- Also called: "long haul networks"
- Ex: DS1 (also known as T1) - 1.544 Mbps each way (duplex)

Shared Media:
- Fiber Distributed Data Interface (FDDI): 100 Mbps Token Ring. ANSI Standard.
- Ethernet: 10, 100, 1000 Mbps.
Point to Point:
- DS-1: 1.544 Mbps, 24 voice channels
- DS-3: 44.736 Mbps, 672 voice channels
- STS-1/OC-1: 51.84 Mbps
- STS-3/OC-3: 155.52 Mbps
- STS-12/OC-12: 622.08 Mbps

STS = Synchronous Transport Signal, SONET = Synchronous Optical Network. ATM is designed specifically for OC-3 and OC-12 speeds.

Todd L. Montgomery (revised 08.23.1999)