Networking Overview
CPSC 330 - Spring 2004

communication

            communications
.--------.      medium      .----------.
| sender |----------------->| receiver |
`--------'                  `----------'

senders and receivers are in general called communication nodes, entities, and/or stations
networking is the case in which the communicating entities are computers
the communications medium can be a pair of twisted copper wires, electrical cables, optical fiber, wireless (i.e., radio), etc.

communication modes

signals
- analog
- digital
clocking
- synchronous - sender and receiver share a clock signal
- asynchronous - no shared clock (e.g., have to add start and stop bits)
direction
- simplex - one direction only
- half-duplex - either direction, but only one direction at a time
- full duplex - both directions at same time
number of recipients
- unicast - send to one receiver
- multicast - send to group of receivers
- broadcast - send to all receivers
number of communication channels within medium
- baseband - one channel
- broadband - multiple channels (although marketing is corrupting the term to mean merely anything faster than a regular telephone line at 56 Kbps)

communication protocols

set of rules for communication between computers
defines the valid sequences of messages and formats of messages

network types -- by geography

These terms often describe distances but may also be used to indicate technologies and protocols. E.g., LAN is now often used as a term meaning a single broadcast domain that is not necessarily limited by physical distance.

PAN - personal area network - 1 m
LAN - local area network - 10 meters to 1 km
MAN - metropolitan area network - 10 km
WAN - wide area network - 100 km to 1000 km, typically subdivided into hosts and communication subnet (such as telephone company or ISP)
internetworking - appears as one seamless communication network, conceals the different technologies and protocols of underlying subnets
Internet - thousands of interconnected networks using TCP/IP

Internet map
Internet growth

network types -- by topology

bus
ring
- token ring
- slotted ring
hierarchical
mesh
star
...

network types -- by technology

circuit-switched - pre-establish a physical circuit for dedicated communication
packet-switched - short messages individually routed over shared networks
- connectionless - indivdual messages
- connection-oriented - establish a session for duplex communication involving multiple message interchanges (called virtual circuit if done at low levels)
some history
- telephone systems traditionally have been circuit switched
- Leonard Kleinrock credited with packet-switching idea (1961 paper)
- ARPAnet was packet-switched over a network with redundant links to provide fault-tolerance
- TCP/IP developed from ARPAnet and became basis for Internet, TCP is connection-oriented and runs on top of connectionless IP
  - TCP - Transmission Control Protocol
  - IP - Internet Protocol
packet-switching history
- X.25, store-and-forward routing
  (then lower error rates and higher transmission rates, from Kbps to Mbps)
- Frame Relay, faster and uses cut-through routing
  used underneath ISDN (integrated services digital network)
  (then, higher transmission rates, to Gbps)
- ATM (asynchronous transfer mode), fixed-length cells
  sometimes called cell relay, used underneath broadband ISDN
ISDN and telephone system are now using virtual circuits on top of ATM
- virtual circuits reserve capacity at switching nodes and can thereby provide bandwidth and latency guarantees
- ATM can be implemented natively or over synchronous optical network (SONET)

communications media

twisted pairs of wire
cable (e.g., null modem)
coaxial cable for LAN
PSTN - public switched telephone network
- analog subscriber line (DS0 - 64 Kbps)
- DSL - digital subscriber line (over regular phone lines using FDM)
  - narrowband ISDN
  - broadband ISDN
cable television
leased or dedicated transmission lines
- T1 (1.5 Mbps)
- T3 (45 Mbps)
fiber optic lines
- OC-1 (50 Mbps)
- ...
- OC-48 (2 Gbps)
- ...
- OC-192 (10 Gbps)
microwave relay
satellite
fixed wireless networks
- within room (e.g., Bluetooth - low-power radio technology)
- within building (e.g., Wi-Fi)
mobile wireless cellular networks

example service providers

AT&T and RBOCs (regional Bell operating companies) - telephone system
MCI (Worldcom) - original system used microwave towers (Worldcom also owns UUnet)
Sprint - buried fiber along railroad right-of-ways (Southern Pacific)
Iridium - low-orbit satellites (started by Motorola)
total US residential consumer ISP subscriptions 73 million in 2003
dial-up service is 70% of the market - controlled by a few national ISPs AOL (25M), MSN (9M), etc., resell Internet access to subscribers (see rankings)
growth in broadband - RBOCs and cable operators, e.g., DSL projected by year-end 2003 to have 9 million subscribers (said to be driven by interactive gaming), cable has 16 million Internet subscribers
Internet stats on US households

home network

wired, e.g., Ethernet
wireless, e.g., IEEE 802.11b
- Microsoft Broadband Networking Wireless Router Model MN-500 ($65) hooks to cable or DSL modem via Ethernet connection, built-in hardware firewall, 1 to 11 Mbps, 160 to 1500 feet (slower with distance)
- notebook adapter card ($45)
- USB adapter ($55)
existing phone lines (doesn't interfere with voice or DSL), standard called HPNA, 10 Mbps, PCI and USB versions
existing electrical power lines, latest spec called HomePlug, runs at 14 Mbps, PCI or USB versions ($125)

standards

OSI layers - separation of concerns

  +-----------------------------+
  | 7 /data/ application layer  | - user applications
  +-----------------------------+
  | 6 /data/ presentation layer | - encryption, protocol translation
  +-----------------------------+
  | 5 /data/ session layer      | - open, maintain, close logical connections
  +-----------------------------+
  | 4 /segments/ transport layer| - end-to-end, reliable communication
  +-----------------------------+
  | 3 /packets/ network layer   | - routing of packets using logical addresses
  +-----------------------------+
  | 2 /frames/ data-link layer  | - final framing, synchronization, flow control
  +-----------------------------+
  | 1 /bits/ physical layer     | - wiring (connector size, # pins), signals
  +-----------------------------+

several acrostics exist (e.g., "All People Seem To Need Data Processing")
example application-level protocols - telnet, ftp, http, ...
example transport-level protocols
- connectionless - UDP
- connection-oriented - TCP
example network-level protocols
- datagram (connectionless packet-switching) - IP
- virtual circuit - X.25, frame relay (ISDN), ATM (broadband ISDN)
example data-link-level protocols - Ethernet, Bluetooth, ATM physical, ...
example physical-link-level standards - RS-232, ...
layers provide flexibility and separation of concerns, but at a performance cost of multiple OS and subroutine calls and parameter passing overhead
wired-network errors now are usually software-related (e.g., buffer overflow) rather than link-related

framing

               sender                                   receiver

             +---------+                               +---------+
             |  data   |                               |  data   |
             +---------+                               +---------+
    frame         V                           frame         ^
    +--------.---------.---------+            +--------.---------.---------+
    | header | payload | trailer | ---------> | header | payload | trailer |
    +--------^---------^---------+            +--------^---------^---------+

    each layer typically adds/removes another header field

IEEE 802 standard divides up data link layer into two sublayers

  +-----------------------------+
  | LLC (logical link control)  | - protocol between peer data link layers
  + - - - - - - - - - - - - - - +
  | MAC (medium access control) | - decides station access to shared medium
  +-----------------------------+     (1) RR (round-robin)
                                      (2) reservation
                                      (3) contention (e.g., CSMA/CD)

name of message unit

LLC - PDU (protocol data unit)

MAC - MAC frame

    +----------+-----------+----------+---------+-----+
    | MAC cntl | dest addr | src addr | LLC PDU | CRC |
    +----------+-----------+----------+---------+-----+

protocols vs. services

     services                                     services
      v   |                                        v   |
  +-----------+                                +-----------+
  | layer n-1 | <-- protocol for layer n-1 --> | layer n-1 |
  +-----------+                                +-----------+
      |   ^                                        |   ^
     services                                     services
      v   |                                        v   |
  +-----------+                                +-----------+
  |  layer n  | <--- protocol for layer n ---> |  layer n  |
  +-----------+                                +-----------+
      |   ^                                        |   ^
     services                                     services

performance

    |<---sender--->|<----transmission time---->|                            |
    |   overhead   | ` .                        ` .                         |
    |              |     ` .                        ` .                     |
    |              |         ` .                        ` .                 |
    |              |<--time of-->|<----transmission time---->|<--receiver-->|
    |              |   flight                                |   overhead   |
    |              |                                         |              |
    |              |<-----------transport latency----------->|              |
    |                                                                       |
    |<----------------------------total latency---------------------------->|

transmission time = packet size / bandwidth
effective bandwidth = payload size / total latency
measures
- bits per second (bps) [note lower case 'b']
- baud - signal change rate, derives from telegraph measures
  - each signal change can carry from 1-16 bits of info
LAN request has latency of 1 msec or less (less than hard disk)
average Internet request has round-trip latency of 100-500 msec

QoS (quality of service)

how stringent are various communication requirements across different applications?

                                               low    low jitter
                      reliability  bandwidth  delay   (variation
                                                       in delay)

          web access       ++          +         +         _
               email       ++          -         -         _
     video on demand        _         ++         _        ++
           telephony        _          -        ++        ++

                      ++ essential
                       + important
                       - desirable

QoS not supported in Ethernet or current IP
ATM provides classes of service (CoS)

extra info -- from RFC1122

[LL means link layer; note, however, that in common usage "frame" and "packet" are often used interchangeably]

  The terms frame, packet, datagram, message, and segment
  are illustrated by the following schematic diagrams:

 
    A. Transmission on connected network:
      _______________________________________________
     | LL hdr | IP hdr |         (data)              |
     |________|________|_____________________________|

      <---------- Frame ----------------------------->
               <----------Packet -------------------->


    B. Before IP fragmentation or after IP reassembly:
               ______________________________________
              | IP hdr | transport| Application Data |
              |________|____hdr___|__________________|

               <--------  Datagram ------------------>
                        <-------- Message ----------->

      or, for TCP:
               ______________________________________
              | IP hdr |  TCP hdr | Application Data |
              |________|__________|__________________|

               <--------  Datagram ------------------>
                        <-------- Segment ----------->

(see also what is a packet?)

[CPSC 330 homepage]

mark@cs.clemson.edu

Networking Overview CPSC 330 - Spring 2004