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
- 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
- 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
see also
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
- 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
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