Computer networks -- 2007-2008 -- info.uvt.ro/Course 10

These pages are somehow outdated and it is recommended to consult the newer version at Computer networks -- 2008-2009 -- info.uvt.ro (by Ciprian Crăciun).

Quick links:

front;
courses 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13;
laboratories agenda, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, evaluation, tools, repository.

WAN communication[edit]

connection types:
- dedicated -- synchronous serial:
  - usually through leased lines;
  - advantages: speed, 100% online;
  - disadvantages: cost, in case of failure;
- circuit switched -- asynchronous serial:
  - usually through the telephony system;
  - either normal modems, ISDN or DSL;
  - advantages: cost and simplicity;
  - disadvantages: speed;
- packet switched -- synchronous serial:
  - usually used to share the bandwidth (and costs) with other companies;

DSL, ADSL, and ISDN[edit]

DSL -- Digital Subscriber Line;
ADSL -- Asymmetric Digital Subscriber Line;
ISDN -- Integrated Services Digital Network:
all work over existing phone lines;
ISDN can be used for both voice and data (at the same time);
links:

HDLC[edit]

HDLC -- High Level Data Link Control:
created by IBM as a simple data link layer protocol;
works with synchronous serial links;
it works at bit level;
it has low overhead;
it is an ISO standard;
it doesn't support multiple network layer protocols;
it doesn't support authentication;
frame structure:
- flag -- 8 bits;
- address -- 8 or more bits;
- control -- 8 or 16 bits;
- data -- 0 or more bits;
- FCS -- Frame Check Sequence -- 16 or 32 bits -- a CRC;
- flag -- 8 bits;
links:
- wikipedia:High-Level Data Link Control;

PPP[edit]

PPP -- Point to Point Protocol:
it is a better version of the HDLC;
works with both synchronous and asynchronous serial links;
it supports multiple network layer protocols;
it allows authentication;
it has two sub-layers:
- LCP -- Link Control Protocol -- used for controlling the connection:
  - authentication;
  - compression;
  - error detection;
  - multilinking;
- NCP -- Network Control Protocol -- used for determining network layer parameters (like IP address);
authentication:
- PAP -- Password authentication protocol:
  - passwords are sent in plain text;
  - the authentication is done only at the beginning of the session;
- CHAP -- Challenge Handshake authentication protocol:
  - passwords are sent as salted hashes;
  - authentication is done both at the beginning and during the session;
links:
miscellaneous:
- wikipedia:Point-to-Point Protocol over ATM -- PPPoA;
- wikipedia:Point-to-Point Protocol over Ethernet -- PPPoE;
- wikipedia:Point-to-point tunneling protocol -- PPTP;

Tunneling and VPN[edit]

VPN -- Virtual Private Network;
used mostly by enterprises;
provides:
- authentication;
- encryption;
advantages:
- security;
disadvantages:
- processing overhead (due to encryption);
- network overhead (due to additional headers);
there are multiple incompatible (most proprietary) standards;
links:

Miscellaneous[edit]

data processing:
data access:
- wikipedia:Proxy server;
- wikipedia:Web cache;
data formating:
- wikipedia:JSON;
- wikipedia:XML;
- wikipedia:Abstract Syntax Notation One -- ASN1;

JSON example:

 {
     "firstName": "John",
     "lastName": "Smith",
     "address": {
         "streetAddress": "21 2nd Street",
         "city": "New York",
         "state": "NY",
         "postalCode": 10021
     },
     "phoneNumbers": [
         "212 555-1234",
         "646 555-4567"
     ]
 }

XML example:

 <recipe name="bread" prep_time="5 mins" cook_time="3 hours">
   <title>Basic bread</title>
   <ingredient amount="3" unit="cups">Flour</ingredient>
   <ingredient amount="0.25" unit="ounce">Yeast</ingredient>
   <ingredient amount="1.5" unit="cups" state="warm">Water</ingredient>
   <ingredient amount="1" unit="teaspoon">Salt</ingredient>
   <instructions>
     <step>Mix all ingredients together.</step>
     <step>Knead thoroughly.</step>
     <step>Cover with a cloth, and leave for one hour in warm room.</step>
     <step>Knead again.</step>
     <step>Place in a bread baking tin.</step>
     <step>Cover with a cloth, and leave for one hour in warm room.</step>
     <step>Bake in the oven at 350(degrees)F for 30 minutes.</step>
   </instructions>
 </recipe>

ASN1 definition example:

FooProtocol DEFINITIONS ::= BEGIN

    FooQuestion ::= SEQUENCE {
        trackingNumber INTEGER,
        question       IA5String
    }

    FooAnswer ::= SEQUENCE {
        questionNumber INTEGER,
        answer         BOOLEAN
    }

END

conf. dr. Victoria Iordan, iordan@info.uvt.ro
lect. drd. Horia Popa, hpopa@info.uvt.ro
Marian Neagul, marian@info.uvt.ro
Ciprian Dorin Crăciun, ccraciun@info.uvt.ro
Mihail Găianu, mgaianu@info.uvt.ro