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A WAN standard usually gives a description of WAN physical layer and WAN data link layer.
A number of organizations have given their standards:
ITU–T – I nternational T elecommunication U nion – T elecommunication Standardization Sector
ISO – I nternational O rganization for S tandardization
IETF – I nternet E ngineering T ask F orce
EIA – E lectronic I ndustries A ssociation
WAN Physical Layer
WAN Physical Layer protocols describe how to provide electrical, mechanical, operational and functional connections for WAN services.
The WAN physical layer describes the interface between
Data Terminal Equipment (DTE)
Data Communicating Equipment (DCE)
WAN Data Link Layer
WAN Data link Layer protocols describe how frames are transmitted in the Data Link Layer.
Several common Data Link Layer standards [only for reference]:
High Level Data Link Control (HDLC)
Point to Point Protocol (PPP)
Integrated Service Digital Network (ISDN)
Simple Data Link Control Protocol (SDLC)
Serial Line Interface Protocol (SLIP)
Link Access Procedure Balance (LAPB)
Link Access Procedure D-channel (LAPD)
Link Access Procedure Frame (LAPF)
Asynchronous Transfer Mode (ATM)
We will study The star-marked topics
WAN Data Link Layer: PPP (Point-to-Point Protocol)
IETF took the initiative to design PPP as the data link layer protocol for point-to-point lines. Their goal was to use it as the official Internet Standard.
Q: What’s Point-to-Point? A: A point-to-point network may consist of more than one connection between two machines: source and destination. A packet sent from source to destination may have to visit one or more intermediate machines. But the source and destination will think that they are connected by a single wire.
In future all the ISP’s (Internet Service Provider) will support PPP.
PPP Frame Format 01111110 11111111 00000011 variable 01111110
ISDN (Integrated Service Digital Network)
History: For more than 100 years, the international telecommunication infrastructure has been public circuit-switched telephone system. It can support only analog voice transmission. Hence it is not enough for modern communication. So the telephone companies of the world decided to design a new, fully digital telephone system. The main goal was to integrate voice and non-voice service . This new system is called ISDN .
Features of ISDN: 1. Voice (the main service) 2. Intercom button for calling secretaries instantly 3. Multiple hotkeys for instantly calling (no delay for call setup) anywhere in the world 4. Caller ID display 5. Telephone can be connected to computer so that caller’s tel. no + name + address can be displayed from database 6. Call forwarding 7. Conferencing worldwide. There are more…
How it works: ISDN
Key Idea : Digital Bit Pipe means a conceptual pipe between the customer and the carrier through which bits flow. ISDN configuration for home/small office is: -
NT ISDN Exchange Customer’s office Carrier’s office Km’s away Digital bit pipe ISDN telephone ISDN Terminal
How it works: ISDN
In the customer’s office up to 8 telephones, computers and other devices can be connected. Similar to LAN.
NT: This is the Network Terminator placed in the customer’s office. It is responsible to send signals to the ISDN Exchange , in the carrier’s office, many kilometers away.
Digital signal is processed inside the carrier’s office and then sent to the destination customer.
Now we would like to look inside the Digital Bit Pipe
How it works: ISDN The ISDN Interface Inside the digital bit pipe: It’s a wonderful world!
There is Time Division Multiplexing ( TDM ) inside the bit pipe. Don’t tell me you don’t know what is TDM!
Three types of bit pipes have been standardized on the basis of TDM:
The Basic Rate digital bit pipe is the replacement for POTS (Plain Old Telephone Service) . Let’s examine it.
How it works: ISDN Basic Rate Digital Pipe 64 kbps digital PCM channel for voice or data 16-kbps out- of-band signal
ATM (Asynchronous Transfer Mode)
This is a connection-oriented network. Hope you know what’s connection-oriented and what’s connectionless.
ATM: This strange name is due to the fact that most transmission is synchronous (closely related to a clock) and ATM is not.
ATM Craze : During 1990s it was believed that ATM was going to solve all the networking problems by merging – voice, data, cable TV, telex, telegraph etc. etc.
Did not work because of politics
Internet (connectionless) versus Telephone companies (connection-oriented)
Don’t think ATM is dead. It is still alive deep inside the telephone network, mostly used by carriers for internal transport.
How it works: ATM Virtual Circuits
ATM is connection-oriented. So before sending data a connection has to be set up.
To set up a connection the source sends a setup-packet to the destination.
As this packet moves through the subnet, it sets up the path from source to destination.
Routers in subnet updates their routing table, when they get this packet.
This kind of connection is called virtual circuits, similar to how electricity passes through an actual circuit.