The size of a network is limited due to size and distance constraints. However networks may be connected over a high speed communications link (called a WAN link) to link them together and thus become a WAN. WAN links are usually:
- Dial up connection
- Dedicated connection – It is a permanent full time connection. When a dedicated connection is used, the cable is leased rather than a part of the cable bandwidth and the user has exclusive use.
- Switched network – Several users share the same line or the bandwidth of the line. There are two types of switched networks:
- Circuit switching – This is a temporary connection between two points such as dial-up or ISDN.
- Packet switching – This is a connection between multiple points. It breaks data down into small packets to be sent across the network. A virtual circuit can improve performance by establishing a set path for data transmission. This will shave some overhead of a packet switching network. A variant of packet switching is called cell-switching where the data is broken into small cells with a fixed length.
WAN Connection Technologies
- X.25 – This is a set of protocols developed by the CCITT/ITU which specifies how to connect computer devices over an internetwork. These protocols use a great deal of error checking for use over unreliable telephone lines. They establish a virtual communication circuit. It uses a store and forward method which can cause about a half second delay in data reception when two way communications are used. Their speed is about 64Kbps. Normally X.25 is used on packed switching PDNs (Public Data Networks). A line must be leased from the LAN to a PDN to connect to an X.25 network. A PAD (packet assembler/disassembler) or an X.25 interface is used on a computer to connect to the X.25 network. CCITT is an abbreviation for International Telegraph and Telephone Consultative Committee. The ITU is the International Telecommunication Union.
- Frame Relay – Error checking is handled by devices at both sides of the connection. Frame relay uses frames of varying length and it operates at the data link layer of the OSI model. A permanent virtual circuit (PVC) is established between two points on the network. Frame relay speed is between 56Kbps and 1.544Mbps. Frame relay networks provide a high-speed connection up to 1.544Mbps using variable-length packet-switching over digital fiber-optic media. Frame relay does not store data and has less error checking than X.25.
- Switched Multi-megabit Data Service (SMDS) – Uses fixed length cell switching and runs at speeds of 1.533 to 45Mbps. It provides no error checking and assumes devices at both ends provide error checking.
- Telephone connections
- Dial up
- Leased lines – These are dedicated analog lines or digital lines. Dedicated digital lines are called digital data service (DDS) lines. A modem is used to connect to analog lines, and a Channel Service Unit/Data Service Unit or Digital Service Unit(CSU/DSU) is used to connect to digital lines. The DSU connects to the LAN and the CSU connects to the line.
- T Carrier lines – Multiplexors are used to allow several channels on one line. The T1 line is basic T Carrier service. The available channels may be used separately for data or voice transmissions or they may be combined for more transmission bandwidth. The 64Kbps data transmission rate is referred to as DS-0 (Digital Signal level 0) and a full T1 line is referred to as DS-1.
Number of equivalent T1 lines
T1 and T3 lines are the most common lines in use today. T1 and T2 lines can use standard copper wire. T3 and T4 lines require fiber-optic cable or other high-speed media. These lines may be leased partially called fractional T1 or fractional T3 which means a customer can lease a certain number of channels on the line. A CSU/DSU and a bridge or router is required to connect to a T1 line.
- Integrated Services Digital Network (ISDN) – Comes in two types and converts analog signals to digital for transmission. It is a dial up service
- Basic Rate ISDN (BRI) – Two 64Kbps B-channels with one 16Kbps D channel. The D-channel is used tor call control and setup.
- Primary Rate ISDN (PRI) – 23 B-channels and one D channel.
A device resembling a modem (called an ISDN modem) is used to connect to ISDN. The computer and telephone line are plugged into it.
- Switched-56 – A switched line similar to a leased line where customers pay for the time they use the line. Speed is 56Kbps. It is not dedicated and will not work to connect a WAN.
- Constant bit rate for data.
- Variable bit rate for audio or video.
- Connection less for data.
- Connection oriented for data.
ATM can be embedded in other protocols such as ATM-25, T1, T3, OC-1, OC-3, OC-12, and OC-48. Some ATM technologies include:
- ATM-25 – 25Mbps speed.
- STS-3 – 155Mbps on fiber or category 5 cable.
- STS-12 – 620 Mbps on fiber cable for campus wide network.
- STS-48 – 2.2 Gbps on fiber cable on a MAN.
- STS-192 – 8.8 Gbps on fiber cable on intercity long distance. This is normally used by phone companies.
WAN Technology comparisons
Switched line, not dedicated
Packet switching, error correction, store and foreward, with round trip delay
56Kbps to 1.544Mbps
Varying length frames with permanent virtual circuit
Fixed cell length with no error checking
24 Multiplexed channels
672 Multiplexed channels
OC2 is 2X OC1, OC3 is 3X OC1
155Mbps to 622 Mbps
Fixed length packets. works on SONET and T carrier lines. Uses virtual circuits
- Circuit switching – Physical switched connection.
- Message switching – A store and forward mechanism where messages are treated as individual units.
- Packet switching – Messages are broken down into smaller packets with individual destination information. Independent routing is used which allows the packets to use any route between the source snd destination. Much RAM and processing power is required to support this switching type.
- Datagram packet switching – Uses independent paths.
- Virtual circuit packet switching – This is used for audio and video streaming. A set path is established between the source and destination and a connection oriented service is made.