Metropolitan and Wide Area Networks. Chapter 8. Introduction. Metropolitan area networks (MANs) typically span from 3 to 30 miles and connect backbone networks (BNs), and LANs. Wide area networks (WANs) connect BNs and MANs across longer distances, often hundreds of miles or more.
Metropolitan area networks (MANs) typically span from 3 to 30 miles and connect backbone networks (BNs), and LANs.
Wide area networks (WANs) connect BNs and MANs across longer distances, often hundreds of miles or more.
Most organizations cannot afford to build their own MANs and WANs, so they rent or lease circuits from common carriers such as AT&T, MCI, BellSouth, PACTEL or NYNEX.
Two approaches to building up a WAN/MAN:
A combination is to build up a Virtual Private Network (VPN) using the Internet.
There are two main problems with dialed circuits.
One alternative is to establish a private dedicated circuit, which the user leases from the common carrier for their exclusive use, 24 hrs/day, 7 days/week.
Dedicate circuits are billed at a flat fee per month and the user has unlimited use of the circuit.
There are six types of dedicated circuits:
Voice grade circuits are analog circuits that work in exactly the same manner as traditional telephone lines, except that you do not dial them.
Dedicated voice grade channels often have conditioning (or equalization) done on them to improve data transmission quality by reducing noise and distortion.
Wideband analog services are similar to voice grade circuits but they provide much greater bandwidth.
Typically wideband analog services provide one 48,000 hertz bandwidth channel for use with frequency division multiplexing or as 12 individual voice grade channels (4000 Hz each).
T Carrier circuits are dedicated digital circuits and are the most commonly used form of dedicated circuit services in North America today.
Instead of a modem, a channel service unit (CSU) or data service unit (DSU) are used to connect the circuit into the network.
T-1 circuit (a.k.a. a DS-1 circuit) provides a data rate of 1.544 Mbps. T-1’s allow 24 simultaneous 64 Kbps channels (with TDM) which transport data, or voice messages using pulse code modulation. (64Kbps x 24 = 1.536Mbps)
T-2 circuit (6.312 Mbps) is basically a multiplexed bundle of four T-1 circuits.
T-3 circuit (44.376 Mbps) is equal to the capacity of 28 T-1 circuits (672 64Kbps channels).
T-4 circuit (274.176 Mbps) is equal to the capacity of 178 T-1s.
Fractional T-1, (FT-1) offers portions of a 1.544 Mbps T-1 for a fraction of its full costs.
T-Carrier Designation DS Designation Speed
DS-1 (24 DS-0)
DS-2 (96 DS-0)
DS-3 (672 DS-0)
DS-4 (178 T-1)
A term for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 Kbps, the bandwidth normally used for one telephone voice channel.
Both the North American T-carrier system and the European E-carrier systems of transmission operate using the DS series as a base multiple. The digital signal is what is carried inside the carrier system.
SONET transmission speeds begin at the OC-1 level (optical carrier level 1) of 51.84 Mbps. Each succeeding rate in the SONET fiber hierarchy is defined as a multiple of OC-1.
Several common carriers (e.g. MCI) now use OC-12 circuits at 622.08 Mbps to carry digitized voice traffic.
SONET Designation SDH Designation Speed
The major problem with dedicated circuit services it that the user must carefully plan all circuits needed.
In contrast, switched circuits work much like dialed circuits. The user buys a connection into the common carrier’s network from the end points of the WAN, without specifying all the interconnecting circuits needed.
The primary differences from dialed circuits is that the circuits are entirely digital and that they offer higher data transmission rates.
The first generation of Integrated services digital network (ISDN), commonly called narrowband ISDN, combines voice, video, and data over the same digital circuit.
ISDN has long been more of a concept than a reliable service in North America.
Acceptance has been slowed because equipment vendors and common carriers conflicting interpretations of ISDN standards.
Narrowband ISDN offers two types of service:
One advantage of BRI is it can be installed over existing telephones lines. (if less than 3.5 miles).
The second generation of ISDN is called Broadband ISDN (B-ISDN).
Packet switched services enable multiple connections to exist simultaneously between computers.
With packet switching users buy a connection into the common carrier network, and connects via a packet assembly/ disassembly device (PAD).
Packet switching splits messages into small segments called packets.
Packets from separate messages are interleaved with other packets for transmission.
Although the packets from one data stream may mix (interleave) with several other data streams during their journey, it is unlikely that packets from two different data streams will travel together during the entire length of their transmission.
There are two methods used to route packets:
Packet switched services are often provided by different common carriers than the one from which organizations get their usual telephone and data services.
Therefore, organizations often lease a dedicated circuits from their offices to the packet switched network point-of-presence (POP).
The oldest packet switched service is X.25, a standard developed by ITU-T. X.25 offers datagram, switched virtual circuit, and permanent virtual circuit services (Data link layer protocol: LAPB (Link Access Procedure-Balanced), network layer protocol PLP).
Although widely used in Europe, X.25 is not widespread in North America. The primary reason is transmission speed, now 2.048 Mbps (up from 64 Kbps).
X.25 Protocol Suite
It maps to the lowest three layers of the OSI model.
Frame relay is a newer packet switching technology that transmits data faster than X.25. It differs from X.25 and traditional networks in three important ways:
1. Frame relay only operates at the data link layer.
2. Frame relay networks do not perform error control.
3. Frame relay defines two connection data rate that are negotiated per connection and for each virtual circuit as it is established: Committed information rate (CIR) and Maximum allowable rate (MAR).
Different common carriers offer frame relay networks with different transmission speeds: 56 Kbps to 45 Mbps.
At present, frame relay suffers from the same problems as ISDN - a lack of standards.
(See Joanie Wexler, BCR, July 1999)
Asynchronous transfer mode (ATM) is one of the fastest growing new technologies, and is similar to frame relay.
All data are packet-switched, and there is no error control at the intermediate computers within the network; error control is the responsibility of the source and destination.
ATM has four important difference from frame relay:
SNI: Subscriber network interface
CPE: Customer premises equipment
SIP is used for communications between CPE and SMDS carrier equipment
In January 1999, AT&T announced the first VPN services to be based on MPLS --- its IP-Enabled Frame Relay service.
Cable & Wireless and Cisco Systems conducted a trial of IP-VPN service based on MPLS with Hongkong Telecom in March, 1999.
MCI/Worldcom Started to offer MPLS-based IP-VPN service in March, 1999.
A 1995 survey of network managers found that:
The most expensive part of the WAN will be the people required to plan, install, and operate it, so pick one that is easy to manage.
It costs more to lease services from common carriers than to buy hardware, so selection decisions should be driven more by the services.
AT&T Network Services (http://www.ipservices.att.com/backbone/)
BBN Planet (GTE)
Cable & Wireless USA
UUNET, a part of MCI WorldCom
A fundamental issue:
Several companies offer value added networks (VANs) that are alternatives to building networks by leasing circuits from common carriers. VANs provide additional services over and above those provided by common carriers.
A new type of VAN, called a virtual private network (VPN), or software defined networks, provide circuits that run over the Internet but appear to the user to be private networks.
A virtual private network (VPN) is an extension of an enterprise’s private intranet across a public network such as the Internet, creating a secure private connection, essentially through a private tunnel. VPN provides cost-effective data transmission with high security.
Figure 8-13 VPN Network
Packet in transmission through the Internet
Packet from the VPN
Figure 8-14 VPN encapsulation of packets
According to industry analyst Forrester Research Inc., when comparing the cost of traditional leased line network versus today\'s Internet-based VPN, the cost differences for 1,000 users are eye-popping.
City Distance (mi.) T1 Fees Internet VPN Fee
SF-Denver 1,267 $13,535 $1,900
Denver-Chicago 1,023 $12,315 $1,900
Chicago-NY 807 $11,235 $1,900
SF-LA 384 $ 5,520 $1,900
Denver-Salt Lake 537 $ 6,285 $1,900
Denver-Dallas 794 $ 7,570 $1,900
NY-DC 235 $ 4,775 $1,900
NY-Boston 194 $ 4,570 $1,900
There are two important disadvantages of VPNs:
Extranet VPNs between a corporation and its strategic partners, customers, and suppliers.
Intranet VPNs between internal corporate departments and branch offices
Remote Access VPNs between a corporation and remote or mobile employees