Chapter 2 WANs
A WAN is a data communications network that operates beyond a LAN's geographic scope. One way that a WAN is different from a LAN is that you must subscribe to an outside WAN service provider, such as a regional Bell operating company (RBOC) to use WAN carrier network services.
WAN technologies function at the three lowest layers of the OSI reference model: the physical layer, the data link layer, and the network layer
Telephone and data services are the most commonly used WAN services. Telephone and data services are connected from the building point of presence (POP) to the WAN provider's central office (CO)
WAN provider services into three main types:
Call setup- sets up and clears calls between telephone users. Also called signaling, call setup uses a separate telephone channel not used for other traffic. The most commonly used call setup is Signaling System 7 (SS7), which uses telephone control messages and signals between the transfer points along the way to the called destination.
Time-division multiplexing (TDM)-Information from many sources has bandwidth allocation on a single medium. Circuit switching uses signaling to determine the call route, which is a dedicated path between the sender and the receiver. By multiplexing traffic into fixed time slots, TDM avoids congested facilities and variable delays. Basic telephone service and ISDN use TDM circuits.
Frame Relay-Information contained in frames shares bandwidth with other WAN Frame Relay subscribers. Frame Relay is statistical multiplexed service, unlike TDM, which uses Layer 2 identifiers and permanent virtual circuits. In addition, Frame Relay packet switching uses Layer 3 routing with sender and receiver addressing contained in the packet.
main parts of WAN services:
Customer premises equipment (CPE) -- Devices physically located on the subscriber's premises. Includes both devices owned by the subscriber and devices leased to the subscriber by the service provider.
Demarcation (or demarc) -- The point at which the CPE ends and the local loop portion of the service begins. Often occurs at the POP of a building.
Local loop (or "last-mile") -- Cabling (usually copper wiring) that extends from the demarc into the WAN service provider's central office.
CO switch -- A switching facility that provides the nearest point of presence for the provider's WAN service.
Toll network -- The collective switches and facilities (called trunks) inside the WAN provider's cloud. The caller's traffic may cross a trunk to a primary center, then to a sectional center, and then to a regional- or international-carrier center as the call travels the long distance to its destination.
A key interface in the customer site occurs between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE)
Typically, the DTE is the router, and the DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility
DCE is the attached modem, channel service unit/data service unit (CSU/DSU), or terminal adapter/network termination 1 (TA/NT1).
The WAN path between the DTEs is called the link, circuit, channel, or line.
A virtual circuit is a logical circuit, as opposed to a point-to-point circuit, created to ensure reliable communication between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs
SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete
Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit-termination phase involves tearing down the virtual circuit between the source and destination devices.
A PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations where data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but increase costs due to constant virtual-circuit availability.
WANs use numerous types of devices, including the following:
Routers, which offer many services, including LAN and WAN interface ports.
WAN switches, which connect to WAN bandwidth for voice, data, and video communication.
Modems, which interface voice-grade services. Modems include CSUs/ DSUs and TA/NT1 devices that interface ISDN services.
Communication servers, which concentrate dial-in and dial-out user communication.
Routers are devices that implement the network service
Routers manage networks by providing dynamic control over resources and supporting the tasks and goals for networks.
A WAN switch is a multiport networking device, which typically switches such traffic as Frame Relay, X.25, and Switched Multimegabit Data Service (SMDS).
WAN switches typically operate at the data link layer of the OSI reference model
A modem is a device that interprets digital and analog signals by modulating and demodulating the signal, enabling data to be transmitted over voice-grade telephone lines.
A CSU/DSU is a digital-interface device-or sometimes two separate digital devices-that adapts the physical interface on a DTE device (such as a terminal) to the interface of a DCE device (such as a switch) in a switched-carrier network.
An ISDN Terminal Adapter (TA) is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces
WAN physical-layer protocols describe how to provide electrical, mechanical, operational, and functional connections for WAN services
the common data-link encapsulations associated with WAN lines, which are:
Frame Relay -- By using simplified encapsulation with no error correction mechanisms over high-quality digital facilities, Frame Relay can transmit data very rapidly compared to the other WAN protocols.
Point-to-Point Protocol (PPP) -- Described by RFC 1661, PPP was developed by the IETF. PPP contains a protocol field to identify the network-layer protocol.
ISDN -- A set of digital services that transmits voice and data over existing phone lines.
Link Access Procedure, Balanced (LAPB) -- For packet-switched networks, LAPB is used to encapsulate packets at Layer 2 of the X.25 stack. It can also be used over a point-to-point link if the link is unreliable or there is an inherent delay associated with the link, such as in a satellite link. LAPB provides reliability and flow control on a point-to-point basis.
Cisco/IETF -- Used to encapsulate Frame Relay traffic. The Cisco option is proprietary and can be used only between Cisco routers.
High-Level Data Link Control (HDLC) -- An ISO standard, HDLC might not be compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations
PPP is a standard serial-line encapsulation method (described in RFC 1332 and RFC 1661). This protocol can, among other things, check for link quality during connection establishment. In addition, there is support for authentication through Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
HDLC is a data link-layer protocol derived from the Synchronous Data Link Control (SDLC) encapsulation protocol. HDLC is Cisco's default encapsulation for serial lines. This implementation is very streamlined; there is no windowing or flow control, and only point-to-point connections are allowed. The address field is always set to all ones. Furthermore, a 2-byte proprietary type code is inserted after the control field, which means that HDLC framing is not interoperable with other vendors' equipment.
two types of WAN link options are available: dedicated lines and switched connections.
Dedicated lines, also called leased lines, provide full-time service. Dedicated lines typically are used to carry data, voice, and occasionally video. In data network design, dedicated lines generally provide core or backbone connectivity between major sites or campuses, as well as LAN-to-LAN connectivity. Dedicated lines generally are considered reasonable design options for WANs.
Different encapsulation methods at the data link layer provide flexibility and reliability for user traffic. Dedicated lines of this type are ideal for high-volume environments with a steady-rate traffic pattern. Use of available bandwidth is a concern because you have to pay for the line to be available even when the connection is idle
Dedicated lines also are referred to as point-to-point links because their established path is permanent and fixed for each remote network reached through the carrier facilities.
A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network
The service provider reserves point-to-point links for the private use of the customer
Packet switching is a WAN switching method in which network devices share a permanent virtual circuit (PVC), which is like a point-to-point link to transport packets from a source to a destination across a carrier network
Frame Relay, SMDS, and X.25 are all examples of packet-switched WAN technologies
Switched networks can carry variable-size frames (packets) or fixed-size cells. The most common packet-switched network type is Frame Relay.
Frame Relay was designed to be used over high-speed, high quality digital facilities.
As a result, Frame Relay does not offer much error checking or reliability, but expects upper-layer protocols to attend to these issues
Frame Relay is called a non-broadcast multi-access technology because it has no broadcast channel. Broadcasts are transmitted through Frame Relay by sending packets to all network destinations
Fully meshed topology -- Every Frame Relay network device has a PVC to every other device on the multipoint WAN. Any update sent by one device is seen by every other. If this design is used, the entire Frame Relay WAN can be treated as one data link.
Partially meshed topology --This is also often called a star topology or hub-and-spokes topology. In a partially meshed topology, not every device on the Frame Relay cloud has a PVC to every other device.
Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session
ISDN is an example of a circuit-switched WAN technology.
Dial-on-demand routing (DDR) is a technique in which a router can dynamically initiate and close circuit-switched sessions when transmitting end stations need them.
When the router receives traffic destined for a remote network, a circuit is established, and the traffic is transmitted normally. The router maintains an idle timer that is reset only when interesting traffic is received. (Interesting traffic refers to traffic the router needs to route.
If the router receives no interesting traffic before the idle timer expires, however, the circuit is terminated. Likewise, if uninteresting traffic is received and no circuit exists, the router drops the traffic. When the router receives interesting traffic, it initiates a new circuit.
DDR enables you to make a standard telephone connection or an ISDN connection only when required by the volume of network traffic
DDR can be used to provide backup load sharing and interface backup
Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN devices include the following:
Terminal Equipment 1 (TE1) -- Designates a device that is compatible with the ISDN network. A TE1 connects to an NT of either Type 1 or Type 2.
Terminal Equipment 2 (TE2) -- Designates a device that is not compatible with ISDN and requires a TA.
TA-Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network.
NT Type 1 (NT1) -- Connects four-wire ISDN subscriber wiring to the conventional two-wire local loop facility.
NT Type 2 (NT2) -- Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating.
There are two ISDN services: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). ISDN BRI operates mostly over the copper twisted-pair telephone wiring in place today. ISDN BRI delivers a total bandwidth of a 144 kbps line into three separate channels. Two of the channels, called B (bearer) channels, operate at 64 kbps and are used to carry voice or data traffic. The third channel, the D (delta) channel, is a 16-kbps signaling channel used to carry instructions that tell the telephone network how to handle each of the B channels. ISDN BRI often is referred to as 2B+D.
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Chapter 3 WAN Design
WAN communication occurs between geographically separated areas. When a local end station wants to communicate with a remote end station
circuit-switched networks offer users dedicated bandwidth that cannot be infringed upon by other users. In contrast, packet switching is a method in which network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network. Packet-switched networks have traditionally offered more flexibility and used network bandwidth more efficiently than circuit-switched networks.
Two primary goals drive WAN design and implementation:
Application availability - Networks carry application information between computers. If the applications are not available to network users, the network is not doing its job.
Total cost of ownership - Information Systems (IS) department budgets often run in the millions of dollars. As large businesses increasingly rely on electronic data for managing business activities, the associated costs of computing resources will continue to rise. A well-designed WAN can help to balance these objectives. When properly implemented, the WAN infrastructure can optimize application availability and allow the cost-effective use of existing network resources.
When designing a WAN, you need to start by gathering data about the business structure and processes. Next, you need to determine who the most important people will be in helping you design the network. You need to speak to major users and find out their geographic location, their current applications, and their projected needs
Hierarchical models for network design allow you to design networks in layers.
Benefits to using a hierarchical model include the following:
Scalability -- Networks that follow the hierarchical model can grow much larger without sacrificing control or manageability because functionality is localized and potential problems can be recognized more easily. An example of a very large-scale hierarchical network design is the Public Switched Telephone Network.
Ease of implementation -- A hierarchical design assigns clear functionality to each layer, thereby making network implementation easier.
Ease of troubleshooting -- Because the functions of the individual layers are well defined, the isolation of problems in the network is less complicated. Temporarily segmenting the network to reduce the scope of a problem also is easier.
Predictability -- The behavior of a network using functional layers is fairly predictable, which makes capacity planning for growth considerably easier; this design approach also facilitates modeling of network performance for analytical purposes.
Protocol support -- The mixing of current and future applications and protocols is much easier on networks that follow the principles of hierarchical design because the underlying infrastructure is already logically organized.
Manageability -- All the benefits listed here contribute to greater manageability of the network.
A hierarchical network design includes the following three layers:
The core layer provides optimal transport between sites
The distribution layer, which provides policy-based connectivity
The access layer, which provides workgroup and user access to the network
Core layer -- The core layer provides fast wide-area connections between geographically remote sites, tying a number of campus networks together in a corporate or enterprise WAN. Core links are usually point-to-point, and there are rarely any hosts in the core layer. Core services (for example, T1/T3, Frame Relay, SMDS) typically are leased from a telecom service provider.
Distribution layer -- The distribution layer gives network services to multiple LANs within a WAN environment. This layer is where the WAN backbone network is found, and it is typically based on Fast Ethernet. This layer is implemented on large sites and is used to interconnect buildings.
Access layer -- The access layer is usually a LAN or a group of LANs, typically Ethernet or Token Ring, that provide users with frontline access to network services. The access layer is where almost all hosts are attached to the network, including servers of all kinds and user workstations.
In the campus environment, access-layer functions can include the following:
Shared bandwidth
Switched bandwidth
MAC-layer filtering
Microsegmentation
================================================================
A WAN is a data communications network that operates beyond a LAN's geographic scope. One way that a WAN is different from a LAN is that you must subscribe to an outside WAN service provider, such as a regional Bell operating company (RBOC) to use WAN carrier network services.
WAN technologies function at the three lowest layers of the OSI reference model: the physical layer, the data link layer, and the network layer
Telephone and data services are the most commonly used WAN services. Telephone and data services are connected from the building point of presence (POP) to the WAN provider's central office (CO)
WAN provider services into three main types:
Call setup- sets up and clears calls between telephone users. Also called signaling, call setup uses a separate telephone channel not used for other traffic. The most commonly used call setup is Signaling System 7 (SS7), which uses telephone control messages and signals between the transfer points along the way to the called destination.
Time-division multiplexing (TDM)-Information from many sources has bandwidth allocation on a single medium. Circuit switching uses signaling to determine the call route, which is a dedicated path between the sender and the receiver. By multiplexing traffic into fixed time slots, TDM avoids congested facilities and variable delays. Basic telephone service and ISDN use TDM circuits.
Frame Relay-Information contained in frames shares bandwidth with other WAN Frame Relay subscribers. Frame Relay is statistical multiplexed service, unlike TDM, which uses Layer 2 identifiers and permanent virtual circuits. In addition, Frame Relay packet switching uses Layer 3 routing with sender and receiver addressing contained in the packet.
main parts of WAN services:
Customer premises equipment (CPE) -- Devices physically located on the subscriber's premises. Includes both devices owned by the subscriber and devices leased to the subscriber by the service provider.
Demarcation (or demarc) -- The point at which the CPE ends and the local loop portion of the service begins. Often occurs at the POP of a building.
Local loop (or "last-mile") -- Cabling (usually copper wiring) that extends from the demarc into the WAN service provider's central office.
CO switch -- A switching facility that provides the nearest point of presence for the provider's WAN service.
Toll network -- The collective switches and facilities (called trunks) inside the WAN provider's cloud. The caller's traffic may cross a trunk to a primary center, then to a sectional center, and then to a regional- or international-carrier center as the call travels the long distance to its destination.
A key interface in the customer site occurs between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE)
Typically, the DTE is the router, and the DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility
DCE is the attached modem, channel service unit/data service unit (CSU/DSU), or terminal adapter/network termination 1 (TA/NT1).
The WAN path between the DTEs is called the link, circuit, channel, or line.
A virtual circuit is a logical circuit, as opposed to a point-to-point circuit, created to ensure reliable communication between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs
SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete
Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit-termination phase involves tearing down the virtual circuit between the source and destination devices.
A PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations where data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but increase costs due to constant virtual-circuit availability.
WANs use numerous types of devices, including the following:
Routers, which offer many services, including LAN and WAN interface ports.
WAN switches, which connect to WAN bandwidth for voice, data, and video communication.
Modems, which interface voice-grade services. Modems include CSUs/ DSUs and TA/NT1 devices that interface ISDN services.
Communication servers, which concentrate dial-in and dial-out user communication.
Routers are devices that implement the network service
Routers manage networks by providing dynamic control over resources and supporting the tasks and goals for networks.
A WAN switch is a multiport networking device, which typically switches such traffic as Frame Relay, X.25, and Switched Multimegabit Data Service (SMDS).
WAN switches typically operate at the data link layer of the OSI reference model
A modem is a device that interprets digital and analog signals by modulating and demodulating the signal, enabling data to be transmitted over voice-grade telephone lines.
A CSU/DSU is a digital-interface device-or sometimes two separate digital devices-that adapts the physical interface on a DTE device (such as a terminal) to the interface of a DCE device (such as a switch) in a switched-carrier network.
An ISDN Terminal Adapter (TA) is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces
WAN physical-layer protocols describe how to provide electrical, mechanical, operational, and functional connections for WAN services
the common data-link encapsulations associated with WAN lines, which are:
Frame Relay -- By using simplified encapsulation with no error correction mechanisms over high-quality digital facilities, Frame Relay can transmit data very rapidly compared to the other WAN protocols.
Point-to-Point Protocol (PPP) -- Described by RFC 1661, PPP was developed by the IETF. PPP contains a protocol field to identify the network-layer protocol.
ISDN -- A set of digital services that transmits voice and data over existing phone lines.
Link Access Procedure, Balanced (LAPB) -- For packet-switched networks, LAPB is used to encapsulate packets at Layer 2 of the X.25 stack. It can also be used over a point-to-point link if the link is unreliable or there is an inherent delay associated with the link, such as in a satellite link. LAPB provides reliability and flow control on a point-to-point basis.
Cisco/IETF -- Used to encapsulate Frame Relay traffic. The Cisco option is proprietary and can be used only between Cisco routers.
High-Level Data Link Control (HDLC) -- An ISO standard, HDLC might not be compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations
PPP is a standard serial-line encapsulation method (described in RFC 1332 and RFC 1661). This protocol can, among other things, check for link quality during connection establishment. In addition, there is support for authentication through Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
HDLC is a data link-layer protocol derived from the Synchronous Data Link Control (SDLC) encapsulation protocol. HDLC is Cisco's default encapsulation for serial lines. This implementation is very streamlined; there is no windowing or flow control, and only point-to-point connections are allowed. The address field is always set to all ones. Furthermore, a 2-byte proprietary type code is inserted after the control field, which means that HDLC framing is not interoperable with other vendors' equipment.
two types of WAN link options are available: dedicated lines and switched connections.
Dedicated lines, also called leased lines, provide full-time service. Dedicated lines typically are used to carry data, voice, and occasionally video. In data network design, dedicated lines generally provide core or backbone connectivity between major sites or campuses, as well as LAN-to-LAN connectivity. Dedicated lines generally are considered reasonable design options for WANs.
Different encapsulation methods at the data link layer provide flexibility and reliability for user traffic. Dedicated lines of this type are ideal for high-volume environments with a steady-rate traffic pattern. Use of available bandwidth is a concern because you have to pay for the line to be available even when the connection is idle
Dedicated lines also are referred to as point-to-point links because their established path is permanent and fixed for each remote network reached through the carrier facilities.
A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network
The service provider reserves point-to-point links for the private use of the customer
Packet switching is a WAN switching method in which network devices share a permanent virtual circuit (PVC), which is like a point-to-point link to transport packets from a source to a destination across a carrier network
Frame Relay, SMDS, and X.25 are all examples of packet-switched WAN technologies
Switched networks can carry variable-size frames (packets) or fixed-size cells. The most common packet-switched network type is Frame Relay.
Frame Relay was designed to be used over high-speed, high quality digital facilities.
As a result, Frame Relay does not offer much error checking or reliability, but expects upper-layer protocols to attend to these issues
Frame Relay is called a non-broadcast multi-access technology because it has no broadcast channel. Broadcasts are transmitted through Frame Relay by sending packets to all network destinations
Fully meshed topology -- Every Frame Relay network device has a PVC to every other device on the multipoint WAN. Any update sent by one device is seen by every other. If this design is used, the entire Frame Relay WAN can be treated as one data link.
Partially meshed topology --This is also often called a star topology or hub-and-spokes topology. In a partially meshed topology, not every device on the Frame Relay cloud has a PVC to every other device.
Circuit switching is a WAN switching method in which a dedicated physical circuit is established, maintained, and terminated through a carrier network for each communication session
ISDN is an example of a circuit-switched WAN technology.
Dial-on-demand routing (DDR) is a technique in which a router can dynamically initiate and close circuit-switched sessions when transmitting end stations need them.
When the router receives traffic destined for a remote network, a circuit is established, and the traffic is transmitted normally. The router maintains an idle timer that is reset only when interesting traffic is received. (Interesting traffic refers to traffic the router needs to route.
If the router receives no interesting traffic before the idle timer expires, however, the circuit is terminated. Likewise, if uninteresting traffic is received and no circuit exists, the router drops the traffic. When the router receives interesting traffic, it initiates a new circuit.
DDR enables you to make a standard telephone connection or an ISDN connection only when required by the volume of network traffic
DDR can be used to provide backup load sharing and interface backup
Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN devices include the following:
Terminal Equipment 1 (TE1) -- Designates a device that is compatible with the ISDN network. A TE1 connects to an NT of either Type 1 or Type 2.
Terminal Equipment 2 (TE2) -- Designates a device that is not compatible with ISDN and requires a TA.
TA-Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network.
NT Type 1 (NT1) -- Connects four-wire ISDN subscriber wiring to the conventional two-wire local loop facility.
NT Type 2 (NT2) -- Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating.
There are two ISDN services: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). ISDN BRI operates mostly over the copper twisted-pair telephone wiring in place today. ISDN BRI delivers a total bandwidth of a 144 kbps line into three separate channels. Two of the channels, called B (bearer) channels, operate at 64 kbps and are used to carry voice or data traffic. The third channel, the D (delta) channel, is a 16-kbps signaling channel used to carry instructions that tell the telephone network how to handle each of the B channels. ISDN BRI often is referred to as 2B+D.
================================================================
Chapter 3 WAN Design
WAN communication occurs between geographically separated areas. When a local end station wants to communicate with a remote end station
circuit-switched networks offer users dedicated bandwidth that cannot be infringed upon by other users. In contrast, packet switching is a method in which network devices share a single point-to-point link to transport packets from a source to a destination across a carrier network. Packet-switched networks have traditionally offered more flexibility and used network bandwidth more efficiently than circuit-switched networks.
Two primary goals drive WAN design and implementation:
Application availability - Networks carry application information between computers. If the applications are not available to network users, the network is not doing its job.
Total cost of ownership - Information Systems (IS) department budgets often run in the millions of dollars. As large businesses increasingly rely on electronic data for managing business activities, the associated costs of computing resources will continue to rise. A well-designed WAN can help to balance these objectives. When properly implemented, the WAN infrastructure can optimize application availability and allow the cost-effective use of existing network resources.
When designing a WAN, you need to start by gathering data about the business structure and processes. Next, you need to determine who the most important people will be in helping you design the network. You need to speak to major users and find out their geographic location, their current applications, and their projected needs
Hierarchical models for network design allow you to design networks in layers.
Benefits to using a hierarchical model include the following:
Scalability -- Networks that follow the hierarchical model can grow much larger without sacrificing control or manageability because functionality is localized and potential problems can be recognized more easily. An example of a very large-scale hierarchical network design is the Public Switched Telephone Network.
Ease of implementation -- A hierarchical design assigns clear functionality to each layer, thereby making network implementation easier.
Ease of troubleshooting -- Because the functions of the individual layers are well defined, the isolation of problems in the network is less complicated. Temporarily segmenting the network to reduce the scope of a problem also is easier.
Predictability -- The behavior of a network using functional layers is fairly predictable, which makes capacity planning for growth considerably easier; this design approach also facilitates modeling of network performance for analytical purposes.
Protocol support -- The mixing of current and future applications and protocols is much easier on networks that follow the principles of hierarchical design because the underlying infrastructure is already logically organized.
Manageability -- All the benefits listed here contribute to greater manageability of the network.
A hierarchical network design includes the following three layers:
The core layer provides optimal transport between sites
The distribution layer, which provides policy-based connectivity
The access layer, which provides workgroup and user access to the network
Core layer -- The core layer provides fast wide-area connections between geographically remote sites, tying a number of campus networks together in a corporate or enterprise WAN. Core links are usually point-to-point, and there are rarely any hosts in the core layer. Core services (for example, T1/T3, Frame Relay, SMDS) typically are leased from a telecom service provider.
Distribution layer -- The distribution layer gives network services to multiple LANs within a WAN environment. This layer is where the WAN backbone network is found, and it is typically based on Fast Ethernet. This layer is implemented on large sites and is used to interconnect buildings.
Access layer -- The access layer is usually a LAN or a group of LANs, typically Ethernet or Token Ring, that provide users with frontline access to network services. The access layer is where almost all hosts are attached to the network, including servers of all kinds and user workstations.
In the campus environment, access-layer functions can include the following:
Shared bandwidth
Switched bandwidth
MAC-layer filtering
Microsegmentation
================================================================
posted by tempus at
9:13 PM
