Wide Area Networks
Asynchronous Transfer Mode (ATM)
*ATM* was designed to be a high-speed communications protocol that doesn't depend on any specific LAN topology. It uses a high-speed-cell-switching technology that can handle data as well as real-time voice and video. The ATM protocol breaks up transmitted data into 53-bytes cells. A cell is analogous to a packet or frame, except that an ATM cell is always fixed in length and is relatively small and fast, whereas a frame's length can vary. Data rates are scalable and start as low at 1.5Mbps, with speeds of 25Mbps, 51Mbps, 100Mbps, 155Mbps, and higher; The common speeds of ATM networks today are 51.84Mbps and 155Mbps; can be used over either copper or fiber-optic cabling. ATM fiber based services can run 10Gbps and are becoming more common. Due to ATMs cells headers it's less efficient than other WAN technologies. ATM networks are fast, but they get bad gas mileage.
Wireless Technologies
*Bluetooth* is actually a wireless protocol that creates *personal are networks (PANs)*. It utilizes short-range communication technology enabling data transmission between fixed and/or mobile devices. Bluetooth uses a radio technology called Frequency Hopping Spread Spectrum that shops up the data being sent and transmits chunks of it through the air on up to 75 different frequencies in the 2.4GHz range. *Microwave radio replay* is a technology for transmitting digital and sometimes even analog signals between 2 locations on a line-of-sight radio path through the atmosphere. During microwave radio relay, radio waves are transmitted between 2 locations with directional antennas. *Communication satellite (comcast)* is an artificial satellite stationed in space for telecommunications purposes. Modern communications satellites use a variety of orbits such as: * Geostationary orbits * Molniya orbits (molniya means lightening in Russian), names after a series of communications satellites from Russia * Low-polar and non-polar Earth orbits from which the satellite can 1st boost communications signals and then send them back 2 earth
Wireless WAN Technologies
*Cellular WAN* - 1st generation cellular (1G) was a voice-only analog network. the 2nd generation (2G) marked the switch to digital, which allowed voice and simple data such as a text message. 4th generation (4G) is still the most widely used globally at this time (2019). 4G allows high-speed voice and data, even Internet. 4G supports most smartphones. GSM, or Global System for Mobile Communications, is a standard developed by the European Telecommunications Standards Institute (ETSI). It delineates protocols for 2G digital cellular networks, which are used by mobile phones, and it's the default global standard for mobile communications and enjoys over 90% market share. 2G is available in over 219 countries and territories worldwide. 4G has 2 variants. LTE and WiMAX. *HSPA+* - *Evolved High Sped Packet Access (HSPA+)* is really considered a 3.5 generation technology. It does include an optional all-IP architecture, which is 1 of 4G's requirements, and it actually has greatly improved data rates over HSPA. Because HSPA+ uses an all IP architecture, it has the potential to evolve into a true 4G someday. It has downlink speeds o 3Mbps to 4Mbps and uplink speeds of 1Mbps to 2Mbps. *WiMAX* - *World Wide Interoperability for Microwave Access (WiMAX)* is considered a true 4G technology, and it's based on the IEEE 802.16 standard. It supports both fixed, tower-to-tower applications and mobile applications. It was originally designed as a last-mile technology to deliver Internet to areas where implementing landlines wasn't possible as an alternative to DSL and cable. WiMAX is not compatible with 2G and 3G technologies, it's pricey, and requires a lot of power. Worse, it also lags behind LTE in speed with downlink speeds of 5Mbps to 6Mbps and uplink speeds of 2Mbps to 3Mbps. *LTE* - *Long Term Evolution (LTE)* is definitely the most promising of all emerging 4G technologies. It uses an all-IP-based core, it offers the highest data rates, and it's compatible with 3G and WiMAX. Has best indoor coverage while maintaining high data rates all the way to the edge of a coverage cell. It also accommodates more devices in a given area and still performs really well by maintaining data rates of 7Mbps to 12Mbps on the downlink and 3Mbps to 5Mbps on the uplink.
Defining WAN Terms
*Customer Premises Equipment (CPE)* - is equipment that's owned by the service provider but located on the subscriber's (your) property. *CSU/DSU* - *Channel service unit/data service unit (CSU/DSU)* is a Layer 1 device that connects the serial ports on your router to the provider's network and connects directly to the demarcation point (demarc) or location. These devices can be external, or they can be internal cards on the router. The CSU/DSU provides clocking of the line to the CPE (in this case your router), and provides other important options, like voltage regulation. *Smart Jack/NIU* - the smart jack or network interface unit is installed between the demarcation location and the CSU/DSU and the customer premises equipment (CPE) location. It can provide signal conversion by converting codes and protocols (i.e. framing types) into something the customer equipment requires. the NIU can buffer and/or regenerate the signal to compensate for signal degradation from line transmission. Smart jacks also provide diagnostic capabilities to the IPS without having a technician to go on site. They can preform loopback test where the signal from the provider is transmitted back to the provider's location. This test brings the line down. *Demarcation Point* - The precise spot where the service provider's or local exchange carrier's responsibility ends and the CPE begins. It's generally a device in a telecommunications closet owned and installed by the telecommunications company (telco). It's your responsibility to cable (*extended demark*) from this box to the CPE, which is usually a connection to a CSU/DSU or ISDN interface. *Copper Line Drivers/Repeaters* - Allows for a demarc extension length of up to 5,000 feet from the telephone company's demarc. These can be used to connect equipment across a campus, between floors of a high-rise office building, even between office buildings with underground cable connections. *Local Loop* - A cable consisting of a pair of copper wires called the *local loop* connects to the demarc to the closest switching office, known as the *central office (CO)*. *Central Office* - A phone company building that connects the customer's network to the provider's switching network to the providers switching network. It's also good to know that a CO is sometimes referred to as a *point of presence (POP). *Toll Network* - Is a trunk line inside a WAN provider's network. This network is a collection of switches and facilities owed by the ISP.
WAN Connection Types
*Dedicated (Leased) Lines* - Usually referred to as *point-to-point* or dedicated connections. A dedicated *leased line* is a pre-established WAN communications path that goes from the CPE through the DCE switch and then over to the CPE of the remote site. The CPE enables DTE networks to communicate at any time with no cumbersome setup procedures. It's expensive because it uses synchronous serial lines up to 45 Mbps. High-Level Data Link (HDLC) and Point-to-Point (PPP) encapsulations are frequently used on leased lines. *Circuit Switching* - Cost advantage. Only pay for the time you actually use. No data can transfer before an end-to-end connection is established. Circuit switching uses dial-up modems or ISDN and is used for low-bandwidth data transfers. *Packet Switching* - Allows you to share bandwidth with other companies to save money. Looks like a leased line but charges like circuit switching. Works best with bursty traffic. Frame Relay and the super-old X.25 are packet-switching technologies with speeds that can range from 56Kbps up to T3 (45Mbps).
WAN Troubleshooting
*Loss of Internet Connectivity* - The most common trouble ticket you'll receive is someone complaining about not being able to connect. This issue can be caused by the WAN link dropping the CSU/DSU or internal wiring, but most of the time, it's due to an ISP issue. A good approach is to double check all your power o all devices, including the smart jack, and run a loop test from the router to the CSU/DSU to verify that specific links come up locally. If you have either a copper line driver or repeaters in your WAN link. You've got to be sure to check those connections too. *Interface Errors/Monitoring* - There are a couple of key interfaces you need to check when verifying your WAN. If you have cable of DSL, you'll need to check into interface errors on the LAN port connecting to your router 1st. *Link Status* The 1st thing to check when there is a trouble ticket or our network management tools alert us of a link error is the link status. This is the 1st line in "show int fa0/x" output. This would be the same on serial links as is it on Ethernet links. "FastEthernet0/0 is up, line protocol is up" The 1st up listed is carrier detected. If this shows down, then you have a physical layer problem locally and you need to get to that port and check the cable and port. the 2nd listed, which is protocol is up, is keepalives from the remote end. If you see up/down, then you know your local end is good but you're not getting a digital signal from the remote end. *Speed and Duplex Settings* - The most common cause of interface errors is a mismatched duplex mode between 2 ends of an Ethernet link. It's vital to verify that the switch and its hosts-PCs, router interfaces, cable modems, and so on-all have the same speed settings. If you have a duplex mismatch, a telling sign is that the late collision counter will increment. *Input Queue Drops* - If the input queue drop counter increments, this tells you that more traffic is being delivered to the router than it can process. If this value is consistently high, try to determine exactly when these counters are increasing and how the events relate to CPU usage. Know that you'll see the ignored and throttle counters increment as well. *Output Queue Drops* - This counter indicates that packets were dropped due to interface congestion, leading to lost data and queuing delays. When this occurs, applications like VoIP will experience performance issues. If you observe this constantly increment, consider QoS as the culprit. *Input Errors* - Input errors often indicate high-level errors such as CRCs. This can point to cabling problems, hardware issues, or duplex mismatches. *Output Errors* - This issue equals the total number of frames that the port tried to transmit when an issue such as a collision occurred. These errors can also be cause by interference of the line itself, which means you'll have to call the ISP to sort them out. On serial interface, start by checking out the physical connection your router. Are you receiving clocking? This comes from the CSU/DSU. "*show controllers s0/0*" >> i.e. *DTE V.35 TX and RX clocks detected*. Next check what the interface tells you "*show int s0/0*" look for line status and errors.
Passive Optical Network
*Passive Optical Network (PON)*, also called *fiber to the premises*, is a new option for connecting homes and business to the internet. It is point-to-multipoint technology with a single fiber strand used for multiple premises (typically 16-28). Unpowered optical splitters are used in the process and are the reason for using the term *passive*. The system consists of an optical line termination (OTL) at the telco's office and a number of optical networks units (ONUs) near end users. These systems typically have downlink speeds of 155Mbps to 655Mbps and uplink bursts to 155Mbps.
The Public Switches Telephone Network
*Publicly* means that, for a fee, anyone can lease the use of the network without being required to run any cabling. *Switched* explains how the phone works.
Wavelength Division Multiplexing
*Wavelength Division Multiplexing (WDM)* is a technology that multiplexes several optical carriers on a single optical fiber by using different wavelengths of the light spectrum is somewhat like using different frequencies in a radio wave. *Dense Wavelength Division Multiplexing (DWDM)* multiplexes within a specific band (1550nm), allowing for the use of erbium-doped fiber amplifiers (EDFAs) that boost the signal. This allows for upgrading the bit rate of a single strand line by simply replacing equipment at either end of the line. The system consists of the following: * DWDM terminal multiplexer * Intermediate line repeater (every 80-100km) * DWDM terminal de-multiplexer *Coarse Wavelength Division Multiplexing (CWDM)* user larger chunks of the light spectrum, and is defined by wavelengths, whereas DWDM is defined by frequencies and fits 40-plus channels into the same frequency range used by just 2 CWDM channels. Why use CWDM at all then? Because CWDM can match the basic capabilities of DWDM at a lower capacity at a significant discount. It allows ISPs to help customers in a MAN physical location where fiber is still to pricey to implement.
Transmission Media
*Wired connections* - Use either copper wire of glass fiber to carry bits as voltages pulses, respectively. Copper can suffer from attenuation over long distance and is limited by distance. Fiber offers a lot more bandwidth and it's a lot less susceptible to noise, but costs a lot more to buy and install. In the USA, the standard for synchronous data transmission on optical fiber is called *Synchronous Optical Network (SONET).* The international equivalent of SONET is called *Synchronous Digital Hierarchy (SDH)*. SONET defines a base data rate, or throughput, of 51.84Mbps, and multiples of this rate are known as optical carrier (OC) levels, like OC-3, OC-12, and on.
Company Security Policy
Another common WAN related problem is the company security policy. i.e. if there's a firewall blocking ports, say, on applications that need to get to the Internet, it mimics a WAN issue when the root of the problem is actually bad configuration on your firewall.
Broadband Services
Cable modems vs DSL as solutions for connecting to WANs: Dedicated broadband services include transmissions over media in a broad range of frequencies. The various forms of *Digital Subscriber Line (DSL)* services are broadband in the sense that digital information is sent over a high-bandwidth channel above the baseband voice channel on a single pair of wires. Ethernet digital signals sent over a *cable modem* from your local cable television service provider compete with DSL service. *Speed* Most people would tell you that cable is faster than DSL Internet, but they wouldn't be right because cable doesn't always win the race in the real world. *Security* DSL and cable are based upon different network-security models, and until recently, cable has been the reputed loser, but now it;s a toss-up, with both offering adequate security to meet the needs of most users. *Popularity* - Cable Internet is definitely "best in show" in the USA, but DSL is beginning to catch up. *Customer Satisfaction* DSL is preferred but no one is really happy with their ISPs.
T-Series Connections speeds
Connection: T1 Maximum Speed: 1.544Mbps Connection: T1C Maximum Speed: 3.152Mbps Connection: T2 Maximum Speed: 6.312Mbps Connection: T3 Maximum Speed: 44.736Mbps Connection: T4 Maximum Speed: 274.176Mbps
Metro Ethernet
Connects offices together via Ethernet the MPLS on IPS network then Ethernet at the other office locations. This is a smart/popular and thrifty solution.
DSL Technology and xDSL
DSL is not a complete end-to-end solution. It is really a Physical Layer transmission technology like dial-up, cable, or wireless. DSL connections are deployed in the *last mile* of a local telephone network or local loop. Last mile and local loop are pretty interchangeable but it basically means means the same thing and defines the physical connection from the customer to the 1st aggregation device of the provider network. A DSL connection is set up between a pair of modems on either end of a copper wire that is between the CPE and the digital subscriber line access multiplexer (DSLAM). A DSLAM is the device located at the provider's CO that concentrates connections from multiple DSL subscribers. xDSL is really a family of technologies that have become popular for data transmission over phone lines because xDSL uses regular PSTN pones wires to transmit digital signals and is extremely inexpensive compared with other digital communications methods. The x in xDSL represents the various letters that refer to different DSL flavors. xDSLs use high-frequency signals, whereas regular phone calls use low-frequency signals over the same lines. Communicating via xDSL required an interface to a PC. All xDSL configurations required a DSL modem called an *endpoint* and a NIC in your computer. The NIC can be connected directly to the DSL modem using a straight-through Ethernet UTP patch cord with standard RJ-45 connectors on each end. If there are other connecting devices between computer and cable modem, you'll need either a special switchable port of and Ethernet crossover cable for things to work well. These cost-effective implementations include the following: *High Bit-Rate Digital Subscriber Line (HDSL)* - HDSL was the 1st DSL technology to use a higher-frequency spectrum of copper twister-pair cables. HDSL was developed in the USA as a better technology for high-speed, synchronous circuits. It was typically used to interconnect local-exchange carrier systems and to carry high-speed corporate data links and voice channels using T1 lines. *Symmetric Digital Subscriber Line (SDSL)* -Symmetric (meaning same rate in both directions) digital subscriber line (SDSL) provides T1/E1 type speeds symmetrically for both uploading and downloading data, but doesn't allow low-frequency phone calls on the same line as asymmetric digital subscriber line (ADSL) does. How much it will set you back ranges between the cost of ADSL and T1s. This option is typically used by small to medium-sized businesses that don't require the higher performance of a leased line for connecting to a server. *Very High Bit-Rate Digital Subscriber Line (VDSL) - Or very high bit-rate DSL (VHDSL), provides faster data transmission over single, flat, untwisted or twister pairs of copper wires. This capacity for blazingly fast speeds mean that VDSL is capable of supporting high-bandwidth applications like HDTV and telephone services like VoIP as well as general Internet access over a single connection. VDSL is deployed over existing wiring used for POTS and lower-sped DSL connections. 2nd generation VDSL2 systems utilize bandwidths of up to 30MHz to provide data rates exceeding 100Mbps and steam and down stream (same time). The maximum available bit rate is achieved at a range of about 300 meters with the single performance degrading as the loop attenuation increases. *Asymmetric Digital Subscriber Line (ADSL)* - Asymmetric (meaning different upload and download speed) DSL has become the most popular xDSL because it focuses on providing reasonably fast upstream transmission speeds (768Kbps) and very fast downstram transmission speeds of up to 9Mbps. (ADSL2+ can get up to 20Mbps). This makes downloading graphics, audio, video, and data files from any remote computer in a snap. The best part is that ADSL works on a single phone line without losing voice calls capability. This is accomplished with something called a *splitter* that enables the use of multiple frequencies on your POTS line.
Cable Modem
Few cable network terms: *Headend* - This is where all cable signals are received, processed, and formatted. The signals are then transmitted over the distribution network from the headend. *Distribution Network* - These are relatively small service areas that usually rage in size from 100 to 2,000 customers. They're typically composed of mixed, fiber-coaxial, or hybrid fiber-coaxial (HFC) architecture, with optical fiber substituting for the distribution network's trunk portion. The fiber forms both the connections from the headend and an optical node that changes light to radio frequency (RF) signals that are then distributed through a coaxial cable throughout the specific service area-that is, your home or office.. *Data over Cable Service Interface Specifications (DOCSIS)* - This specification provides the interface requirements for a data-over-cable system, including that of high-speed data transfer to an existing cable TV system. All cable modems and similar devices have to measure up to this standard. Maximum download speeds of 20-50Mbps.
WAN Protocols
Focusing on: * ISDN * Frame Relay * PPP * ATM * MPLS *Integrated Services Digital Network* - Is a point-to-point WAN technology capable of maximum transmission speeds of about 2Mbp (*Primary Rate Interface [PRI]*), although speeds of 128Kbps (*Basic Rate Interface [BRI]*) are more the reality within a SOHO environment. ISDN uses the same UTP wiring as POTS, yet it can transmit data at much higher speeds. The main difference of ISDN vs regular POTS line is how it utilizes th copper wiring. Instead of carrying an analog voice signal, it carries digital signals. 1st, a computer connects to the 128Kbps ISDN line via an ISDN *terminal adapter (TA)* that's often incorrectly referred to as an ISDN modem. At ISDN TA is not a modem because it doesn't convert a digital signal from the computer to an analog signal on the subscriber line-ISDN signals are digital on the subscriber line. A TA is technically an ISDN compatible device that has 1 or more non-ISDN ports for devices like computer serial interfaces and RJ-11 analog phones, which work to give these non-ISDN devices access to the ISDN network. 2nd, an ISDN line has 2 types of channels. The data is carries on a special *Bearer channel*, or *B channels*, each of which can carry 64Kbps of data. A BRI ISDN line has 2 B channels, and a PRI has 23 64Kbps channels. 1 channel can be used from a voice call while the other can be used for data transmissions, and it's all made possible by time-division multiplexing (TDM) on 1 pair of copper wires. The other type of ISDN is also multiplexed onto only 1 copper pair. It's used for call setup and link management and is known as the *signaling channel*, *D channel* or *Delta channel* This channel has only 16Kbps of bandwidth in BRI and 64Kbps in PRI. To maximize throughput, the 2 B channels are often combined into 1 data connection for a total bandwidth of 128Kbps. This is known as *Bandwidth on Demand Interoperability Group (BONDING)* or *inverse multiplexing*. This still leaves the D channel free for signaling purposes. In rare cases, you may see user data, such as credit-card verification, on the D channel. This was introduced as an additional feature of ISDN, but hasn't caught on. Some of the main advantages of ISDN: * A fast connection * It offers higher bandwidth than POTS. BONDING yields 128Kbps bandwidth. * There is no conversion from digital to analog. ISDN has the following disadvantages: * It's more expensive than POTS * Specialized equipment is required both at the phone company and at the remote computer. * ISDN equipment isn't compatible to connect to every other type of equipment out there. * Why use ISDN if you can get DSL or cable? * It's just a plain outdated technology.
The T1 Connection
Is a 1.544Mbps digital connection that's typically carried over 2 pairs of copper wires. This 1.544Mbps connection uses DS1 and aggregates 24, 64Kbps channels that use DS0, which refers to the time slots within a channel. Each channel can carry either voice or data. In POTS world, T1 lines are used to convert and bundle analog phone conversations over great distances due to the better quality of a digital signal and the use of a great deal less wiring than would be needed if each pair carried only 1 call. This splitting into independent channels also allows a company to combine voice and data over 1 T1 connection or to use the T1 as if it were an unchannelized 1.544Mbps pipe. You can also order a fraction T1 (FT1) circuit that's delivered on a T1 but doesn't allow the use of all 24 channels. The European version of the T1 is the E1, which operates at 2.048Mbps and uses 32 64Kbps channels (32 DS0s). It was designed based on the T1, and is a little bigger. You'll also find the J1, which is the Japanese version of the T1 and operates at 1.544Mbps, just like a T1.
Point-to-Point Protocol
Is a Data Link layer protocol that can be used over either asynchronous serial (dial-up) or synchronous serial (ISDN) media. It relies on Link Control Protocol (LCP) to build and maintain data-link connections. Network Control Protocol (NCP) enables multiple Network Layer protocols (routed protocols) to be used on a point-to-point connections. Because HDLC is the default serial encapsulation on Cisco serial links and it works great, why would you choose to use PPP? PPP is non-proprietary so if you have something other than Cisco router you will need to use PPP. PPP contains 4 main components: *HDLC* - A method for encapsulating datagrams over serial links. *LCP* - A method of establishing, configuring, maintaining, and terminating point-to-point connections. It also provides features such as authentication. *NCP* - A method of establishing and configuring different Network layer protocols for transport across the PPP link. NCP is designed to allow the simultaneous use of multiple Network layer protocols. 2 examples of protocols here are Internet Protocol Control Protocol (IPCP) and Cisco Discovery Protocol Control Protocol (CDPCP). PPP protocol stack is specified at the Physical and Data Link layers only. NCP is used to allow communication of multiple Network Layer protocols by identifying and encapsulating the protocols across a PPP data link
MPLS
Is a data carrying mechanism that emulates some properties of a circuit-switched network over a packet-switched network. So MPLS is a switching mechanism that imposes labels (numbers) to packets and then uses them to forward packets. The labels are assigned on the edge of the MPLS network, and forwarding inside the MPLS network is carried out solely based on the labels. The labels usually correspond to a path to Layer 3 destination addresses, which is on par with IP destination-based routing. Edge routers reform a routing lookup and core routers forward packets on labels making routing fast. You can use Ethernet with MPLS to connect a WAN, and this is called Ethernet over MPLS, or EoMPLS.
The T3 Connection
It carries 44.736Mbps. This is the equivalent to 28 T1 circuits and 672 DS0 channels. It uses a signal known as Digital Signal 3 (DS3), not the same as a DS1, which is generally delivered over fiber-optic cable. As it goes a T1, the T3's European counterpart is the E3, which operates at 34.368Mbps. The Japanese J3 circuit, which operates at 32.064Mbps. *Why does E1 have more capacity and a T1 but a T3 has more capacity than an E3? E and T lines are incremented differently. A T1 is 28 T1s, while an E3 is only 16 E1s.*
Link Control Protocol (LCP) Configuration Options
LCP offers different PPP encapsulation options: *Authentication* - This option tells the calling side of the link to send information that can identify the user. the 2 methods fo this task are PAP and CHAP *Compression* - This is used to increase the throughput of PPP connections by compressing the data or payload prior to transmission. PPP decompresses the data on the receiving end. *Error Detection* - PPP uses Quality and Magic Number options to ensure a reliable, loop-free data link. *Multilink* - The multilink option makes several separate physical paths appear to be 1 logical path at Layer 3. This means that the 2 T1s running multilink PPP would show up as a single 3Mbps path to a Layer 3 routing protocol. *PPP Callback* - On a dial-up connection, PPP can be configured to call back after successful authentication. *PPP callback* can be a very good thing because it allows us to keep track of usage based upon access charges for accounting records and a bunch of other reason. With callback enabled, a calling router (client) will contact a remote router (server) and authenticate. Predictably, both routers have to be configured for the callback feature for this to work. Once authentication is completed, the remote router will terminate the connection and then reinitiate a connection to the calling router.
Common Optical Carrier Levels (OC-x)
Level: OC-1 Data rate: 51.84Mbps Level: OC-3 Data rate: 155.52Mbps Level: OC-12 Data rate: 662.08Mbps Level: OC-48 Data rate: 2.488Gbps Level: OC-192 Data rate: 9.953Gbps
Router Configurations
Make sure to verify your default route to the ISP when trouble shooting using "*show ip route*"
Point-to-Point Protocol stack
OSI Layer: 3 Upper-Layer Protocols (such as IP adn IPv6): Network Control Protocol (NCP) (specifit to each Network layer protocol) OSI Layer: 2 Upper-Layer Protocols (such as IP adn IPv6): Link Control Protocol (LCP) OSI Layer: 2 Upper-Layer Protocols (such as IP adn IPv6): High-Level Data Link Control (HDLC) OSI Layer: 1 Upper-Layer Protocols (such as IP adn IPv6): Physical Layer (such as EIA/TIA-232, V.24, V.35, ISDN)
Bandwidth or Speed
Slowest WAN connection is dail-on-demand dail-up connection. Modern dial-up modems typically have a maximum theoretical transfer speed of 56k, in most cased 40Kbps to 50Kbps. Modems are required to modulate/demodulate the signal, which means translating the analog signal our ears hear into a digital steam for transfer across a digital network. Some connections may be as slow as 20Kbps in noisy environment like hotel rooms where phone line is shared. *Megabyte (MB)* and *gigabyte (GB)* usually refer to the amount of storage capacity available, whereas *bandwidth* and *speed* refer to units that measure how much data (bits) can be sent per second. Speed is essentially the measurement of how fast the data flows (Hz) and also refers to how fast data flows within memory systems. Sometimes Bandwidth and speed are used interchangeably.
Split Horizon
Split horizon usually happens when using Frame Relay in an environment where you have multiple PVCs coming into a single serial WAN interface. This configuration makes the routing protocol think that it's receiving routes on the same interface that they were being sent out of, which in the case would result in the routes being dropped. A great way to solve this problem is to create subinterfaces (logical interfaces) on the serial interface to make the routing protocol believe there are multiple interfaces-1 for each subnet-so the routing advertisement will be received.
DNS Issues
The 1st step in solving a DNS issue is to understand exactly how your DNS is set up on your specific network. Do you have a local DNS server or are you using the ISPs DNS server? If you can ping a site, such as 74.125.228.50, but can't ping www.google.com from an internal host, you know you're dealing with a DNS resolution issue. Start by verifying the local DNS server, if you have 1, and if that checks out, call the ISP
T-Series Connections
The basic, entry-level in bandwidth or speed for leased lines that provide synchronous connections between sites is known as the T1. It serves up 24 Digital Signal 0 (DS0) 64Kbps channels in the USA, Japan, and South Korea. There's a slightly bigger/faster version with 32 DS0 channels that's available in Europe and called the E1 or E carrier line. T1s use Digital Signal 1 (DS1) bit patterns to transmit packets; DS1 has to do with the service to be sent over a T1. Originally, 24 digitized voice channels. The terms T1 and DS1 have become synonymous and include a bunch of different services from voice to data to clear-channel pipes. The line speed is always consistent at 1.544Mbps, but the payload can vary. *24 of these channels are a composite of 1.536Mbps, not 1.544Mbps. The reason is that after each byte of data is sent from each channel (24x8=192), there is an extra bit used for synchronizing called a frame bit. Hence 193 bits are sent, and this increase of 1 bit per 192 causes the speed to increase to 1.544Mbps.* T-series connections are digital that you can lease from the telephone company. They can use copper pairs like regular phone lines, or be brought in as part of a backbone, which is also called a trunk line. T-series connections use time-division multiplexing, or TDM, to devide the bandwidth into channels of equal bit rate. Most commonly used T-series lines are T1 and T3.
PPP Authentication Methods
There are 2 types of authentication that can be used with PPP links: *Password Authentication Protocol (PAP)* - Is the less secure of the 2 methods. Passwords are sent in clear text and PAP is performed only upon the initial link establishment. When the PPP link is 1st established, the remote node sends the username and password back to the originating target router until authentication is acknowledged. *Challenge Handshake Authentication Protocol (CHAP)* - Is used at the initial startup of a link and at periodic checkups on the link to ensure that the router is still communicating with the same host. After PPP finishes its initial link-establishment phase, the local router sends a challenge request to the remote device. The remote device sends a value calculated using a one-way hash function called MD5. The local router checks this hash value to make sure it matches. If it doesn't match the link is terminated.
Frame Relay Technology
WAN technology where packets are transmitted by switching. *Packet switching* involves breaking messages into chunks at the sending device. Each packet can be sent over any number of routes on its way to its destination. Because they are packet-switched the exact path is unknown and a cloud is used in diagrams to illustrate how data travels through this type of service. * Frame relay doesn't work like a point-to-point leased line (although it can be made to look and act like 1). * Frame Relay is usually les expensive than leased lines are, but there are some sacrifices to make to get that savings. Need to know the following for Net+: *Committed Information Rate* Frames Relay allows users to exceed their guaranteed bandwidth usage is resources on the telco network happen to be available. There are 2 separate bandwidth specifications with Frame Relay: *Access Rate* - The maximum speed at which the Frame Relay interface can transmit. *Committed Information Rate (CIR)* - The maximum bandwidth of data guaranteed to be delivered. Any traffic sent beyond what was ordered is *burst* traffic. Any excess burst size known as the *Maximum Burst Rate (MBR), exceeds the access rate and will be dropped. *Virtual Circuits* - Frame Relay operates using *virtual circuits* vs to the actual circuits that leased lines use. 2 types of VC-Permanent and switched: *Permanent Virtual Circuits (PVCs)* are by far the most common type in use today. Permanent means that the telco creates the mappings inside its gear and as long as you pay the bill, they'll remain "permanently" in place. *Switched Virtual Circuits (SVCs)* are more like phone calls. The virtual circuit is established when data needs to be transmitted, and it's taken down when the data transfer is complete. *Data Link Connections Identifiers (DLCI)* Frame Relay ISP assigned DLCI values to differentiate between VC. Because many VCs can be terminated on 1 multipoint Frame Relay interface, many DLCI are often affiliated with it.
PPP Session Establishment
When PPP connections are started, the links go through 3 phases of session establishment: 1: *Link-Establishment Phase* - LCP packets are sent by each PPP device to configure and test the link. These packets contain a field called Configuration Option that allows each device to see the size of the data, the compression, and authentication, if no Configuration Option field is present, then the default configurations will be used. 2: *Authentication Phase* - If required, either CHAP or PAP can be used to authenticate a link. Authentication takes place before Network layer protocol information is read, and it's also possible that link-quality determination will occur simultaneously. 3: *Network Layer Protocol Phase* - PPP uses the *Network Control Protocol (NCP)* to allow multiple Network layer protocols to be encapsulated and sent over a PPP data link. Each Network layer protocol (i.e. IP, IPv6) establishes a service with NCP.
