The SDLC data link allows a reliable exchange of information over a communication facility between SNA devices. The protocol synchronizes receivers and transmitters and detects transmission errors. It accomplishes these functions by acknowledging frame receipt and by performing a cyclic redundancy check (CRC) on the data.
SDLC node types:
- Primary station- Controls operations of other stations, manages links, polls secondary stations.
- Secondary station - controlled by primary stations, cannot communicate with each-other, can also communicate with primary station, only when polled by primary station.
HDLC - High-Level Data Link Control is the default encapsulation type on Cisco router over synchronous serial link.
Cisco provides HDLC serial encapsulation for serial lines. This encapsulation method provides the synchronous framing and error detection functions of HDLC without windowing or retransmission.
Cisco uses its own proprietary implementation of HDLC which is not compatible with other Non-Cisco routers.
To enable HDLC encapsulation type encapsulation hdlc from the interface configuration mode
HDLC supports the following transfer modes:
- Normal Response mode - NRM
- Asynchronous response mode - ARM
- Asynchronous balanced mode - ABM
- Link Access Procedure Balance - LAPB
Dial-Up wan connections.
The most common type of dial-up wan connections are modems and ISDN.
POTS - Plain Old Telephone Service.
Terminology associated with POTS
- CO - Central office. The location of telephone company's equipment where the phone line connects to the high speed line (trunk).
- Local loop - The wiring from customer's location to the nearest CO.
- CPE - Customer premise equipment. Could me modem, CSU/DSU, wiring on the customer's location.
ISDN - Integrated Services Digital Line.
ISDN was developed in mid-80'. What makes ISDN different from POTS is that ISDN allows data to be transmitted digitally, unlike modem connection, where data is converted into analog signal for transmission, and then converted back into digital signal.
ISDN line can simultaneously transmit voice, data and video over the same physical line.
ISDN used Bearer Channels (B channels) and Data channels (D channels).
- B channels occupy 64Kbps bandwidth and can transmit voice and data simultaneously.
- D channel is used for signaling to pass dial string information and occupies 16Kbps on a slow ISDN, and 64Kbps on a T1 line.
There are two types of ISDN service - Basic Rate Interface (BRI) and Primary Rate Interface (PRI). The difference between them is the number of B channels.
- BRI - 2 B channels and 1 D channel for a total of 144 Kbps. Since the D channel is used for signaling, and does not participate in actual data transfer, the bandwidth of the BRI is 128Kbps. BRI can operate with almost all types of Cisco routers, like 1000, 2500, 3000, and 4000 series routers.
- PRI consists of 23 B channels and one 64Kbps D channel for a total bandwidth of 1.536 Kbps.
In Europe PRI consists of 30 B channels and one 64Kbps D channel, for a total of 1,984 Kbps bandwidth.
A BRI customer has the ability to add more channels for an extra cost to accommodate the need for more bandwidth.
B channels are aggregated into a single logical high-speed connection using channel aggregation protocol, such as B channel bonding or Multilink-PPP.
ISDN supports many different protocols, like TCP/IP, IPX, AppleTalk.Available encapsulations for ISDN designs are PPP, HDLC, X.25, and V.120. X.25 can also be used for datagram delivery over the D channel. Most internetworking designs use PPP as the encapsulation.
The distance limitation of ISDN is 18,000 feet from carrier's CO. This distance can be increase using repeaters. Using repeaters may increase cost of service.
At customer premises ISDN line terminates on the device called Terminal Adapter (TA), or sometimes referred to as ISDN modem.One side of the TA connects to the ISDN line, and the other side connects to the serial interface on the router.A Cisco router can have a BRI interface installed, in which case you can connect it directly to the ISDN line.A router with built-in BRI interface is called TE (Terminal End-point1), and router that requires TA is called TE2.
There are 3 types of protocols associated with ISDN:
- E-series - telephone network
- I-series - Concepts, aspects, interfaces
- Q-series - Switching and signaling
Configuring ISDN interfaces
BRI configuration involves configuration of ISDN switch-type, and ISDN SPIDs.To see the list of available switch types, enter isdn switch type ? from the global configuration mode. In North America, the most common types are 5ESS, DMS100, and NI-1. For proper ISDN operation, it is imperative that the correct switch type is configured on the ISDN device.
A service profile identifier (SPID) is a number provided by the ISDN carrier to identify the line configuration of the BRI service. If SPIDs are necessary but not configured or configured incorrectly on the device, the Layer 3 initialization fails, and the ISDN services cannot be used. There is no standard format for SPID numbers. As a result, SPID numbers vary depending on the switch vendor and the carrier.
This is an example of configuring BRI interface:Router(config)#isdn switch-type basic-dms100After we configured the switch type, we must configure encapsulation type and assign SPID numbers from the interface configuration mode. To enter BRI interface configuration mode, type int bri0.Router(config-if)#encap pppRouter(config-if)#isdn spid1 0835866201 8358662Router(config-if)#isdn spid2 0835866401 8358664
To confirm BRI operations, use the show isdn status command to inspect the status of your BRI interfaces.
Troubleshooting SPID problems is done with the debug isdn q921 command.
PRI configuration Cisco IOS routers support PRI interfaces using MultiChannel Interface Processor (MIP) cards. MIP cards can support Channelized T1/E1 or PRI timeslots.
On Cisco routers with IOS 11.2 and older, you cannot have both BRI and PRI interface cards. Cisco IOS 11.3T or later will provide support for multiple switch-types in a single Cisco IOS chassis.
Configure the ISDN switch-type for the PRI interface using the isdn switch-type [switch type] command.
Then enter interface configuration mode by typingRouter(config)#controller T1 1/0Router(config-if)#framing esfRouter(config-if)#linecode b8zsRouter(config-if)#pri-group timeslots 1-24
Note that PRI channels 0-23 map to pri-group timeslots 1-24. The same +1 mapping is used on E1-based PRI.
The state of the T1 controller is inspected with the Cisco IOS exec command: show controller t1
To verify ISDN is operational, use the Cisco IOS exec command show isdn status
Inspect B channel status with the show isdn service exec command The Simple Network Management Protocol (SNMP) uses management information bases (MIBs) to store information about network events. Currently, no industry-standard ISDN MIB is available, but as of Cisco IOS Software Release 10.3(3), two Cisco ISDN MIBs are available. With these MIBs, SNMP-compliant management platforms (for example, HP OpenView or SunNet Manager) can query Cisco routers for ISDN-related statistics.
X.25
X.25 was developed by the common carriers (telephone companies, essentially) rather than any single commercial enterprise. The specification is therefore designed to work well regardless of a user's system type or manufacturer.
The X.25 specification defines a point-to-point interaction between data terminal equipment (DTE) and data circuit-terminating equipment (DCE). DTEs (terminals and hosts in the user's facilities) connect to DCEs (modems, packet switches, and other ports into the PDN, generally located in the carrier's facilities), which connect to packet switching exchanges (PSEs, or switches) and other DCEs inside a PSN and, ultimately, to another DTE.
X.25 defines a telephone network for data communications. To begin communication, one computer calls another to request a communication session. The called computer can accept or refuse the connection. If the call is accepted, the two systems can begin full-duplex information transfer. Either side can terminate the connection at any time.
The X.25 specification maps to Layers 1 through 3 of the OSI reference model. Layer 3 X.25 describes packet formats and packet exchange procedures between peer Layer 3 entities. Layer 2 X.25 is implemented by Link Access Procedure, Balanced (LAPB). LAPB defines packet framing for the DTE/DCE link. Layer 1 X.25 defines the electrical and mechanical procedures for activating and deactivating the physical medium connecting the DTE and the DCE.
Layer 2 X.25 is implemented by LAPB, which allows each side (the DTE and the DCE) to initiate communication with the other. During information transfer, LAPB checks that the frames arrive at the receiver in the correct sequence and free of errors.
Layer 1 X.25 uses the X.21 bis physical-layer protocol
End-to-end communication between DTEs is accomplished through a bidirectional association called a virtual circuit. Virtual circuits permit communication between distinct network elements through any number of intermediate nodes without the dedication of portions of the physical medium that characterizes physical circuits. Virtual circuits can be either permanent or switched (temporary). Permanent virtual circuits (PVCs) are typically used for the most often used data transfers, whereas switched virtual circuits (SVCs) are used for sporadic data transfers. Layer 3 X.25 is concerned with end-to-end communication involving both PVCs and SVCs.
When a virtual circuit is established, the DTE sends a packet to the other end of the connection by sending it to the DCE using the proper virtual circuit. The DCE looks at the virtual circuit number to determine how to route the packet through the X.25 network. The Layer 3 X.25 protocol multiplexes between all the DTEs served by the DCE on the destination side of the network, and the packet is delivered to the destination DTE.
Configuring X.25 From the interface configuration mode set the encapsulation type on this interface to X.25 by typing Router(config-if)#encapsulation x25Set the X.121 address of this interface by typingRouter(config-if)#x25 address 123456Set up the LAN protocols-to-remote-host mapping for IP and X.25.Router(config-if)#x25 map protocol address [protocol2 address2[...[protocol9 address9]]] x121-address [option]For exampleRouter(config-if)#x25 map ip 172.16.26.2 2345 broadcastBecause most datagram routing protocols rely on broadcasts or multicasts to send routing information to their neighbors, the broadcast keyword is needed to run such routing protocols over X.25.
Use the x25 win interface configuration command to change the default maximum number of unacknowledged incoming packets for the interface (Sliding window size).
x25 win packetsPacket count that can range from 1 to one less than the window modulus The default is 2 packets.
The following example specifies that five packets must be received before sending an X.25 acknowledgment:interface serial 1x25 win 5
Use the x25 wout interface configuration command to change the default maximum number of unacknowledged packets to allow.
This command determines the default number of packets the communication server can send before waiting for an X.25 acknowledgment. To maintain high bandwidth utilization, assign this limit the largest number that the network allows.
Set x25 win and x25 wout to the same value unless your network supports asymmetry between input and output window sizes
The show interfaces serial command provides important information useful for identifying problems in X.25 internetworks. The following fields provide especially important information:
- REJs---Number of rejects
- SABMs---Number of Set Asynchronous Balance Mode requests
- RNRs---Number of Receiver Not Ready events
- FRMRs---Number of protocol frame errors
- RESTARTs---Number of restarts
- DISCs---Number of disconnects
To display all traffic for interfaces using LAPB encapsulation use the debug lapb exec command .
If no SABMs are being sent, disable the debug lapb command and enable the debug x25 events privileged exec command.
To check that X.25 has been correctly configured use the show running-config privileged exec command to view the router configuration. Look for x25 map interface configuration command entries.
Frame relay
Frame Relay is a method of packet-switching that is used for communication between user devices (such as routers, bridges, and host machines) and network devices (such as switching nodes and modems). User devices are called data terminal equipment (DTE) and network devices are called data circuit-terminating equipment (DCE).
Frame relay became very popular wan technology due to its low cost and wide availability. On a frame relay connection the link between the customer and the ISP's router can be shared by other customer applications. This is called many-to-one connectivity over a single physical line.
Frame Relay services can be provided by either a public network or a network of privately owned equipment serving a single enterprise
The major difference between X.25 and Frame relay is that Frame relay does not do its own error checking, and relies on the higher layer protocols for error checking. This makes it a faster protocol, but requires better connection lines. Today many Frame relay connections run over fiber optic lines.
Frame Relay uses a simple congestion notification mechanism to inform user devices when the network become congested. Congestion notification alerts the higher-layer protocols that flow control is needed.
Frame relay protocol operates at Physical and Data Link layers of the OSI model.
Fame relay can use both PVC (Private Virtual Circuit) and SVC (Switched Virtual Circuit). SVC is a temporary connection line established only for the duration of communication, similar to the phone line. PVC is a permanent line similar to the dedicated line.
Current Frame Relay standards support permanent virtual circuits (PVCs) that are configured and managed in a Frame Relay network. Cisco IOS software releases 11.2 and later support switched virtual circuits (SVCs) for DTE interfaces.
The Frame relay is less expensive then the dedicated line like a T1. When subscribing to Frame Relay, the ISP will guarantee you a minimum bandwidth that will always be available to you. This is called Committed Information Rate - CIR. When the line is not saturated with traffic, you will be able to get a much higher transfer rate then the CIR.
Frame Relay supports encapsulation of all supported protocols in conformance with RFC 1490, allowing interoperability between multiple vendors.
Enabling Frame Relay encapsulation on the serial interface requires the following steps:
Step 1 Enter interface configuration mode and select the serial interface to configure.
Step 2 Enable encapsulation on the interface.
Router(config-if)#encapsulation frame-relay [type]Sets the encapsulation method on this interface to Frame Relay. There are two types of Frame relay encapsulation: Cisco and IETF. The default is Cisco. This will work only if you have Cisco router on both sides of Frame Relay circuit. If you have another vendor's router, you must use IETF. Also use IETF when configuring Frame Relay on ATM switch router.
To verify the Frame relay configuration, confirming an active permanent virtual circuit (PVC) is active on the Frame Relay line by typing show frame-relay pvc from the privileged exec mode. Confirm that the "PVC STATUS=ACTIVE" message appears in the command output.
If there is no output after entering the command, use the show interface serial0 command to determine whether or not the serial interface is active.
The next step in configuring a Frame relay is to assign DLCI number to a serial interface from the interface configuration mode. The DLCI numbers are usually assigned by the ISPs that offer Frame Relay service. If you work on your own end-to-end Frame relay network, you can make up your own DLCI numbers. Use this command to assign DLCI:Router(config-if)#frame-relay interface-dlci [number]
DLCI numbers are from 16-1007. After you enter the above command and DLCI number, the Router prompt changes to this:Router(config-fr-dlci)#
To find out your DLCI number on an existing Frame relay network, use show frame-relay pvc from the privileged exec mode.
LMI - Local Management Interface. LMI is a set of enhancements to the Frame Relay protocol. When configuring a Cisco router with IOS prior to 11.2, you must specify the LMI type. IOS 11.2 and newer are set to autodetect LMI type. Check with your Frame Relay ISP to find out the LMI type. There are 3 types of LMI:
Cisco
ANSI
q933a
The Local Management Interface (LMI) provides a set of enhancements to the basic Frame Relay specification, including support for a keepalive mechanism and statistics. You can configure the following LMI-related parameters on the Frame Relay interface:
LMI type
LMI keepalive interval
LMI polling and timer intervals (optional)
LMI type is configured from the interface configuration mode.
A keepalive interval must be set to configure the LMI. By default, this interval is 10 seconds
Configuring the keepalive interval requires the following steps:
Step 1 Enter interface configuration mode and select the serial interface to configure.
Step 2 Specify a keepalive interface for the interface.
Router(config-if)#frame-relay keepalive 15This sets keepalive interval to 15 seconds.
Keepalive specifies how long the connection is kept active in between the bursts of packets.
A single serial interface can have multiple virtual circuits. To configure multiple virtual circuits, you enter a subinterface configuration mode. Subinterface is a logical interface that acts as a separate physical interface. Subinterfaces operate independently of each other. You can have one subinterface configured for IP routing and another one for IPX routing. This is similar to binding different protocols to a single NIC card on a Windows OS.
There are two types of Subinterfaces: point-to-point and multipoint. Point-to-point is used with a single virtual circuit (PVC or SVC), and multipoint is used when the interface connects to many virtual circuits.
To assign a subinterface type, first enter subinterface configuration mode by typingRouter(config)#int s0.17 You can use different number then 17, it does not matter. Then specify the interface type by enteringinterface Serial0.17 point-to-pointThis command sets the subinterface Serial 0.17 as point-to-point type.
The IP address of the interface (or subinterface) must be mapped to the DLCI number.
The syntax for mapping is: frame-relay map ip [ip address] [dlci number] [encapsulation type]This must be entered from the interface or subinterface configuration mode.
PPP - Point-to-Point Protocol
PPP provides a standard method for transporting multiprotocol packets over point-to-point links. PPP is defined in RFC 1661.Multiprotocol packets means that PPP can transport many different protocols, like IP, IPX, AppleTalk, DECnet, and several more protocols.
PPP is a Data link protocol, and therefore referred to as Layer 2 protocol.
PPP supports compression, authentication, dynamic addressing.
PPP uses Link Control Packets to exchange information between devices. The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection. Before any network-layer datagrams (for example, IP) can be exchanged, LCP must first open the connection and negotiate configuration parameters. This phase is complete when a configuration acknowledgment frame has been both sent and received.
The PPP authentication protocols (PAP and CHAP) are defined in RFC 1334. After LCP has established the PPP connection, an optional authentication protocol can be implemented before proceeding to the negotiation and establishment of the Network Control Protocols. If authentication is desired, it must be negotiated as an option at the LCP establishment phase. Authentication can be bidirectional (both sides authenticate the other) or unidirectional (one side, typically the called side, authenticates the other).
Multilink PPP (MP) is a standard for aggregating multiple PPP links that allows for multivendor interoperability, and is defined in RFC 1717. MP defines a way of sequencing and transmitting packets over multiple physical interfaces. To reduce potential latency issues, MP also defines a method of fragmenting and reassembling large packets.
The Point-to-Point (PPP) Compression Control Protocol (CCP) is an Internet Engineering Task Force (IETF) draft RFC that defines a method for negotiating data compression over PPP links. These links can be either leased lines or circuit-switched WAN links, including ISDN. Compression increases throughput and shortens file transfer times.
