WAN overview
The software of the router provides the ability for holding a large range of network , which can meet the demand of various network circumstances.This chapter introduce some common guidances to configure following :
This chapter only give brief
descriptions of these technologies, if you want to know more details, please
refer to corresponding chapters.
Frame relay of the router’s
software support IP route and private line now.
The software provides such functions:
Support three kinds of
generally implemented specifications of Frame Relay Local
Management Interfaces (LMIs):
frame relay interfaces linking specifications named by Northern
Telecom, Digital Equipment Corporation, StrataCom, and Cisco Systems
T1.617 Annex D , Frame Relay signal specification adopted by ANSI.
Q.933 annex A, Frame relay signal specification adopted by (ITU-T).
accord ITU-T I-series (ISDN) recommendation “I122”,
"framework for annex Packet
Mode Services."
T1.618,
frame relay encapsulation specifications adopted by ANSI
Q.922 annex A, frame relay encapsulation specifications
adopted by ITU-T
accord RFC 2427 Internet
engineering task force(IETF) encapsulation, except bridging
support
a keepalive mechanism, a multicast group, and a status message as follows:
The
keepalive mechanism provides an exchange of information between network
server and switch to validate data fluidity.
The multiple mechanism
provides for network server a local data connection identifier(DLCI) and a
multicast DLCI. This feature is special to our realization of the frame
relay connecting specification.
a status mechanism provides
a status message reporting DLCI which is known by switch is running.
support inverse ARP as described in RFC 1293. It permits a router running Frame Relay to discover the protocol address of a device associated with the virtual circuit.
support frame relay
switching, where packets are switched based on DLCI(a
Frame Relay equivalent of a media access control (MAC)). Routers can be
configured as a hybrid DTE switch or pure Frame relay DCE access node in the
frame relay network. Following configurations are permitted to realize frame
relay switching .
Switching
on IP channel
from
network-network interface(NNI) to other frame relay switch
local
serial-serial switching
When all flux received by a DLCI can be transmit to the same next hop address by another DLCI, you can use frame relay switching. In such a case, the software of the router does not need to check every frame to assure target address, so,it will reduce processing burden of the router.
X.25 is one of a group of specifications defined by ITU-T, these specifications
are international standards, and they are called recommendations before.
ITU-T X.25 has defined how DTE and DCE are mainted for remote
terminal access and computer computer communication.
The X.25 specification has defined two-layer protocols of the
Open Systems Interconnection (OSI) reference model. The data link layer protocol
is defined as Link Access Procedure, Balanced (LAPB). The network layer protocol
is sometimes called the packet level protocol(PLP), but it is always called the
X.25 protocol(although less correctly) .
The ITU-T updates its Recommendations periodically. The specifications published in 1980 and 1984 are most common versions. In addition,the international standards organization (ISO) has published ISO 7776:1986 which is equal to the LAPB standard, and ISO 8208:1989 which is equal to the ITU-T 1984 X.25 recommendation packet layer.
The router’s x.25 follows the ITU-T 1982 x.25
recommendation,but its defence data network(DDN) and blacker front end (BFE)
operation follow the ITU-T 1980 X.25 recommendation.
Note:the ITU-T executes the functions of the former
consultative committee for international telegraph and teleohone(ccitt).The 1988
X.25 standard was the last CCITT recommendation. The first ITU-T recommendation
was the 1993 version.
Except providing remote terminal access, the router’s X.25
software provides transport for IP(LAN protocols).
In a brief, the X.25 software provides following functions:
LAPB datagram transport-LAPB is a protocol that work at level
2(the data link layer) of the OSI reference model. It offers a relioable
connection for switch data(its unit is frame) between two hosts.
X.25 datagram transport--X.25 can setup connections with
several hosts; these connections are named “virtual circuits”.
Protocol datagrams (IP) are encapsulated in packets on an X.25 virtual
circuit. Maps between a host's X.25 address and its datagram protocol address
permit these datagrams to route through an X.25 network, so, allow an X.25
public data network (PDN) to transport LAN protocols.
X.25 switch –X.25 calls can be routed by a serial interface
on the same router based on their X.25 addresses to another router.
PAD--User sessions can be carried across an X.25 network
using the packet assembler/disassembler (PAD) protocols defined by the ITU-T
Recommendations X.3 and X.29
The
router’s software X.25 realization does not support quick switching.
High-level data link control protocol(HDLC)is derived from
synchronization data link control protocol (SDLC) in SNA of IBM.After developing
SDLC, IBM provides it to ANSI AND ISO, dividing them to America standard and
international standard. And ANSI modified this protocol to advanced data
communication CONTROL protocol(ADCCP);ISO modified this protocol to HDLC.
Later,CCITT adopted and modified HDLC, and became a part of link access protocol
(LAP) and X.25 interface standard. HDLC is a protocol facing to byte ,which
guarantees clarities of data.
HDLC is a data packet protocol, it defines an linking
encapsulation to IP packet on synchronization, and run TCP/IP on point-to-point
serial line.
HDLC is usually used to DDN lines, and it is a simple and
high-efficiency protocol.
Whatever it is a router-router linking way or it is a
dialing-host to router linking way, point-point data linking layer protocol(such
as SLIP,PPP) is need in line to encapsulate frame and control errors.
SLIP is a data packeted protocol, it defines a series of
strings to encapsulate IP packet on serial line, and it is used to run TCP/IP on
point-point serial line.
The realization of SLIP accord to RFC 1055, support TCP/IP header compression recommendated by RFC 1144 ,and SLIP supports that various network devices and hosts realize connection with each other through SLIP protocol.
PPP provides multiple-protocol transportation on point-point
link. The router’s main fucntions are:
①.It obeys RFC 1661 and supports link controlling
protocol(LCP),is used to setup,configure and test data link.
②.It obeys RFC 1662 and supports IP encapsulation on PPP, and
it can realize IPCP on NCP.
③.It obeys RFC 1334 and support several common authentication
protocol ,including PAP,CHAP,MC-CHAP and TACACS+.
④.It obeys RFC 1144 and support TCP/IP header compression, and
it can advance efficiency data throughput.
⑤.It provides broad chooses of items, can adapt many
conditions, and it support to link to all kinds of network equipments and hosts
through PPP.
⑥.It support synchronism and asynchronism PPP .
⑦.It support PPP multilink ,can realize multiple-link binding.
⑧.It can support “call back”, provides more safety.
⑨.It can support RADIUS ,can realize authentication to user ,authorization and accouting when the router is used as a dialing server. The information of users are saved in a host, which switch information with routers through RADIUS.