23. WAN
ROX™ v2.2 User Guide
237
RuggedBackbone™ RX1500
23. WAN
23.1. T1/E1 Fundamentals
A T1 line is a communications circuit using the Digital Signal 1 (DS1) signalling scheme. DS1 allows 24
“timeslots” of 64 Kbps DS0 information, along with 8 Kbps of signalling information, to be multiplexed
onto a 1544 Kbps circuit.
The 24 DS0s can be used individually as standalone channels, bonded into groups of channels, or can
be bonded to form a single 1536 Kbps channel, referred to as a clear channel. Not all channels need
be used. It is quite common to purchase a number of channels of 64Kbps bandwidth and leave the
remainder unused; this is known as fractional T1.
The telephone network terminates the T1 line and maps each of the channels through the T1 network
to a chosen T1 line. Individual and bonded DS0s from more than one remote T1 can be aggregated
into a full T1 line. This is referred to as central site concentration.
The T1 line itself is referred to as the physical interface. Groups of DS0s form channels and the protocols
that run on the channels are known as logical interfaces. The RuggedBackbone™ provides the ability
to operate Frame Relay or PPP over logical interfaces.
An E1 line is a communications circuit conforming to European standards with 32 64 Kbps channels,
one of which is usually reserved for signalling information.
23.1.1. Frame Relay
Frame Relay is a packet switching protocol for use over the WAN. The RuggedBackbone™ provides
the ability to construct point-to-point IP network connections over Frame Relay.
Each Frame Relay interface provides a link between a local and a peer station. One of the stations
must be configured as a Data Communications Equipment (DCE) device, often referred to as a Switch.
The the peer station must be configured as a Data Terminal Equipment (DTE) device, often referred
to as Customer Premises Equipment (CPE). The DCE is responsible for managing the link, advertising
connections to the DTE, and switching packets between connections. The DTE raises individual
connections and sends data on them. A frame relay switch is usually configured as DCE, and the
RX1500 is configured as DTE.
When using a T1/E1 line to access a public Frame Relay provider, configure the router as a DTE.
Unlike PPP, a Frame Relay link can provide multiple connections. Each connection is identified by
a Data Link Connection Identifier (DLCI) and must match at the DCE and DTE. The use of multiple
connections can support meshed network interconnections and disaster recovery.
23.1.2. RX1500 and Frame Relay Encapsulation
The RX1500 supports IETF encapsulation for frame relay connections. When connecting the RX1500
to a Cisco router, you must configure the Cisco router to use IETF encapsulation. How you configure
IETF encapsulation depends on the number of Data Link Connection Identifiers (DLCI) in use:
• When using a single physical Frame Relay interface and connecting to the RX1500 with one Data
Link Connection Identifier (DLCI), enable IETF encapsulation as follows:
Cisco#conf terminal
Cisco(config)#interface serial0/0
Cisco(config-if)#encapsulation frame-relay IETF
Cisco(config-if)#frame map ip ipaddress dlci broadcast
Where
ipaddress
is the remote IP address, and
dlci
is the DLCI number.
